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40. Operating System Interface

This chapter is about starting and getting out of Emacs, access to values in the operating system environment, and terminal input, output, and flow control.

See section E.1 Building Emacs, for related information. See also 38. Emacs Display, for additional operating system status information pertaining to the terminal and the screen.

40.1 Starting Up Emacs  Customizing Emacs startup processing.
40.2 Getting Out of Emacs  How exiting works (permanent or temporary).
40.3 Operating System Environment  Distinguish the name and kind of system.
40.4 User Identification  Finding the name and user id of the user.
40.5 Time of Day  Getting the current time.
40.6 Time Conversion  Converting a time from numeric form to a string, or to calendrical data (or vice versa).
40.7 Timers for Delayed Execution  Setting a timer to call a function at a certain time.
40.8 Terminal Input  Recording terminal input for debugging.
40.9 Terminal Output  Recording terminal output for debugging.
40.10 Sound Output  Playing sounds on the computer's speaker.
40.11 System-Specific X11 Keysyms  Defining system-specific key symbols for X.
40.12 Flow Control  How to turn output flow control on or off.
40.13 Batch Mode  Running Emacs without terminal interaction.

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40.1 Starting Up Emacs

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

40.1.1 Summary: Sequence of Actions at Startup  Sequence of actions Emacs performs at startup.
40.1.2 The Init File, `.emacs'  Details on reading the init file (`.emacs').
40.1.3 Terminal-Specific Initialization  How the terminal-specific Lisp file is read.
40.1.4 Command-Line Arguments  How command-line arguments are processed, and how you can customize them.

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40.1.1 Summary: Sequence of Actions at Startup

The order of operations performed (in `startup.el') by Emacs when it is started up is as follows:

  1. It adds subdirectories to load-path, by running the file named `subdirs.el' in each directory in the list. Normally this file adds the directory's subdirectories to the list, and these will be scanned in their turn. The files `subdirs.el' are normally generated automatically by Emacs installation.

  2. It sets the language environment and the terminal coding system, if requested by environment variables such as LANG.

  3. It loads the initialization library for the window system, if you are using a window system. This library's name is `term/windowsystem-win.el'.

  4. It processes the initial options. (Some of them are handled even earlier than this.)

  5. It initializes the window frame and faces, if appropriate.

  6. It runs the normal hook before-init-hook.

  7. It loads the library `site-start', unless the option `-no-site-file' was specified. The library's file name is usually `site-start.el'.

  8. It loads your init file (usually `~/.emacs'), unless `-q', `-no-init-file', or `-batch' was specified on the command line. The `-u' option can specify another user whose home directory should be used instead of `~'.

  9. It loads the library `default', unless inhibit-default-init is non-nil. (This is not done in `-batch' mode or if `-q' was specified on the command line.) The library's file name is usually `default.el'.

  10. It runs the normal hook after-init-hook.

  11. It sets the major mode according to initial-major-mode, provided the buffer `*scratch*' is still current and still in Fundamental mode.

  12. It loads the terminal-specific Lisp file, if any, except when in batch mode or using a window system.

  13. It displays the initial echo area message, unless you have suppressed that with inhibit-startup-echo-area-message.

  14. It processes the action arguments from the command line.

  15. It runs emacs-startup-hook and then term-setup-hook.

  16. It calls frame-notice-user-settings, which modifies the parameters of the selected frame according to whatever the init files specify.

  17. It runs window-setup-hook. See section 38.19 Window Systems.

  18. It displays copyleft, nonwarranty, and basic use information, provided there were no remaining command-line arguments (a few steps above), the value of inhibit-startup-message is nil, and the buffer is still empty.

User Option: inhibit-startup-message
This variable inhibits the initial startup messages (the nonwarranty, etc.). If it is non-nil, then the messages are not printed.

This variable exists so you can set it in your personal init file, once you are familiar with the contents of the startup message. Do not set this variable in the init file of a new user, or in a way that affects more than one user, because that would prevent new users from receiving the information they are supposed to see.

