This section describes what Emacs does when it is started, and how you can customize these actions.

(setq inhibit-startup-echo-area-message
      "YOUR-LOGIN-NAME")

(load (concat term-file-prefix (getenv "TERM")))

-OPTION

(OPTION . HANDLER-FUNCTION)

There are two ways to get out of Emacs: you can kill the Emacs job, which exits permanently, or you can suspend it, which permits you to reenter the Emacs process later. (In a graphical environment, you can of course simply switch to another application without doing anything special to Emacs, then switch back to Emacs when you want.)

(add-hook 'suspend-hook
          (lambda () (or (y-or-n-p "Really suspend?")
                         (error "Suspend canceled"))))
(add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
                                 (sit-for 2)))

---------- Buffer: Minibuffer ----------
Really suspend? y
---------- Buffer: Minibuffer ----------

---------- Parent Shell ----------
bash$ pwd
/home/username
bash$ fg

---------- Echo Area ----------
Resumed!

Emacs provides access to variables in the operating system environment through various functions. These variables include the name of the system, the user's UID, and so on.

system-configuration

This variable holds the standard GNU configuration name for the hardware/software configuration of your system, as a string. For example, a typical value for a 64-bit GNU/Linux system is "x86_64-unknown-linux-gnu".

system-type

The value of this variable is a symbol indicating the type of operating system Emacs is running on. The possible values are:

aix

IBM's AIX.

berkeley-unix

Berkeley BSD and its variants.

cygwin

Cygwin, a POSIX layer on top of MS-Windows.

darwin

Darwin (macOS).

gnu

The GNU system (using the GNU kernel, which consists of the HURD and Mach).

gnu/linux

A GNU/Linux system—that is, a variant GNU system, using the Linux kernel. (These systems are the ones people often call "Linux", but actually Linux is just the kernel, not the whole system.)

gnu/kfreebsd

A GNU (glibc-based) system with a FreeBSD kernel.

haiku

The Haiku operating system, a derivative of the Be Operating System.

hpux

Hewlett-Packard HPUX operating system.

nacl

Google Native Client (NaCl) sandboxing system.

android

The Open Handset Alliance's Android operating system.

ms-dos

Microsoft's DOS. Emacs compiled with DJGPP for MS-DOS binds system-type to ms-dos even when you run it on MS-Windows.

usg-unix-v

AT&T Unix System V.

windows-nt

Microsoft Windows NT, 9X and later. The value of system-type is always windows-nt, e.g., even on Windows 10.

We do not wish to add new symbols to make finer distinctions unless it is absolutely necessary! In fact, we hope to eliminate some of these alternatives in the future. If you need to make a finer distinction than system-type allows for, you can test system-configuration, e.g., against a regexp.

system-name

This function returns the name of the machine you are running on, as a string.

mail-host-address

If this variable is non-nil, it is used instead of system-name for purposes of generating email addresses. For example, it is used when constructing the default value of user-mail-address. User Identification.

Command getenv

This function returns the value of the environment variable var, as a string. var should be a string. If var is undefined in the environment, getenv returns nil. It returns "" if var is set but null. Within Emacs, a list of environment variables and their values is kept in the variable process-environment.

(getenv "USER")
     => "lewis"

The shell command printenv prints all or part of the environment:

bash$ printenv
PATH=/usr/local/bin:/usr/bin:/bin
USER=lewis
TERM=xterm
SHELL=/bin/bash
HOME=/home/lewis
...

Command setenv

This command sets the value of the environment variable named variable to value. variable should be a string. Internally, Emacs Lisp can handle any string. However, normally variable should be a valid shell identifier, that is, a sequence of letters, digits and underscores, starting with a letter or underscore. Otherwise, errors may occur if subprocesses of Emacs try to access the value of variable. If value is omitted or nil (or, interactively, with a prefix argument), setenv removes variable from the environment. Otherwise, value should be a string. If the optional argument substitute is non-nil, Emacs calls the function substitute-env-vars to expand any environment variables in value. setenv works by modifying process-environment; binding that variable with let is also reasonable practice. setenv returns the new value of variable, or nil if it removed variable from the environment.

with-environment-variables

This macro sets the environment variables according to variables temporarily when executing body. The previous values are restored when the form finishes. The argument variables should be a list of pairs of strings of the form (VAR VALUE), where var is the name of the environment variable and value is that variable's value.

(with-environment-variables (("LANG" "C")
                             ("LANGUAGE" "en_US:en"))
  (call-process "ls" nil t))

process-environment

This variable is a list of strings, each describing one environment variable. The functions getenv and setenv work by means of this variable.

process-environment
=> ("PATH=/usr/local/bin:/usr/bin:/bin"
    "USER=lewis"
    "TERM=xterm"
    "SHELL=/bin/bash"
    "HOME=/home/lewis"
    ...)

If process-environment contains multiple elements that specify the same environment variable, the first of these elements specifies the variable, and the others are ignored.

initial-environment

This variable holds the list of environment variables Emacs inherited from its parent process when Emacs started.

path-separator

This variable holds a string that says which character separates directories in a search path (as found in an environment variable). Its value is ":" for Unix and GNU systems, and ";" for MS systems.

path-separator

This function returns the connection-local value of variable path-separator. That is ";" for MS systems and a local default-directory, and ":" for Unix and GNU systems, or a remote default-directory.

parse-colon-path

This function takes a search path string such as the value of the PATH environment variable, and splits it at the separators, returning a list of directories. nil in this list means the current directory. Although the function's name says "colon", it actually uses the value of variable path-separator.

(parse-colon-path ":/foo:/bar")
     => (nil "/foo/" "/bar/")

invocation-name

This variable holds the program name under which Emacs was invoked. The value is a string, and does not include a directory name.

invocation-directory

This variable holds the directory in which the Emacs executable was located when it was run, or nil if that directory cannot be determined.

installation-directory

If non-nil, this is a directory within which to look for the lib-src and etc subdirectories. In an installed Emacs, it is normally nil. It is non-nil when Emacs can't find those directories in their standard installed locations, but can find them in a directory related somehow to the one containing the Emacs executable (i.e., invocation-directory).

load-average

This function returns the current 1-minute, 5-minute, and 15-minute system load averages, in a list. The load average indicates the number of processes trying to run on the system. By default, the values are integers that are 100 times the system load averages, but if use-float is non-nil, then they are returned as floating-point numbers without multiplying by 100. If it is impossible to obtain the load average, this function signals an error. On some platforms, access to load averages requires installing Emacs as setuid or setgid so that it can read kernel information, and that usually isn't advisable. If the 1-minute load average is available, but the 5- or 15-minute averages are not, this function returns a shortened list containing the available averages.

(load-average)
     => (169 48 36)
(load-average t)
     => (1.69 0.48 0.36)

The shell command uptime returns similar information.

emacs-pid

This function returns the process ID of the Emacs process, as an integer.

tty-erase-char

This variable holds the erase character that was selected in the system's terminal driver, before Emacs was started.

null-device

This variable holds the system null device. Its value is "/dev/null" for Unix and GNU systems, and "NUL" for MS systems.

null-device

This function returns the connection-local value of variable null-device. That is "NUL" for MS systems and a local default-directory, and "/dev/null" for Unix and GNU systems, or a remote default-directory.

init-file-user

This variable says which user's init files should be used by Emacs—or nil if none. "" stands for the user who originally logged in. The value reflects command-line options such as -q or -u USER. Lisp packages that load files of customizations, or any other sort of user profile, should obey this variable in deciding where to find it. They should load the profile of the user name found in this variable. If init-file-user is nil, meaning that the -q, -Q, or -batch option was used, then Lisp packages should not load any customization files or user profile.

user-mail-address

This holds the email address of the user who is using Emacs.

user-login-name

This function returns the name under which the user is logged in. It uses the environment variables LOGNAME or USER if either is set. Otherwise, the value is based on the effective UID, not the real UID. If you specify uid (a number), the result is the user name that corresponds to uid, or nil if there is no such user.

user-real-login-name

This function returns the user name corresponding to Emacs's real UID. This ignores the effective UID, and the environment variables LOGNAME and USER.

user-full-name

This function returns the full name of the logged-in user—or the value of the environment variable NAME, if that is set. If the Emacs process's user-id does not correspond to any known user (and provided NAME is not set), the result is "unknown". If uid is non-nil, then it should be a number (a user-id) or a string (a login name). Then user-full-name returns the full name corresponding to that user-id or login name. If you specify a user-id or login name that isn't defined, it returns nil.