User Option: inhibit-startup-echo-area-message
This variable controls the display of the startup echo area message. You can suppress the startup echo area message by adding text with this form to your init file:

(setq inhibit-startup-echo-area-message

Emacs explicitly checks for an expression as shown above in your init file; your login name must appear in the expression as a Lisp string constant. Other methods of setting inhibit-startup-echo-area-message to the same value do not inhibit the startup message.

This way, you can easily inhibit the message for yourself if you wish, but thoughtless copying of your init file will not inhibit the message for someone else.

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40.1.2 The Init File, `.emacs'

When you start Emacs, it normally attempts to load your init file, a file in your home directory. Its normal name is `.emacs', but you can alternatively call it `.emacs.el', which enables you to byte-compile it (see section 16. Byte Compilation); then the actual file loaded will be `.emacs.elc'.

The command-line switches `-q' and `-u' control whether and where to find the init file; `-q' says not to load an init file, and `-u user' says to load user's init file instead of yours. See section `Entering Emacs' in The GNU Emacs Manual. If neither option is specified, Emacs uses the LOGNAME environment variable, or the USER (most systems) or USERNAME (MS systems) variable, to find your home directory and thus your init file; this way, even if you have su'd, Emacs still loads your own init file. If those environment variables are absent, though, Emacs uses your user-id to find your home directory.

A site may have a default init file, which is the library named `default.el'. Emacs finds the `default.el' file through the standard search path for libraries (see section 15.1 How Programs Do Loading). The Emacs distribution does not come with this file; sites may provide one for local customizations. If the default init file exists, it is loaded whenever you start Emacs, except in batch mode or if `-q' is specified. But your own personal init file, if any, is loaded first; if it sets inhibit-default-init to a non-nil value, then Emacs does not subsequently load the `default.el' file.

Another file for site-customization is `site-start.el'. Emacs loads this before the user's init file. You can inhibit the loading of this file with the option `-no-site-file'.

Variable: site-run-file
This variable specifies the site-customization file to load before the user's init file. Its normal value is "site-start". The only way you can change it with real effect is to do so before dumping Emacs.

See section `Init File Examples' in The GNU Emacs Manual, for examples of how to make various commonly desired customizations in your `.emacs' file.

User Option: inhibit-default-init
This variable prevents Emacs from loading the default initialization library file for your session of Emacs. If its value is non-nil, then the default library is not loaded. The default value is nil.

Variable: before-init-hook
This normal hook is run, once, just before loading all the init files (the user's init file, `default.el', and/or `site-start.el'). (The only way to change it with real effect is before dumping Emacs.)

Variable: after-init-hook
This normal hook is run, once, just after loading all the init files (the user's init file, `default.el', and/or `site-start.el'), before loading the terminal-specific library and processing the command-line arguments.

Variable: emacs-startup-hook
This normal hook is run, once, just after handling the command line arguments, just before term-setup-hook.

Variable: user-init-file
This variable holds the file name of the user's init file. If the actual init file loaded is a compiled file, such as `.emacs.elc', the value refers to the corresponding source file.

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40.1.3 Terminal-Specific Initialization

Each terminal type can have its own Lisp library that Emacs loads when run on that type of terminal. The library's name is constructed by concatenating the value of the variable term-file-prefix and the terminal type (specified by the environment variable TERM). Normally, term-file-prefix has the value "term/"; changing this is not recommended. Emacs finds the file in the normal manner, by searching the load-path directories, and trying the `.elc' and `.el' suffixes.

The usual function of a terminal-specific library is to enable special keys to send sequences that Emacs can recognize. It may also need to set or add to function-key-map if the Termcap entry does not specify all the terminal's function keys. See section 40.8 Terminal Input.

When the name of the terminal type contains a hyphen, only the part of the name before the first hyphen is significant in choosing the library name. Thus, terminal types `aaa-48' and `aaa-30-rv' both use the `term/aaa' library. If necessary, the library can evaluate (getenv "TERM") to find the full name of the terminal type.

Your init file can prevent the loading of the terminal-specific library by setting the variable term-file-prefix to nil. This feature is useful when experimenting with your own peculiar customizations.

You can also arrange to override some of the actions of the terminal-specific library by setting the variable term-setup-hook. This is a normal hook which Emacs runs using run-hooks at the end of Emacs initialization, after loading both your init file and any terminal-specific libraries. You can use this variable to define initializations for terminals that do not have their own libraries. See section 23.6 Hooks.

Variable: term-file-prefix
If the term-file-prefix variable is non-nil, Emacs loads a terminal-specific initialization file as follows:

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

You may set the term-file-prefix variable to nil in your init file if you do not wish to load the terminal-initialization file. To do this, put the following in your init file: (setq term-file-prefix nil).

On MS-DOS, if the environment variable TERM is not set, Emacs uses `internal' as the terminal type.

Variable: term-setup-hook
This variable is a normal hook that Emacs runs after loading your init file, the default initialization file (if any) and the terminal-specific Lisp file.

You can use term-setup-hook to override the definitions made by a terminal-specific file.

See window-setup-hook in 38.19 Window Systems, for a related feature.

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40.1.4 Command-Line Arguments

You can use command-line arguments to request various actions when you start Emacs. Since you do not need to start Emacs more than once per day, and will often leave your Emacs session running longer than that, command-line arguments are hardly ever used. As a practical matter, it is best to avoid making the habit of using them, since this habit would encourage you to kill and restart Emacs unnecessarily often. These options exist for two reasons: to be compatible with other editors (for invocation by other programs) and to enable shell scripts to run specific Lisp programs.

This section describes how Emacs processes command-line arguments, and how you can customize them.

Function: command-line
This function parses the command line that Emacs was called with, processes it, loads the user's init file and displays the startup messages.

Variable: command-line-processed
The value of this variable is t once the command line has been processed.

If you redump Emacs by calling dump-emacs, you may wish to set this variable to nil first in order to cause the new dumped Emacs to process its new command-line arguments.

Variable: command-switch-alist
The value of this variable is an alist of user-defined command-line options and associated handler functions. This variable exists so you can add elements to it.

A command-line option is an argument on the command line, which has the form:


The elements of the command-switch-alist look like this:

(option . handler-function)

The CAR, option, is a string, the name of a command-line option (not including the initial hyphen). The handler-function is called to handle option, and receives the option name as its sole argument.

In some cases, the option is followed in the command line by an argument. In these cases, the handler-function can find all the remaining command-line arguments in the variable command-line-args-left. (The entire list of command-line arguments is in command-line-args.)

The command-line arguments are parsed by the command-line-1 function in the `startup.el' file. See also section `Command Line Switches and Arguments' in The GNU Emacs Manual.

Variable: command-line-args
The value of this variable is the list of command-line arguments passed to Emacs.

Variable: command-line-functions
This variable's value is a list of functions for handling an unrecognized command-line argument. Each time the next argument to be processed has no special meaning, the functions in this list are called, in order of appearance, until one of them returns a non-nil value.

These functions are called with no arguments. They can access the command-line argument under consideration through the variable argi, which is bound temporarily at this point. The remaining arguments (not including the current one) are in the variable command-line-args-left.

When a function recognizes and processes the argument in argi, it should return a non-nil value to say it has dealt with that argument. If it has also dealt with some of the following arguments, it can indicate that by deleting them from command-line-args-left.

If all of these functions return nil, then the argument is used as a file name to visit.

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40.2 Getting Out of Emacs

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. As a practical matter, you seldom kill Emacs--only when you are about to log out. Suspending is much more common.

40.2.1 Killing Emacs  Exiting Emacs irreversibly.
40.2.2 Suspending Emacs  Exiting Emacs reversibly.

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40.2.1 Killing Emacs

Killing Emacs means ending the execution of the Emacs process. The parent process normally resumes control. The low-level primitive for killing Emacs is kill-emacs.

Function: kill-emacs &optional exit-data
This function exits the Emacs process and kills it.

If exit-data is an integer, then it is used as the exit status of the Emacs process. (This is useful primarily in batch operation; see 40.13 Batch Mode.)