The symbols user-login-name, user-real-login-name and user-full-name are variables as well as functions. The functions return the same values that the variables hold. These variables allow you to fake out Emacs by telling the functions what to return. The variables are also useful for constructing frame titles (Frame Titles).

user-real-uid

This function returns the real UID of the user.

user-uid

This function returns the effective UID of the user.

file-user-uid

This function returns the connection-local value for the user's effective UID. If default-directory is local, this is equivalent to user-uid, but for remote files (Remote Files), it will return the UID for the user associated with that remote connection; if the remote connection has no associated user, it will instead return -1.

group-real-gid

This function returns the real GID of the Emacs process.

group-gid

This function returns the effective GID of the Emacs process.

file-group-gid

This function returns the connection-local value for the user's effective GID. Similar to file-user-uid, if default-directory is local, this is equivalent to group-gid, but for remote files (Remote Files), it will return the GID for the user associated with that remote connection; if the remote connection has no associated user, it will instead return -1.

system-users

This function returns a list of strings, listing the user names on the system. If Emacs cannot retrieve this information, the return value is a list containing just the value of user-real-login-name.

system-groups

This function returns a list of strings, listing the names of user groups on the system. If Emacs cannot retrieve this information, the return value is nil.

group-name

This function returns the group name that corresponds to the numeric group ID gid, or nil if there is no such group.

This section explains how to determine the current time and time zone. Many functions like current-time and file-attributes return Lisp timestamp values that count seconds, and that can represent absolute time by counting seconds since the epoch of 1970-01-01 00:00:00 UTC (Coordinated Universal Time). Typically these counts ignore leap seconds; however, GNU and some other operating systems can be configured to count leap seconds. Although traditionally Lisp timestamps were integer pairs, their form has evolved and programs ordinarily should not depend on the current default form. If your program needs a particular timestamp form, you can use the time-convert function to convert it to the needed form. Time Conversion. There are currently three forms of Lisp timestamps, each of which represents a number of seconds:

• An integer. Although this is the simplest form, it cannot represent subsecond timestamps.
• A pair of integers (TICKS . HZ), where hz is positive. This represents ticks///hz seconds, which is the same time as plain ticks if hz is 1. A common value for hz is 1000000000, for a nanosecond-resolution clock.
• A list of four integers (/high/ /low/ /micro/ /pico/), where 0@leq{}/low/<65536, 0@leq{}/micro/<1000000, and 0@leq{}/pico/<1000000. This represents the number of seconds using the formula: high * 2**16 + low + micro * 10**−6 + pico * 10**−12. If current-time-list is t, some functions may default to returning two- or three-element lists, with omitted micro and pico components defaulting to zero. On all current machines pico is a multiple of 1000, but this may change as higher-resolution clocks become available.

Function arguments, e.g., the time argument to format-time-string, accept a more-general time value format, which can be a Lisp timestamp, nil for the current time, a finite floating-point number for seconds, or a list (HIGH LOW MICRO) or (HIGH LOW) that is a truncated list timestamp with missing elements taken to be zero. Time values can be converted to and from calendrical and other forms. Some of these conversions rely on operating system functions that limit the range of possible time values, and signal an error such as "Specified time is not representable" if the limits are exceeded. For instance, a system might not support timestamps before the epoch, or years far in the future. You can convert a time value into a human-readable string using format-time-string, into a Lisp timestamp using time-convert, and into other forms using decode-time and float-time. These functions are described in the following sections.

current-time-string

This function returns the current time and date as a human-readable string. The format does not vary for the initial part of the string, which contains the day of week, month, day of month, and time of day in that order: the number of characters used for these fields is always the same, although (unless you require English weekday or month abbreviations regardless of locale) it is typically more convenient to use format-time-string than to extract fields from the output of current-time-string, as the year might not have exactly four digits, and additional information may some day be added at the end. The argument time, if given, specifies a time to format, instead of the current time. The optional argument zone defaults to the current time zone rule. Time Zone Rules. The operating system limits the range of time and zone values.

(current-time-string)
     => "Fri Nov  1 15:59:49 2019"

current-time-list

This boolean variable is a transition aid. If t, current-time and related functions return timestamps in list form, typically (HIGH LOW MICRO PICO); otherwise, they use (TICKS . HZ) form. Currently this variable defaults to t, for behavior compatible with previous Emacs versions. Developers are encouraged to test timestamp-related code with this variable set to nil, as it will default to nil in a future Emacs version, and will be removed in some version after that.

current-time

This function returns the current time as a Lisp timestamp. If current-time-list is nil, the timestamp has the form (TICKS . HZ) where ticks counts clock ticks and hz is the clock ticks per second. Otherwise, the timestamp has the list form (HIGH LOW USEC PSEC). You can use (time-convert nil t) or (time-convert nil 'list) to obtain a particular form regardless of the value of current-time-list. Time Conversion.

float-time

This function returns the current time as a floating-point number of seconds since the epoch. The optional argument time, if given, specifies a time to convert instead of the current time. Warning: Since the result is floating point, it may not be exact. Do not use this function if precise time stamps are required. For example, on typical systems (float-time '(1 . 10)) displays as 0.1 but is slightly greater than 1/10. time-to-seconds is an alias for this function.

current-cpu-time

Return the current CPU time along with its resolution. The return value is a pair (CPU-TICKS . TICKS-PER-SEC). The CPU-TICKS counter can wrap around, so values cannot be meaningfully compared if too much time has passed between them.

The default time zone is determined by the TZ environment variable. System Environment. For example, you can tell Emacs to default to Universal Time with (setenv "TZ" "UTC0"). If TZ is not in the environment, Emacs uses system wall clock time, which is a platform-dependent default time zone. The set of supported TZ strings is system-dependent. GNU and many other systems support TZDB timezones, e.g., "America/New_York" specifies the time zone and daylight saving time history for locations near New York City. GNU and most other systems support POSIX-style TZ strings, e.g., "EST5EDT specifies the rules used in New York from 1987 through 2006. All systems support the string "UTC0" meaning Universal Time. Functions that convert to and from local time accept an optional time zone rule argument, which specifies the conversion's time zone and daylight saving time history. If the time zone rule is omitted or nil, the conversion uses Emacs's default time zone. If it is t, the conversion uses Universal Time. If it is wall, the conversion uses the system wall clock time. If it is a string, the conversion uses the time zone rule equivalent to setting TZ to that string. If it is a list (offset abbr), where offset is an integer number of seconds east of Universal Time and abbr is a string, the conversion uses a fixed time zone with the given offset and abbreviation. An integer offset is treated as if it were (offset abbr), where abbr is a numeric abbreviation on POSIX-compatible platforms and is unspecified on MS-Windows.

current-time-zone

This function returns a list describing the time zone that the user is in. The value has the form (OFFSET ABBR). Here offset is an integer giving the number of seconds ahead of Universal Time (east of Greenwich). A negative value means west of Greenwich. The second element, abbr, is a string giving an abbreviation for the time zone, e.g., "CST" for China Standard Time or for U.S. Central Standard Time. Both elements can change when daylight saving time begins or ends; if the user has specified a time zone that does not use a seasonal time adjustment, then the value is constant through time. If the operating system doesn't supply all the information necessary to compute the value, the unknown elements of the list are nil. The argument time, if given, specifies a time value to analyze instead of the current time. The optional argument zone defaults to the current time zone rule. The operating system limits the range of time and zone values.

These functions convert time values (Time of Day) to Lisp timestamps, or into calendrical information and vice versa. Many operating systems use 64-bit signed integers to count seconds, and can represent times far in the past or future. However, some are more limited. For example, old-fashioned operating systems that use 32-bit signed integers typically handle only times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 Universal Time. Calendrical conversion functions use the Gregorian calendar even for dates before the Gregorian calendar was introduced, and for dates in the far distant past or future for which the Gregorian calendar is wildly inaccurate and disagrees with common practice in scientific fields like astronomy and paleontology, which use Julian-calendar year lengths. Year numbers count since the year 1 BCE, and do not skip zero as traditional Gregorian years do; for example, the year number −37 represents the Gregorian year 38 BCE.

time-convert

This function converts a time value into a Lisp timestamp. The form argument specifies the timestamp form to be returned. If form is the symbol integer, this function returns an integer count of seconds. If form is a positive integer, it specifies a clock frequency and this function returns an integer-pair timestamp (TICKS . FORM). If form is t, this function treats it as a positive integer suitable for representing the timestamp; for example, it is treated as 1000000000 if time is nil and the platform timestamp has nanosecond resolution. If form is list, this function returns an integer list (HIGH LOW MICRO PICO). Although a nil form currently acts like list, this is planned to change in a future Emacs version, so callers requiring list timestamps should pass list explicitly. If time is not a time value, this function signals an error. Otherwise, if time cannot be represented exactly, conversion truncates it toward minus infinity. When form is t, conversion is always exact so no truncation occurs, and the returned clock resolution is no less than that of time. By way of contrast, although float-time can also convert any time value without signaling an error, the result might not be exact. Time of Day. For efficiency this function might return a value that is eq to time, or that otherwise shares structure with time. Although (time-convert nil nil) is equivalent to (current-time), the latter may be a bit faster.