If exit-data is a string, its contents are stuffed into the terminal input buffer so that the shell (or whatever program next reads input) can read them.

All the information in the Emacs process, aside from files that have been saved, is lost when the Emacs process is killed. Because killing Emacs inadvertently can lose a lot of work, Emacs queries for confirmation before actually terminating if you have buffers that need saving or subprocesses that are running. This is done in the function save-buffers-kill-emacs.

Variable: kill-emacs-query-functions
After asking the standard questions, save-buffers-kill-emacs calls the functions in the list kill-emacs-query-functions, in order of appearance, with no arguments. These functions can ask for additional confirmation from the user. If any of them returns nil, Emacs is not killed.

Variable: kill-emacs-hook
This variable is a normal hook; once save-buffers-kill-emacs is finished with all file saving and confirmation, it runs the functions in this hook.

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40.2.2 Suspending Emacs

Suspending Emacs means stopping Emacs temporarily and returning control to its superior process, which is usually the shell. This allows you to resume editing later in the same Emacs process, with the same buffers, the same kill ring, the same undo history, and so on. To resume Emacs, use the appropriate command in the parent shell--most likely fg.

Some operating systems do not support suspension of jobs; on these systems, "suspension" actually creates a new shell temporarily as a subprocess of Emacs. Then you would exit the shell to return to Emacs.

Suspension is not useful with window systems, because the Emacs job may not have a parent that can resume it again, and in any case you can give input to some other job such as a shell merely by moving to a different window. Therefore, suspending is not allowed when Emacs is using a window system (X or MS Windows).

Function: suspend-emacs string
This function stops Emacs and returns control to the superior process. If and when the superior process resumes Emacs, suspend-emacs returns nil to its caller in Lisp.

If string is non-nil, its characters are sent to be read as terminal input by Emacs's superior shell. The characters in string are not echoed by the superior shell; only the results appear.

Before suspending, suspend-emacs runs the normal hook suspend-hook.

After the user resumes Emacs, suspend-emacs runs the normal hook suspend-resume-hook. See section 23.6 Hooks.

The next redisplay after resumption will redraw the entire screen, unless the variable no-redraw-on-reenter is non-nil (see section 38.1 Refreshing the Screen).

In the following example, note that `pwd' is not echoed after Emacs is suspended. But it is read and executed by the shell.

     => nil

(add-hook 'suspend-hook
          (function (lambda ()
                      (or (y-or-n-p
                            "Really suspend? ")
                          (error "Suspend cancelled")))))
     => (lambda nil
          (or (y-or-n-p "Really suspend? ")
              (error "Suspend cancelled")))
(add-hook 'suspend-resume-hook
          (function (lambda () (message "Resumed!"))))
     => (lambda nil (message "Resumed!"))
(suspend-emacs "pwd")
     => nil
---------- Buffer: Minibuffer ----------
Really suspend? y
---------- Buffer: Minibuffer ----------

---------- Parent Shell ----------
lewis@slug[23] % /user/lewis/manual
lewis@slug[24] % fg

---------- Echo Area ----------

Variable: suspend-hook
This variable is a normal hook that Emacs runs before suspending.

Variable: suspend-resume-hook
This variable is a normal hook that Emacs runs on resuming after a suspension.

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40.3 Operating System Environment

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.

Variable: system-configuration
This variable holds the GNU configuration name for the hardware/software configuration of your system, as a string. The convenient way to test parts of this string is with string-match.

Variable: system-type
The value of this variable is a symbol indicating the type of operating system Emacs is operating on. Here is a table of the possible values:

VMS on the Alpha.


Berkeley BSD.

Data General DGUX operating system.

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

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.)

Hewlett-Packard HPUX operating system.

Silicon Graphics Irix system.

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

NeXT Mach-based system.

Masscomp RTU, UCB universe.

UniSoft UniPlus.

AT&T System V.


Microsoft windows NT. The same executable supports Windows 9X, but the value of system-type is windows-nt in either case.

SCO Xenix 386.