(setq a (time-convert nil t))
=> (1564826753904873156 . 1000000000)
(time-convert a 100000)
=> (156482675390487 . 100000)
(time-convert a 'integer)
=> 1564826753
(time-convert a 'list)
=> (23877 23681 904873 156000)

decode-time

This function converts a time value into calendrical information. If you don't specify time, it decodes the current time, and similarly zone defaults to the current time zone rule. Time Zone Rules. The operating system limits the range of time and zone values. The form argument controls the form of the returned seconds element, as described below. The return value is a list of nine elements, as follows:

(SECONDS MINUTES HOUR DAY MONTH YEAR DOW DST UTCOFF)

Here is what the elements mean:

seconds

The number of seconds past the minute, with form described below.

minutes

The number of minutes past the hour, as an integer between 0 and 59.

hour

The hour of the day, as an integer between 0 and 23.

day

The day of the month, as an integer between 1 and 31.

month

The month of the year, as an integer between 1 and 12.

year

The year, an integer typically greater than 1900.

dow

The day of week, as an integer between 0 and 6, where 0 stands for Sunday.

dst

t if daylight saving time is effect, nil if it is not in effect, and −1 if this information is not available.

utcoff

An integer indicating the Universal Time offset in seconds, i.e., the number of seconds east of Greenwich.

The seconds element is a Lisp timestamp that is nonnegative and less than 61; it is less than 60 except during positive leap seconds (assuming the operating system supports leap seconds). If the optional form argument is t, seconds uses the same precision as time; if form is integer, seconds is truncated to an integer. For example, if time is the timestamp (1566009571321 . 1000), which represents 2019-08-17 02:39:31.321 UTC on typical systems that lack leap seconds, then (decode-time TIME t t) returns ((31321 . 1000) 39 2 17 8 2019 6 nil 0), whereas (decode-time TIME t 'integer) returns (31 39 2 17 8 2019 6 nil 0). If form is omitted or nil, it currently defaults to integer but this default may change in future Emacs releases, so callers requiring a particular form should specify form. Common Lisp Note: Common Lisp has different meanings for dow, dst and utcoff, and its second is an integer between 0 and 59 inclusive. To access (or alter) the elements in the calendrical information, the decoded-time-second, decoded-time-minute, decoded-time-hour, decoded-time-day, decoded-time-month, decoded-time-year, decoded-time-weekday, decoded-time-dst and decoded-time-zone accessors can be used.

encode-time

This function converts time to a Lisp timestamp. It can act as the inverse of decode-time. Ordinarily the first argument is a list (SECOND MINUTE HOUR DAY MONTH YEAR IGNORED DST ZONE) that specifies a decoded time in the style of decode-time. For the meanings of these list elements, see the table under decode-time. In particular, dst says how to interpret timestamps during a daylight saving fallback when timestamps are repeated. If dst is −1, the DST value is guessed; if it is t or nil the timestamp with that DST value is returned, with an error signaled if no such timestamp exists. Unfortunately a dst value of t or nil does not disambiguate timestamps duplicated when a TZDB-based timezone moves further west of Greenwich, such as disambiguating the two standard-time timestamps 2020-12-27 01:30 when zone is "Europe/Volgograd", which at 02:00 that day changed standard time from 4 to 3 hours east of Greenwich; if you need to handle situations like this you can use a numeric zone to disambiguate instead. The first argument can also be a list (SECOND MINUTE HOUR DAY MONTH YEAR), which is treated like the list (SECOND MINUTE HOUR DAY MONTH YEAR nil -1 nil). As an obsolescent calling convention, this function can be given six or more arguments. The first six arguments second, minute, hour, day, month, and year specify most of the components of a decoded time. If there are more than six arguments the last argument is used as zone and any other extra arguments are ignored, so that (apply #'encode-time (decode-time ...)) works. In this obsolescent convention, dst is −1 and zone defaults to the current time zone rule (Time Zone Rules). When modernizing an obsolescent caller, ensure that the more-modern list equivalent contains 9 elements with a dst element that is −1, not nil. Year numbers less than 100 are not treated specially. If you want them to stand for years above 1900, or years above 2000, you must alter them yourself before you call encode-time. The operating system limits the range of time and zone values. However, timestamps ranging from the epoch to the near future are always supported. The encode-time function acts as a rough inverse to decode-time. For example, you can pass the output of the latter to the former as follows:

(encode-time (decode-time ...))

You can perform simple date arithmetic by using out-of-range values for seconds, minutes, hour, day, and month; for example, day 0 means the day preceding the given month. Take care when doing so, as it is common for this to fail in some cases. For example:

;; Try to compute the time one month from now.
;; Watch out; this might not work as expected.
(let ((time (decode-time)))
  (setf (decoded-time-month time)
        (+ (decoded-time-month time) 1))
  time)

Unfortunately, this code might not work as expected if the resulting time is invalid due to month length differences, daylight saving transitions, time zone changes, or missing leap days or leap seconds. For example, if executed on January 30 this code yields a nonexistent date February 30, which encode-time would adjust to early March. Similarly, adding four years to February 29, 2096 would yield the nonexistent date February 29, 2100; and adding one hour to 01:30 on March 13, 2022 in New York would yield a timestamp 02:30 that does not exist because clocks sprang forward from 02:00 to 03:00 that day. To avoid some (though not all) of the problem, you can base calculations on the middle of the affected unit, e.g., start at the 15th of the month when adding months. Alternatively, you can use the calendar and time-date libraries.

These functions convert time values to text in a string, and vice versa. Time values are either represented as a Lisp timestamp (Time of Day) or a decoded time structure (Time Conversion).

date-to-time

This function parses the time-string string and returns the corresponding Lisp timestamp. The argument string should represent a date-time, and should be in one of the forms recognized by parse-time-string (see below). This function assumes Universal Time if string lacks explicit time zone information, and assumes earliest values if string lacks month, day, or time. The operating system limits the range of time and zone values.

parse-time-string

This function parses the time-string string into a decoded time structure (Time Conversion). The argument string should resemble an RFC 822 (or later) or ISO 8601 string, like "Fri, 25 Mar 2016 16:24:56 +0100" or "1998-09-12T12:21:54-0200", but this function will attempt to parse less well-formed time strings as well. Note that, unlike decode-time (Time Conversion), this function does not interpret the time string, and in particular the values of daylight-saving and timezone or UTC offset parts of the string argument do not affect the returned value of date and time, they only affect the last two members of the returned decoded time structure. For example, if the time-zone information is present in string, the decoded time structure will include it; otherwise the time-zone member of the returned value will be nil. In other words, this function simply parses the textual representation of date and time into separate numerical values, and doesn't care whether the input time is local or UTC. If a Lisp program passes the return value of this function to some other time-related API, it should make sure the nil members of the decoded time structure are interpreted correctly, and in particular the lack of time-zone information is interpreted as UTC or local time, according to the needs of the calling program.

iso8601-parse

For a more strict function (that will error out upon invalid input), Lisp programs can use this function instead. It can parse all variants of the ISO 8601 standard, so in addition to the formats mentioned above, it also parses things like "1998W45-3" (week number) and "1998-245" (ordinal day number). To parse durations, there's iso8601-parse-duration, and to parse intervals, there's iso8601-parse-interval. All these functions return decoded time structures, except the final one, which returns three of them (the start, the end, and the duration). Like parse-time-string, this function does not interpret the time string, and in particular the time-zone designator or UTC offset that is part of the string argument does not affect the returned value of date and time, it only affects the last two members of the returned decoded time structure. The ISO 8601 standard specifies that date/time strings that do not include information about UTC relation are assumed to be in local time, but this function does not do that, because it doesn't interpret the time values. For example, if the time-zone information is present in string, the decoded time structure will include it; otherwise the time-zone member of the returned value will be nil. In other words, this function simply parses the textual representation of date and time into separate numerical values, and doesn't care whether the input time is local or UTC.

format-time-string

This function converts time (which should be a Lisp timestamp, and defaults to the current time if time is omitted or nil) to a string according to format-string. The conversion uses the time zone rule zone, which defaults to the current time zone rule. Time Zone Rules. The argument format-string may contain %-sequences which say to substitute parts of the time. Here is a table of what the %-sequences mean:

%a

This stands for the abbreviated name of the day of week.

%A

This stands for the full name of the day of week.

%b

This stands for the abbreviated name of the month.

%B

This stands for the full name of the month.

%c

This is a synonym for %x %X.

%C

This stands for the century, that is, the year divided by 100, truncated toward zero. The default field width is 2.

%d

This stands for the day of month, zero-padded.

%D

This is a synonym for %m/%d/%y.

%e

This stands for the day of month, blank-padded.

%F

This stands for the ISO 8601 date format, which is like %+4Y-%m-%d except that any flags or field width override the + and (after subtracting 6) the 4.

%g

This stands for the year without century (00–99) corresponding to the current ISO week number. ISO weeks start on Monday and end on Sunday. If an ISO week begins in one year and ends in another, the rules regarding which year %g will produce are complex and will not be described here; however, in general, if most of the week's days are in the ending year, %g will produce that year.

%G

This stands for the year with century corresponding to the current ISO week number.

%h

This is a synonym for %b.

%H

This stands for the hour (00–23).

%I

This stands for the hour (01–12).

%j

This stands for the day of the year (001–366).

%k

This stands for the hour (0–23), blank padded.

%l

This stands for the hour (1–12), blank padded.