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. We recommend using system-configuration to distinguish between different operating systems.

Function: system-name
This function returns the name of the machine you are running on.
     => "www.gnu.org"

The symbol system-name is a variable as well as a function. In fact, the function returns whatever value the variable system-name currently holds. Thus, you can set the variable system-name in case Emacs is confused about the name of your system. The variable is also useful for constructing frame titles (see section 29.4 Frame Titles).

Variable: 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. See section 40.4 User Identification. (Since this is done when Emacs starts up, the value actually used is the one saved when Emacs was dumped. See section E.1 Building Emacs.)

Command: getenv var
This function returns the value of the environment variable var, as a string. Within Emacs, the environment variable values are kept in the Lisp variable process-environment.

(getenv "USER")
     => "lewis"

lewis@slug[10] % printenv

Command: setenv variable value
This command sets the value of the environment variable named variable to value. Both arguments should be strings. This function works by modifying process-environment; binding that variable with let is also reasonable practice.

Variable: 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.

=> ("l=/usr/stanford/lib/gnuemacs/lisp"

Variable: path-separator
This variable holds a string which 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-DOS and MS-Windows.

Function: parse-colon-path path
This function takes a search path string such as would be the value of the PATH environment variable, and splits it at the separators, returning a list of directory names. nil in this list stands for "use the current directory." Although the function's name says "colon," it actually uses the value of path-separator.

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

Variable: 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.

Variable: invocation-directory
This variable holds the directory from which the Emacs executable was invoked, or perhaps nil if that directory cannot be determined.

Variable: installation-directory
If non-nil, this is a directory within which to look for the `lib-src' and `etc' subdirectories. This 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.

Function: load-average &optional use-float
This function returns the current 1-minute, 5-minute, and 15-minute load averages, in a list.

By default, the values are integers that are 100 times the system load averages, which indicate the average number of processes trying to run. If use-float is non-nil, then they are returned as floating point numbers and without multiplying by 100.

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

lewis@rocky[5] % uptime
 11:55am  up 1 day, 19:37,  3 users,
 load average: 1.69, 0.48, 0.36

Function: emacs-pid
This function returns the process ID of the Emacs process.

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

Function: setprv privilege-name &optional setp getprv
This function sets or resets a VMS privilege. (It does not exist on other systems.) The first argument is the privilege name, as a string. The second argument, setp, is t or nil, indicating whether the privilege is to be turned on or off. Its default is nil. The function returns t if successful, nil otherwise.

If the third argument, getprv, is non-nil, setprv does not change the privilege, but returns t or nil indicating whether the privilege is currently enabled.

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40.4 User Identification

Variable: init-file-user
This variable says which user's init files should be used by Emacs--or nil if none. 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' option was used, then Lisp packages should not load any customization files or user profile.

Variable: user-mail-address
This holds the nominal email address of the user who is using Emacs. Emacs normally sets this variable to a default value after reading your init files, but not if you have already set it. So you can set the variable to some other value in your init file if you do not want to use the default value.

Function: user-login-name &optional uid
If you don't specify uid, this function returns the name under which the user is logged in. If the environment variable LOGNAME is set, that value is used. Otherwise, if the environment variable USER is set, that value is used. Otherwise, the value is based on the effective UID, not the real UID.

If you specify uid, the value is the user name that corresponds to uid (which should be an integer).

     => "lewis"

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

Function: user-full-name &optional uid
This function returns the full name of the logged-in user--or the value of the environment variable NAME, if that is set.

     => "Bil Lewis"

If the Emacs job's user-id does not correspond to any known user (and provided NAME is not set), the value is "unknown".

If uid is non-nil, then it should be an integer (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 (see section 29.4 Frame Titles).

Function: user-real-uid
This function returns the real UID of the user.

     => 19

Function: user-uid
This function returns the effective UID of the user.

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40.5 Time of Day

This section explains how to determine the current time and the time zone.

Function: current-time-string &optional time-value
This function returns the current time and date as a human-readable string. The format of the string is unvarying; the number of characters used for each part is always the same, so you can reliably use substring to extract pieces of it. It is wise to count the characters from the beginning of the string rather than from the end, as additional information may some day be added at the end.