%m

This stands for the month (01–12).

%M

This stands for the minute (00–59).

%n

This stands for a newline.

%N

This stands for the nanoseconds (000000000–999999999). To ask for fewer digits, use %3N for milliseconds, %6N for microseconds, etc. Any excess digits are discarded, without rounding.

%p

This stands for AM or PM, as appropriate.

%q

This stands for the calendar quarter (1–4).

%r

This is a synonym for %I:%M:%S %p.

%R

This is a synonym for %H:%M.

%s

This stands for the integer number of seconds since the epoch.

%S

This stands for the second (00–59, or 00–60 on platforms that support leap seconds).

%t

This stands for a tab character.

%T

This is a synonym for %H:%M:%S.

%u

This stands for the numeric day of week (1–7). Monday is day 1.

%U

This stands for the week of the year (01–52), assuming that weeks start on Sunday. If January 1 is not a Sunday, the first partial week is week zero.

%V

This stands for the week of the year according to ISO 8601. Note that, unlike %U and %W, the week according to ISO 8601 does not roll over to 1 on January 1, but keeps its last number.

%w

This stands for the numeric day of week (0–6). Sunday is day 0.

%W

This stands for the week of the year (01–52), assuming that weeks start on Monday. If January 1 is not a Monday, the first partial week is week zero.

%x

This has a locale-specific meaning. In the default locale (named C), it is equivalent to %D.

%X

This has a locale-specific meaning. In the default locale (named C), it is equivalent to %T.

%y

This stands for the year without century (00–99).

%Y

This stands for the year with century.

%Z

This stands for the time zone abbreviation (e.g., EST).

%z

This stands for the time zone numerical offset. The z can be preceded by one, two, or three colons; if plain %z stands for -0500, then %:z stands for -05:00, %::z stands for -05:00:00, and %:::z is like %::z except it suppresses trailing instances of :00 so it stands for -05 in the same example.

%%

This stands for a single %.

One or more flag characters can appear immediately after the %. 0 pads with zeros, + pads with zeros and also puts + before nonnegative year numbers with more than four digits, _ pads with blanks, - suppresses padding, ^ upper-cases letters, and # reverses the case of letters. You can also specify the field width and type of padding for any of these %-sequences. This works as in printf: you write the field width as digits in a %-sequence, after any flags. For example, %S specifies the number of seconds since the minute; %03S means to pad this with zeros to 3 positions, %_3S to pad with spaces to 3 positions. Plain %3S pads with zeros, because that is how %S normally pads to two positions. The characters E and O act as modifiers when used after any flags and field widths in a %-sequence. E specifies using the current locale's alternative version of the date and time. In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns. E is allowed in %Ec, %EC, %Ex, %EX, %Ey, and %EY. O means to use the current locale's alternative representation of numbers, instead of the ordinary decimal digits. This is allowed with most letters, all the ones that output numbers. To help debug programs, unrecognized %-sequences stand for themselves and are output as-is. Programs should not rely on this behavior, as future versions of Emacs may recognize new %-sequences as extensions. This function uses the C library function strftime (Formatting Calendar Time) to do most of the work. In order to communicate with that function, it first converts time and zone to internal form; the operating system limits the range of time and zone values. This function also encodes format-string using the coding system specified by locale-coding-system (Locales); after strftime returns the resulting string, this function decodes the string using that same coding system.

format-seconds

This function converts its argument seconds into a string of years, days, hours, etc., according to format-string. The argument format-string may contain %-sequences which control the conversion. Here is a table of what the %-sequences mean:

%y, %Y

The integer number of 365-day years.

%d, %D

The integer number of days.

%h, %H

The integer number of hours.

%m, %M

The integer number of minutes.

%s, %S

The number of seconds. If the optional = parameter is used, it's a floating point number, and the number after the = specifies how many decimals to be used. % means "use two decimals".

%z

Non-printing control flag. When it is used, other specifiers must be given in the order of decreasing size, i.e., years before days, hours before minutes, etc. Nothing will be produced in the result string to the left of %z until the first non-zero conversion is encountered. For example, the default format used by emacs-uptime (emacs-uptime) "%Y means that the number of seconds will always be produced, but years, days, hours, and minutes will only be shown if they are non-zero.

%x

Non-printing control flag that works along the same lines as %z, but instead suppresses printing of trailing zero-value time elements.

%%

Produces a literal %.

Upper-case format sequences produce the units in addition to the numbers, lower-case formats produce only the numbers. You can also specify the field width by following the % with a number; shorter numbers will be padded with blanks. An optional period before the width requests zero-padding instead. For example, "%.3Y" might produce "004 years".

Emacs provides several functions and primitives that return time, both elapsed and processor time, used by the Emacs process.

Command emacs-uptime

This function returns a string representing the Emacs uptime—the elapsed wall-clock time this instance of Emacs is running. The string is formatted by format-seconds according to the optional argument format. For the available format descriptors, see format-seconds. If format is nil or omitted, it defaults to "%Y. When called interactively, it prints the uptime in the echo area.

get-internal-run-time

This function returns the processor run time used by Emacs, as a Lisp timestamp (Time of Day). Note that the time returned by this function excludes the time Emacs was not using the processor, and if the Emacs process has several threads, the returned value is the sum of the processor times used up by all Emacs threads. If the system doesn't provide a way to determine the processor run time, get-internal-run-time returns the same time as current-time.

Command emacs-init-time

This function returns the duration of the Emacs initialization (Startup Summary) in seconds, as a string. When called interactively, it prints the duration in the echo area.

These functions perform calendrical computations using time values (Time of Day). As with any time value, a value of nil for any of their time-value arguments stands for the current system time, and a finite number stands for the number of seconds since the epoch.

time-less-p

This returns t if the time value t1 is less than the time value t2.

time-equal-p

This returns t if the two time values t1 and t2 are equal. The result is nil if either argument is a NaN. For the purpose of comparison, a nil argument represents the current time with infinite resolution, so this function returns nil if one argument is nil and the other is not, and callers can therefore use nil to represent an unknown time value that does not equal any timestamp.

time-subtract

This returns the time difference t1 − t2 between two time values, as a Lisp timestamp. The result is exact and its clock resolution is no worse than the worse of its two arguments' resolutions. If you need the difference in units of elapsed seconds, you can convert it with time-convert or float-time. Time Conversion.

time-add

This returns the sum of two time values, using the same conversion rules as time-subtract. One argument should represent a time difference rather than a point in time, as a time value that is often just a single number of elapsed seconds. Here is how to add a number of seconds to a time value:

(time-add TIME SECONDS)

time-to-days

This function returns the number of days between the beginning of year 1 and time-value, assuming the default time zone. The operating system limits the range of time and zone values.

days-to-time

This is not quite the inverse of the time-to-days function, as it uses the Emacs epoch (instead of the year 1) for historical reasons. To get the inverse, subtract (time-to-days 0) from days, in which case days-to-time may return nil if days is negative.

time-to-day-in-year

This returns the day number within the year corresponding to time-value, assuming the default time zone. The operating system limits the range of time and zone values.

date-leap-year-p

This function returns t if year is a leap year.

date-days-in-month

Return the number of days in month in year. For instance, February 2020 has 29 days.

date-ordinal-to-time

Return the date of ordinal in year as a decoded time structure. For instance, the 120th day in 2004 is April 29th.

You can set up a timer to call a function at a specified future time or after a certain length of idleness. A timer is a special object that stores the information about the next invocation times and the function to invoke.

timerp

This predicate function returns non-nil if object is a timer.

Emacs cannot run timers at any arbitrary point in a Lisp program; it can run them only when Emacs could accept output from a subprocess: namely, while waiting or inside certain primitive functions such as sit-for or read-event which can wait. Therefore, a timer's execution may be delayed if Emacs is busy. However, the time of execution is very precise if Emacs is idle. Emacs binds inhibit-quit to t before calling the timer function, because quitting out of many timer functions can leave things in an inconsistent state. This is normally unproblematical because most timer functions don't do a lot of work. Indeed, for a timer to call a function that takes substantial time to run is likely to be annoying. If a timer function needs to allow quitting, it should use with-local-quit (Quitting). For example, if a timer function calls accept-process-output to receive output from an external process, that call should be wrapped inside with-local-quit, to ensure that C-g works if the external process hangs. It is usually a bad idea for timer functions to alter buffer contents. When they do, they usually should call undo-boundary both before and after changing the buffer, to separate the timer's changes from user commands' changes and prevent a single undo entry from growing to be quite large. Timer functions should also avoid calling functions that cause Emacs to wait, such as sit-for (Waiting). This can lead to unpredictable effects, since other timers (or even the same timer) can run while waiting. If a timer function needs to perform an action after a certain time has elapsed, it can do this by scheduling a new timer. If a timer function performs a remote file operation, it can be in conflict with an already running remote file operation of the same connection. Such conflicts are detected, and they result in a remote-file-error error (Standard Errors). This should be protected by wrapping the timer function body with

(ignore-error 'remote-file-error
  ...)

If a timer function calls functions that can change the match data, it should save and restore the match data. Saving Match Data.

Command run-at-time

This sets up a timer that calls the function function with arguments args at time time. If repeat is a number (integer or floating point), the timer is scheduled to run again every repeat seconds after time. If repeat is nil, the timer runs only once. time may specify an absolute or a relative time. Absolute times may be specified using a string with a limited variety of formats, and are taken to be times today, even if already in the past. The recognized forms are XXXX, X:XX, or XX:XX (military time), and XXam, XXAM, XXpm, XXPM, XX:XXam, XX:XXAM, XX:XXpm, or XX:XXPM. A period can be used instead of a colon to separate the hour and minute parts. To specify a relative time as a string, use numbers followed by units. For example:

1 min

denotes 1 minute from now.

1 min 5 sec

denotes 65 seconds from now.

1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year

denotes exactly 103 months, 123 days, and 10862 seconds from now.

For relative time values, Emacs considers a month to be exactly thirty days, and a year to be exactly 365.25 days. Not all convenient formats are strings. If time is a number (integer or floating point), that specifies a relative time measured in seconds. The result of encode-time can also be used to specify an absolute value for time. In most cases, repeat has no effect on when first call takes place—time alone specifies that. There is one exception: if time is t, then the timer runs whenever the time is a multiple of repeat seconds after the epoch. This is useful for functions like display-time. For instance, the following will make function run at every "whole" minute (e.g., 11:03:00, 11:04:00, etc):

(run-at-time t 60 FUNCTION)

If Emacs didn't get any CPU time when the timer would have run (for example if the system was busy running another process or if the computer was sleeping or in a suspended state), the timer will run as soon as Emacs resumes and is idle. The function run-at-time returns a timer value that identifies the particular scheduled future action. You can use this value to call cancel-timer (see below).

Command run-with-timer

This is exactly the same as run-at-time (so see that definition for an explanation of the parameters; secs is passed as time to that function), but is meant to be used when the delay is specified in seconds.

A repeating timer nominally ought to run every repeat seconds, but remember that any invocation of a timer can be late. Lateness of one repetition has no effect on the scheduled time of the next repetition. For instance, if Emacs is busy computing for long enough to cover three scheduled repetitions of the timer, and then starts to wait, it will immediately call the timer function three times in immediate succession (presuming no other timers trigger before or between them). If you want a timer to run again no less than n seconds after the last invocation, don't use the repeat argument. Instead, the timer function should explicitly reschedule the timer.

timer-max-repeats

This variable's value specifies the maximum number of times to repeat calling a timer function in a row, when many previously scheduled calls were unavoidably delayed.

with-timeout

Execute body, but give up after seconds seconds. If body finishes before the time is up, with-timeout returns the value of the last form in body. If, however, the execution of body is cut short by the timeout, then with-timeout executes all the timeout-forms and returns the value of the last of them. This macro works by setting a timer to run after seconds seconds. If body finishes before that time, it cancels the timer. If the timer actually runs, it terminates execution of body, then executes timeout-forms. Since timers can run within a Lisp program only when the program calls a primitive that can wait, with-timeout cannot stop executing body while it is in the midst of a computation—only when it calls one of those primitives. So use with-timeout only with a body that waits for input, not one that does a long computation.

The function y-or-n-p-with-timeout provides a simple way to use a timer to avoid waiting too long for an answer. Yes-or-No Queries.

cancel-timer

This cancels the requested action for timer, which should be a timer—usually, one previously returned by run-at-time or run-with-idle-timer. This cancels the effect of that call to one of these functions; the arrival of the specified time will not cause anything special to happen.

The list-timers command lists all the currently active timers. The command c (timer-list-cancel) will cancel the timer on the line under point. You can sort the list by column using the command S (tabulated-list-sort).

Here is how to set up a timer that runs when Emacs is idle for a certain length of time. Aside from how to set them up, idle timers work just like ordinary timers.

Command run-with-idle-timer

Set up a timer which runs the next time Emacs is idle for secs seconds. The value of secs may be a number or a value of the type returned by current-idle-time. If repeat is nil, the timer runs just once, the first time Emacs remains idle for a long enough time. More often repeat is non-nil, which means to run the timer each time Emacs remains idle for secs seconds. The function run-with-idle-timer returns a timer value which you can use in calling cancel-timer (Timers).

Emacs becomes idle when it starts waiting for user input (unless it waits for input with a timeout, Reading One Event), and it remains idle until the user provides some input. If a timer is set for five seconds of idleness, it runs approximately five seconds after Emacs first becomes idle. Even if repeat is non-nil, this timer will not run again as long as Emacs remains idle, because the duration of idleness will continue to increase and will not go down to five seconds again. Emacs can do various things while idle: garbage collect, autosave or handle data from a subprocess. But these interludes during idleness do not interfere with idle timers, because they do not reset the clock of idleness to zero. An idle timer set for 600 seconds will run when ten minutes have elapsed since the last user command was finished, even if subprocess output has been accepted thousands of times within those ten minutes, and even if there have been garbage collections and autosaves. When the user supplies input, Emacs becomes non-idle while executing the input. Then it becomes idle again, and all the idle timers that are set up to repeat will subsequently run another time, one by one. Do not write an idle timer function containing a loop which does a certain amount of processing each time around, and exits when (input-pending-p) is non-nil. This approach seems very natural but has two problems:

• It blocks out all process output (since Emacs accepts process output only while waiting).
• It blocks out any idle timers that ought to run during that time.

Similarly, do not write an idle timer function that sets up another idle timer (including the same idle timer) with secs argument less than or equal to the current idleness time. Such a timer will run almost immediately, and continue running again and again, instead of waiting for the next time Emacs becomes idle. The correct approach is to reschedule with an appropriate increment of the current value of the idleness time, as described below.

current-idle-time

If Emacs is idle, this function returns the length of time Emacs has been idle, using the same format as current-time (Time of Day). When Emacs is not idle, current-idle-time returns nil. This is a convenient way to test whether Emacs is idle.

The main use of current-idle-time is when an idle timer function wants to "take a break" for a while. It can set up another idle timer to call the same function again, after a few seconds more idleness. Here's an example:

(defvar my-resume-timer nil
  "Timer for `my-timer-function' to reschedule itself, or nil.")

(defun my-timer-function ()
  ;; If the user types a command while my-resume-timer
  ;; is active
  ;; its main idle timer
  (when my-resume-timer
    (cancel-timer my-resume-timer))
  ...DO THE WORK FOR A WHILE...
  (when TAKING-A-BREAK
    (setq my-resume-timer
          (run-with-idle-timer
            ;; Compute an idle time BREAK-LENGTH
            ;; more than the current value.
            (time-add (current-idle-time) BREAK-LENGTH)
            nil
            'my-timer-function))))

This section describes functions and variables for recording or manipulating terminal input. See Display, for related functions.

The terminal output functions send output to a text terminal, or keep track of output sent to the terminal. The variable baud-rate tells you what Emacs thinks is the output speed of the terminal.

baud-rate

This variable's value is the output speed of the terminal, as far as Emacs knows. Setting this variable does not change the speed of actual data transmission, but the value is used for calculations such as padding. It also affects decisions about whether to scroll part of the screen or repaint on text terminals. Forcing Redisplay, for the corresponding functionality on graphical terminals. The value is measured in baud.

If you are running across a network, and different parts of the network work at different baud rates, the value returned by Emacs may be different from the value used by your local terminal. Some network protocols communicate the local terminal speed to the remote machine, so that Emacs and other programs can get the proper value, but others do not. If Emacs has the wrong value, it makes decisions that are less than optimal. To fix the problem, set baud-rate.

send-string-to-terminal

This function sends string to terminal without alteration. Control characters in string have terminal-dependent effects. (If you need to display non-ASCII text on the terminal, encode it using one of the functions described in Explicit Encoding.) This function operates only on text terminals. terminal may be a terminal object, a frame, or nil for the selected frame's terminal. In batch mode, string is sent to stdout when terminal is nil. One use of this function is to define function keys on terminals that have downloadable function key definitions. For example, this is how (on certain terminals) to define function key 4 to move forward four characters (by transmitting the characters C-u C-f to the computer):

(send-string-to-terminal "\eF4\^U\^F")
     => nil

Command open-termscript

This function is used to open a termscript file that will record all the characters sent by Emacs to the terminal. It returns nil. Termscript files are useful for investigating problems where Emacs garbles the screen, problems that are due to incorrect Termcap entries or to undesirable settings of terminal options more often than to actual Emacs bugs. Once you are certain which characters were actually output, you can determine reliably whether they correspond to the Termcap specifications in use.

(open-termscript "../junk/termscript")
     => nil

You close the termscript file by calling this function with an argument of nil. See also open-dribble-file in Recording Input.

To play sound using Emacs, use the function play-sound. Only certain systems are supported; if you call play-sound on a system which cannot really do the job, it gives an error. The sound must be stored as a file in RIFF-WAVE format (.wav) or Sun Audio format (.au).

play-sound

This function plays a specified sound. The argument, sound, has the form (sound PROPERTIES...), where the properties consist of alternating keywords (particular symbols recognized specially) and values corresponding to them. Here is a table of the keywords that are currently meaningful in sound, and their meanings:

:file FILE

This specifies the file containing the sound to play. If the file name is not absolute, it is expanded against the directory data-directory.

:data DATA

This specifies the sound to play without need to refer to a file. The value, data, should be a string containing the same bytes as a sound file. We recommend using a unibyte string.

:volume VOLUME

This specifies how loud to play the sound. It should be a number in the range of 0 to 1. The default is to use whatever volume has been specified before.

:device DEVICE

This specifies the system device on which to play the sound, as a string. The default device is system-dependent.

Before actually playing the sound, play-sound calls the functions in the list play-sound-functions. Each function is called with one argument, sound.

Command play-sound-file

This function is an alternative interface to playing a sound file specifying an optional volume and device.

play-sound-functions

A list of functions to be called before playing a sound. Each function is called with one argument, a property list that describes the sound.

To define system-specific X11 keysyms, set the variable system-key-alist.

system-key-alist

This variable's value should be an alist with one element for each system-specific keysym. Each element has the form (CODE . SYMBOL), where code is the numeric keysym code (not including the vendor-specific bit, −2**28), and symbol is the name for the function key. For example (168 . mute-acute) defines a system-specific key (used by HP X servers) whose numeric code is −2**28 + 168. It is not crucial to exclude from the alist the keysyms of other X servers; those do no harm, as long as they don't conflict with the ones used by the X server actually in use. The variable is always local to the current terminal, and cannot be buffer-local. Multiple Terminals.

You can specify which keysyms Emacs should use for the Control, Meta, Alt, Hyper, and Super modifiers by setting these variables:

x-ctrl-keysym

@defvarx x-alt-keysym @defvarx x-meta-keysym @defvarx x-hyper-keysym @defvarx x-super-keysym The name of the keysym that should stand for the Control modifier (respectively, for Alt, Meta, Hyper, and Super). For example, here is how to swap the Meta and Alt modifiers within Emacs:

(setq x-alt-keysym 'meta)
(setq x-meta-keysym 'alt)

The command-line option -batch causes Emacs to run noninteractively. In this mode, Emacs does not read commands from the terminal, it does not alter the terminal modes, and it does not expect to be outputting to an erasable screen. The idea is that you specify Lisp programs to run; when they are finished, Emacs should exit. The way to specify the programs to run is with -l FILE, which loads the library named file, or -f FUNCTION, which calls function with no arguments, or --eval=FORM.

noninteractive

This variable is non-nil when Emacs is running in batch mode.

If the specified Lisp program signals an unhandled error in batch mode, Emacs exits with a non-zero exit status after invoking the Lisp debugger which shows the Lisp backtrace (Invoking the Debugger) on the standard error stream:

$ emacs -Q --batch --eval '(error "foo")'; echo $?

Error: error ("foo")
mapbacktrace(#f(compiled-function (evald func args flags) #<bytecode -0x4f85c5
7c45e2f81>))
debug-early-backtrace()
debug-early(error (error "foo"))
signal(error ("foo"))
error("foo")
eval((error "foo") t)
command-line-1(("--eval" "(error \"foo\")"))
command-line()
normal-top-level()
foo
255

Any Lisp program output that would normally go to the echo area, either using message, or using prin1, etc., with t as the stream (Output Streams), goes instead to Emacs's standard descriptors when in batch mode: message writes to the standard error descriptor, while prin1 and other print functions write to the standard output. Similarly, input that would normally come from the minibuffer is read from the standard input descriptor. Thus, Emacs behaves much like a noninteractive application program. (The echo area output that Emacs itself normally generates, such as command echoing, is suppressed entirely.) Non-ASCII text written to the standard output or error descriptors is by default encoded using locale-coding-system (Locales) if it is non-nil; this can be overridden by binding coding-system-for-write to a coding system of you choice (Explicit Encoding). In batch mode, Emacs will enlarge the value of the gc-cons-percentage variable from the default of 0.1 up to 1.0. Batch jobs that are supposed to run for a long time should adjust the limit back down again, because this means that less garbage collection will be performed by default (and more memory consumed).

Emacs supports the X Session Management Protocol, which is used to suspend and restart applications. In the X Window System, a program called the session manager is responsible for keeping track of the applications that are running. When the X server shuts down, the session manager asks applications to save their state, and delays the actual shutdown until they respond. An application can also cancel the shutdown. When the session manager restarts a suspended session, it directs these applications to individually reload their saved state. It does this by specifying a special command-line argument that says what saved session to restore. For Emacs, this argument is --smid SESSION.

emacs-save-session-functions

Emacs supports saving state via a hook called emacs-save-session-functions. Emacs runs this hook when the session manager tells it that the window system is shutting down. The functions are called with no arguments, and with the current buffer set to a temporary buffer. Each function can use insert to add Lisp code to this buffer. At the end, Emacs saves the buffer in a file, called the session file. Subsequently, when the session manager restarts Emacs, it loads the session file automatically (Loading). This is performed by a function named emacs-session-restore, which is called during startup. Startup Summary. If a function in emacs-save-session-functions returns non-nil, Emacs tells the session manager to cancel the shutdown.

Here is an example that just inserts some text into *scratch* when Emacs is restarted by the session manager.

(add-hook 'emacs-save-session-functions 'save-yourself-test)

(defun save-yourself-test ()
  (insert
   (format "%S" '(with-current-buffer "*scratch*"
                   (insert "I am restored"))))
  nil)

Emacs is able to send notifications on systems that support the freedesktop.org Desktop Notifications Specification, MS-Windows, Haiku, and Android. In order to use this functionality on POSIX hosts, Emacs must have been compiled with D-Bus support, and the notifications library must be loaded. D-Bus. The following function is supported when D-Bus support is available:

notifications-notify

This function sends a notification to the desktop via D-Bus, consisting of the parameters specified by the params arguments. These arguments should consist of alternating keyword and value pairs. The supported keywords and values are as follows:

:bus BUS

The D-Bus bus. This argument is needed only if a bus other than :session shall be used.

:title TITLE

The notification title.

:body TEXT

The notification body text. Depending on the implementation of the notification server, the text could contain HTML markups, like "<b>bold text</b>", hyperlinks, or images. Special HTML characters must be encoded, as "Contact <postmaster@localhost>!".

:app-name NAME

The name of the application sending the notification. The default is notifications-application-name.

:replaces-id ID

The notification id that this notification replaces. id must be the result of a previous notifications-notify call.

:app-icon ICON-FILE

The file name of the notification icon. If set to nil, no icon is displayed. The default is notifications-application-icon. If the value is a string, the function interprets it as a file name and converts to absolute by using expand-file-name; if it is a symbol, the function will use its name (which is useful when using the Icon Naming Specification (For more information about icon naming convention see Icon Naming Specification)).

:actions (KEY TITLE KEY TITLE ...)

A list of actions to be applied. key and title are both strings. The default action (usually invoked by clicking the notification) should have a key named "default". The title can be anything, though implementations are free not to display it.

:timeout TIMEOUT

The timeout time in milliseconds since the display of the notification at which the notification should automatically close. If −1, the notification's expiration time is dependent on the notification server's settings, and may vary for the type of notification. If 0, the notification never expires. Default value is −1.

:urgency URGENCY

The urgency level. It can be low, normal, or critical.

:action-items

When this keyword is given, the title string of the actions is interpreted as icon name.

:category CATEGORY

The type of notification this is, a string. See the Desktop Notifications Specification for a list of standard categories.

:desktop-entry FILENAME

This specifies the name of the desktop filename representing the calling program, like "emacs".

:image-data (WIDTH HEIGHT ROWSTRIDE HAS-ALPHA BITS CHANNELS DATA)

This is a raw data image format that describes the width, height, rowstride, whether there is an alpha channel, bits per sample, channels and image data, respectively.

:image-path PATH

This is represented either as a URI (file:// is the only URI schema supported right now) or a name in a freedesktop.org-compliant icon theme from $XDG_DATA_DIRS/icons.

:sound-file FILENAME

The path to a sound file to play when the notification pops up.

:sound-name NAME

A themable named sound from the freedesktop.org sound naming specification from $XDG_DATA_DIRS/sounds, to play when the notification pops up. Similar to the icon name, only for sounds. An example would be "message-new-instant".

:suppress-sound

Causes the server to suppress playing any sounds, if it has that ability.

:resident

When set the server will not automatically remove the notification when an action has been invoked. The notification will remain resident in the server until it is explicitly removed by the user or by the sender. This hint is likely only useful when the server has the :persistence capability.

:transient

When set the server will treat the notification as transient and by-pass the server's persistence capability, if it should exist.

:x POSITION, :y POSITION

Specifies the X, Y location on the screen that the notification should point to. Both arguments must be used together.

:on-action FUNCTION

Function to call when an action is invoked. The notification id and the key of the action are passed as arguments to the function.

:on-close FUNCTION

Function to call when the notification has been closed by timeout or by the user. The function receive the notification id and the closing reason as arguments:

expired if the notification has expired

dismissed if the notification was dismissed by the user

close-notification if the notification was closed by a call to

notifications-close-notification

undefined if the notification server hasn't provided a reason

Which parameters are accepted by the notification server can be checked via notifications-get-capabilities. This function returns a notification id, an integer, which can be used to manipulate the notification item with notifications-close-notification or the :replaces-id argument of another notifications-notify call. For example:

(defun my-on-action-function (id key)
  (message "Message %d, key \"%s\" pressed" id key))
     => my-on-action-function

(defun my-on-close-function (id reason)
  (message "Message %d, closed due to \"%s\"" id reason))
     => my-on-close-function

(notifications-notify
 :title "Title"
 :body "This is <b>important</b>."
 :actions '("Confirm" "I agree" "Refuse" "I disagree")
 :on-action 'my-on-action-function
 :on-close 'my-on-close-function)
     => 22

A message window opens on the desktop.  Press "I agree".
     => Message 22, key "Confirm" pressed
        Message 22, closed due to "dismissed"

notifications-close-notification

This function closes a notification with identifier id. bus can be a string denoting a D-Bus connection, the default is :session.

notifications-get-capabilities

Returns the capabilities of the notification server, a list of symbols. bus can be a string denoting a D-Bus connection, the default is :session. The following capabilities can be expected:

:actions

The server will provide the specified actions to the user.

:body

Supports body text.

:body-hyperlinks

The server supports hyperlinks in the notifications.

:body-images

The server supports images in the notifications.

:body-markup

Supports markup in the body text.

:icon-multi

The server will render an animation of all the frames in a given image array.

:icon-static

Supports display of exactly 1 frame of any given image array. This value is mutually exclusive with :icon-multi.

:persistence

The server supports persistence of notifications.

:sound

The server supports sounds on notifications.

Further vendor-specific caps start with :x-vendor, like :x-gnome-foo-cap.

notifications-get-server-information

Return information on the notification server, a list of strings. bus can be a string denoting a D-Bus connection, the default is :session. The returned list is (NAME VENDOR VERSION SPEC-VERSION).

name

The product name of the server.

vendor

The vendor name. For example, "KDE", "GNOME".

version

The server's version number.

spec-version

The specification version the server is compliant with.

If spec_version is nil, the server supports a specification prior to "1.0". When Emacs runs on MS-Windows as a GUI session, it supports a small subset of the D-Bus notifications functionality via a native primitive:

w32-notification-notify

This function displays an MS-Windows tray notification as specified by params. MS-Windows tray notifications are displayed in a balloon from an icon in the notification area of the taskbar. Value is the integer unique ID of the notification that can be used to remove the notification using w32-notification-close, described below. If the function fails, the return value is nil. The arguments params are specified as keyword/value pairs. All the parameters are optional, but if no parameters are specified, the function will do nothing and return nil. The following parameters are supported:

:icon ICON

Display icon in the system tray. If icon is a string, it should specify a file name from which to load the icon; the specified file should be a .ico Windows icon file. If icon is not a string, or if this parameter is not specified, the standard Emacs icon will be used.

:tip TIP

Use tip as the tooltip for the notification. If tip is a string, this is the text of a tooltip that will be shown when the mouse pointer hovers over the tray icon added by the notification. If tip is not a string, or if this parameter is not specified, the default tooltip text is Emacs notification. The tooltip text can be up to 127 characters long (63 on Windows versions before W2K). Longer strings will be truncated.

:level LEVEL

Notification severity level, one of info, warning, or error. If given, the value determines the icon displayed to the left of the notification title, but only if the :title parameter (see below) is also specified and is a string.

:title TITLE

The title of the notification. If title is a string, it is displayed in a larger font immediately above the body text. The title text can be up to 63 characters long; longer text will be truncated.

:body BODY

The body of the notification. If body is a string, it specifies the text of the notification message. Use embedded newlines to control how the text is broken into lines. The body text can be up to 255 characters long, and will be truncated if it's longer. Unlike with D-Bus, the body text should be plain text, with no markup.

Note that versions of Windows before W2K support only :icon and :tip. The other parameters can be passed, but they will be ignored on those old systems. There can be at most one active notification at any given time. An active notification must be removed by calling w32-notification-close before a new one can be shown. To remove the notification and its icon from the taskbar, use the following function:

w32-notification-close

This function removes the tray notification given by its unique id.

When Emacs runs under Haiku as a GUI program, it is also provides a restricted pastiche of the D-Bus desktop notifications interface previously addressed. The principle capabilities absent from the function detailed below are call-back functions such as :actions, :on-action and :on-close.

haiku-notifications-notify

This function sends a notification to the desktop notification server, incorporating a number of parameters that are akin to some of those accepted by notifications-notify. The parameters are:

:title TITLE

:body BODY

:replaces-id REPLACES-ID

:urgency URGENCY

These have the same meaning as they do when used in calls to notifications-notify.

:app-icon APP-ICON

This should be the file name designating an image file to use as the icon for the notification displayed. If nil, the icon presented will instead be Emacs's app icon.

Its return value is a number identifying the notification, which can be exploited as the :replaces-id parameter to a subsequent call to this function. When Emacs is built as an Android application package, displaying notifications is facilitated by the function android-notifications-notify. This function does not feature call-backs, and has several idiosyncrasies, when compared to notifications-notify.

android-notifications-notify

This function displays a desktop notification. params is a list of parameters analogous to its namesake in notifications-notify. The parameters are:

:title TITLE

:body BODY

:replaces-id REPLACES-ID

:on-action ON-ACTION

:on-cancel ON-CLOSE

:actions ACTIONS

:timeout TIMEOUT

:resident RESIDENT

These have the same meaning as they do when used in calls to notifications-notify, except that no more than three non-default actions will be displayed.

:urgency URGENCY

The set of accepted values for urgency is the same as with notifications-notify, but the urgency applies to all notifications displayed with the defined group, except under Android 7.1 and earlier.

:group GROUP

group is a string that designates a category to which the notification sent will belong. This category is reproduced within the system's notification settings menus, but is ignored under Android 7.1 and earlier. If group is nil or not present within params, it is replaced by the string "Desktop Notifications". Callers should provide one stable combination of urgency and group for each kind of notification they send, given that the system may elect to disregard urgency if it does not match that of any notification previously delivered to group.

:icon ICON

This parameter controls the symbolic icon the notification will be displayed with. Its value is a string designating an icon within the android.R.drawable system package. See R.drawable | Android Developers for a list of such icons. If it is not provided within params or icon does not exist, it defaults to "ic_dialog_alert".

It returns a number identifying the notification, which may be supplied as the :replaces-id parameter to a later call to this function. If Emacs is not afforded the permission to display notifications (Android Environment) under Android 13 and later, any notifications sent will be silently disregarded.

Several operating systems support watching of filesystems for changes to files or their attributes. If configured properly, Emacs links a respective library like inotify, kqueue, gfilenotify, or w32notify statically. These libraries enable watching of filesystems on the local machine. It is also possible to watch filesystems on remote machines, Remote Files. This does not depend on one of the libraries linked to Emacs. Since all these libraries emit different events upon notified file changes, Emacs provides a special library filenotify which presents a unified interface to applications. Lisp programs that want to receive file notifications should always use this library in preference to the native ones. This section documents the filenotify library functions and variables.

file-notify-add-watch

Add a watch for filesystem events pertaining to file. This arranges for filesystem events pertaining to file to be reported to Emacs. The returned value is a descriptor for the added watch. Its type depends on the underlying library, and in general cannot be assumed to be an integer as in the example below. It should be used for comparison by equal only. If the file cannot be watched for some reason, this function signals a file-notify-error error. Sometimes, mounted filesystems cannot be watched for file changes. This is not detected by this function, and so a non-nil return value does not guarantee that changes on file will be actually notified. If file is a symlink, it doesn't follow that link. Just file itself will be watched. flags is a list of conditions to set what will be watched for. It can include the following symbols:

change

watch for changes in file's contents

attribute-change

watch for changes in file attributes, like permissions or modification time

If file is a directory, change watches for file creation and deletion in that directory. Some of the native file notification libraries also report file changes in that case. This does not work recursively. When any event happens, Emacs will call the callback function passing it a single argument event, which is of the form

(DESCRIPTOR ACTION FILE [FILE1])

descriptor is the same object as the one returned by this function. action is the description of the event. It could be any one of the following symbols:

created

file was created

deleted

file was deleted

changed

file's contents has changed; with w32notify library, reports attribute changes as well

renamed

file has been renamed to file1

attribute-changed

a file attribute was changed

stopped

watching file has stopped

Note that the w32notify library does not report attribute-changed events. When some file's attribute, like permissions or modification time, has changed, this library reports a changed event. Likewise, the kqueue library does not reliably report file attribute changes when watching a directory. The stopped event means that watching the file has been discontinued. This could be because file-notify-rm-watch was called (see below), or because the file being watched was deleted, or because the filesystem of the file being watched was unmounted, or due to another error reported from the underlying library which makes further watching impossible. file and file1 are the name of the file(s) whose event is being reported. For example:

(require 'filenotify)
     => filenotify

(defun my-notify-callback (event)
  (message "Event %S" event))
     => my-notify-callback

(file-notify-add-watch
  "/tmp" '(change attribute-change) 'my-notify-callback)
     => 35025468

(write-region "foo" nil "/tmp/foo")
     => Event (35025468 created "/tmp/.#foo")
        Event (35025468 created "/tmp/foo")
        Event (35025468 changed "/tmp/foo")
        Event (35025468 deleted "/tmp/.#foo")

(write-region "bla" nil "/tmp/foo")
     => Event (35025468 created "/tmp/.#foo")
        Event (35025468 changed "/tmp/foo")
        Event (35025468 deleted "/tmp/.#foo")

(set-file-modes "/tmp/foo" (default-file-modes) 'nofollow)
     => Event (35025468 attribute-changed "/tmp/foo")

Whether the action renamed is returned depends on the used watch library. Otherwise, the actions deleted and created could be returned in a random order.

(rename-file "/tmp/foo" "/tmp/bla")
     => Event (35025468 renamed "/tmp/foo" "/tmp/bla")

(delete-file "/tmp/bla")
     => Event (35025468 deleted "/tmp/bla")

file-notify-rm-watch

Removes an existing file watch specified by its descriptor. descriptor should be an object returned by file-notify-add-watch.

Command file-notify-rm-all-watches

Removes all existing file notification watches from Emacs. Use this command with caution, because it could have unexpected side effects on packages relying on file watches. It is intended mainly for debugging purposes, or when Emacs has been stalled.

file-notify-valid-p

Checks a watch specified by its descriptor for validity. descriptor should be an object returned by file-notify-add-watch. A watch can become invalid if the file or directory it watches is deleted, or if the watcher thread exits abnormally for any other reason. Removing the watch by calling file-notify-rm-watch also makes it invalid.

(make-directory "/tmp/foo")
     => Event (35025468 created "/tmp/foo")

(setq desc
      (file-notify-add-watch
        "/tmp/foo" '(change) 'my-notify-callback))
     => 11359632

(file-notify-valid-p desc)
     => t

(write-region "bla" nil "/tmp/foo/bla")
     => Event (11359632 created "/tmp/foo/.#bla")
        Event (11359632 created "/tmp/foo/bla")
        Event (11359632 changed "/tmp/foo/bla")
        Event (11359632 deleted "/tmp/foo/.#bla")

;; Deleting a file in the directory doesn't invalidate the watch.
(delete-file "/tmp/foo/bla")
     => Event (11359632 deleted "/tmp/foo/bla")

(write-region "bla" nil "/tmp/foo/bla")
     => Event (11359632 created "/tmp/foo/.#bla")
        Event (11359632 created "/tmp/foo/bla")
        Event (11359632 changed "/tmp/foo/bla")
        Event (11359632 deleted "/tmp/foo/.#bla")

;; Deleting the directory invalidates the watch.
;; Events arrive for different watch descriptors.
(delete-directory "/tmp/foo" 'recursive)
     => Event (35025468 deleted "/tmp/foo")
        Event (11359632 deleted "/tmp/foo/bla")
        Event (11359632 deleted "/tmp/foo")
        Event (11359632 stopped "/tmp/foo")

(file-notify-valid-p desc)
     => nil

A dynamically loaded library is a library that is loaded on demand, when its facilities are first needed. Emacs supports such on-demand loading of support libraries for some of its features.

dynamic-library-alist

This is an alist of dynamic libraries and external library files implementing them. Each element is a list of the form (LIBRARY FILES...), where the car is a symbol representing a supported external library, and the rest are strings giving alternate filenames for that library. Emacs tries to load the library from the files in the order they appear in the list; if none is found, the Emacs session won't have access to that library, and the features it provides will be unavailable. Image support on some platforms uses this facility. Here's an example of setting this variable for supporting images on MS-Windows:

(setq dynamic-library-alist
      '((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
        (png "libpng12d.dll" "libpng12.dll" "libpng.dll"
             "libpng13d.dll" "libpng13.dll")
        (jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
              "jpeg.dll")
        (tiff "libtiff3.dll" "libtiff.dll")
        (gif "giflib4.dll" "libungif4.dll" "libungif.dll")
        (svg "librsvg-2-2.dll")
        (gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
        (glib "libglib-2.0-0.dll")
        (gobject "libgobject-2.0-0.dll")))

Note that image types pbm and xbm do not need entries in this variable because they do not depend on external libraries and are always available in Emacs. Also note that this variable is not meant to be a generic facility for accessing external libraries; only those already known by Emacs can be loaded through it. This variable is ignored if the given library is statically linked into Emacs.

Like any application, Emacs can be run in a secure environment, where the operating system enforces rules about access and the like. With some care, Emacs-based applications can also be part of a security perimeter that checks such rules. Although the default settings for Emacs work well for a typical software development environment, they may require adjustment in environments containing untrusted users that may include attackers. Here is a compendium of security issues that may be helpful if you are developing such applications. It is by no means complete; it is intended to give you an idea of the security issues involved, rather than to be a security checklist.

File local variables

A file that Emacs visits can contain variable settings that affect the buffer visiting that file; File Local Variables. Similarly, a directory can specify local variable values common to all files in that directory; see Directory Local Variables. Although Emacs takes some effort to protect against misuse of these variables, a security hole can be created merely by a package setting safe-local-variable too optimistically, a problem that is all too common. To disable this feature for both files and directories, set enable-local-variables to nil.

Access control

Although Emacs normally respects access permissions of the underlying operating system, in some cases it handles accesses specially. For example, file names can have handlers that treat the files specially, with their own access checking. Magic File Names. Also, a buffer can be read-only even if the corresponding file is writable, and vice versa, which can result in messages such as File passwd is write-protected; try to save anyway? (yes or no). Read Only Buffers.

Authentication

Emacs has several functions that deal with passwords, e.g., read-passwd. Reading a Password. Although these functions do not attempt to broadcast passwords to the world, their implementations are not proof against determined attackers with access to Emacs internals. For example, even if Elisp code uses clear-string to scrub a password from its memory after using it, remnants of the password may still reside in the garbage-collected free list. Modifying Strings.

Code injection

Emacs can send commands to many other applications, and applications should take care that strings sent as operands of these commands are not misinterpreted as directives. For example, when using a shell command to rename a file a to b, do not simply use the string mv A B, because either file name might start with -, or might contain shell metacharacters like ;. Although functions like shell-quote-argument can help avoid this sort of problem, they are not panaceas; for example, on a POSIX platform shell-quote-argument quotes shell metacharacters but not leading -. On MS-Windows, quoting for % assumes none of the environment variables have ^ in their name. Shell Arguments. Typically it is safer to use call-process than a subshell. Synchronous Processes. And it is safer yet to use builtin Emacs functions; for example, use (rename-file "A" "B" t) instead of invoking mv. Changing Files.

Coding systems

Emacs attempts to infer the coding systems of the files and network connections it accesses. Coding Systems. If Emacs infers incorrectly, or if the other parties to the network connection disagree with Emacs's inferences, the resulting system could be unreliable. Also, even when it infers correctly, Emacs often can use bytes that other programs cannot. For example, although to Emacs the null byte is just a character like any other, many other applications treat it as a string terminator and mishandle strings or files containing null bytes.

Environment and configuration variables

POSIX specifies several environment variables that can affect how Emacs behaves. Any environment variable whose name consists entirely of uppercase ASCII letters, digits, and the underscore may affect the internal behavior of Emacs. Emacs uses several such variables, e.g., EMACSLOADPATH. Library Search. On some platforms some environment variables (e.g., PATH, POSIXLY_CORRECT, SHELL, TMPDIR) need to have properly-configured values in order to get standard behavior for any utility Emacs might invoke. Even seemingly-benign variables like TZ may have security implications. System Environment. Emacs has customization and other variables with similar considerations. For example, if the variable shell-file-name specifies a shell with nonstandard behavior, an Emacs-based application may misbehave.

Installation

When Emacs is installed, if the installation directory hierarchy can be modified by untrusted users, the application cannot be trusted. This applies also to the directory hierarchies of the programs that Emacs uses, and of the files that Emacs reads and writes.

Network access

Emacs often accesses the network, and you may want to configure it to avoid network accesses that it would normally do. For example, unless you set tramp-mode to nil, file names using a certain syntax are interpreted as being network files, and are retrieved across the network. The Tramp Manual.

Race conditions

Emacs applications have the same sort of race-condition issues that other applications do. For example, even when (file-readable-p "foo.txt") returns t, it could be that foo.txt is unreadable because some other program changed the file's permissions between the call to file-readable-p and now. Testing Accessibility.

Resource limits

When Emacs exhausts memory or other operating system resources, its behavior can be less reliable, in that computations that ordinarily run to completion may abort back to the top level. This may cause Emacs to neglect operations that it normally would have done.