The argument time-value, if given, specifies a time to format instead of the current time. The argument should be a list whose first two elements are integers. Thus, you can use times obtained from current-time (see below) and from file-attributes (see section 25.6.4 Other Information about Files).

     => "Wed Oct 14 22:21:05 1987"

Function: current-time
This function returns the system's time value as a list of three integers: (high low microsec). The integers high and low combine to give the number of seconds since 0:00 January 1, 1970 (local time), which is high * 2**16 + low.

The third element, microsec, gives the microseconds since the start of the current second (or 0 for systems that return time with the resolution of only one second).

The first two elements can be compared with file time values such as you get with the function file-attributes. See section 25.6.4 Other Information about Files.

Function: current-time-zone &optional time-value
This function returns a list describing the time zone that the user is in.

The value has the form (offset name). Here offset is an integer giving the number of seconds ahead of UTC (east of Greenwich). A negative value means west of Greenwich. The second element, name, is a string giving the name of the time zone. Both elements change when daylight savings 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, both elements of the list are nil.

The argument time-value, if given, specifies a time to analyze instead of the current time. The argument should be a cons cell containing two integers, or a list whose first two elements are integers. Thus, you can use times obtained from current-time (see above) and from file-attributes (see section 25.6.4 Other Information about Files).

Function: float-time &optional time-value
This function returns the current time as a floating-point number of seconds since the epoch. The argument time-value, if given, specifies a time to convert instead of the current time. The argument should have the same form as for current-time-string (see above), and it also accepts the output of current-time and file-attributes.

Warning: Since the result is floating point, it may not be exact. Do not use this function if precise time stamps are required.

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40.6 Time Conversion

These functions convert time values (lists of two or three integers) to strings or to calendrical information. There is also a function to convert calendrical information to a time value. You can get time values from the functions current-time (see section 40.5 Time of Day) and file-attributes (see section 25.6.4 Other Information about Files).

Many operating systems are limited to time values that contain 32 bits of information; these systems typically handle only the times from 1901-12-13 20:45:52 UTC through 2038-01-19 03:14:07 UTC. However, some operating systems have larger time values, and can represent times far in the past or future.

Time conversion functions always use the Gregorian calendar, even for dates before the Gregorian calendar was introduced. Year numbers count the number of years since the year 1 B.C., and do not skip zero as traditional Gregorian years do; for example, the year number -37 represents the Gregorian year 38 B.C.

Function: format-time-string format-string &optional time universal
This function converts time (or the current time, if time is omitted) to a string according to format-string. The argument format-string may contain `%'-sequences which say to substitute parts of the time. Here is a table of what the `%'-sequences mean:

This stands for the abbreviated name of the day of week.
This stands for the full name of the day of week.
This stands for the abbreviated name of the month.
This stands for the full name of the month.
This is a synonym for `%x %X'.
This has a locale-specific meaning. In the default locale (named C), it is equivalent to `%A, %B %e, %Y'.
This stands for the day of month, zero-padded.
This is a synonym for `%m/%d/%y'.
This stands for the day of month, blank-padded.
This is a synonym for `%b'.
This stands for the hour (00-23).
This stands for the hour (01-12).
This stands for the day of the year (001-366).
This stands for the hour (0-23), blank padded.
This stands for the hour (1-12), blank padded.
This stands for the month (01-12).
This stands for the minute (00-59).
This stands for a newline.
This stands for `AM' or `PM', as appropriate.
This is a synonym for `%I:%M:%S %p'.
This is a synonym for `%H:%M'.
This stands for the seconds (00-59).
This stands for a tab character.
This is a synonym for `%H:%M:%S'.
This stands for the week of the year (01-52), assuming that weeks start on Sunday.
This stands for the numeric day of week (0-6). Sunday is day 0.
This stands for the week of the year (01-52), assuming that weeks start on Monday.
This has a locale-specific meaning. In the default locale (named `C'), it is equivalent to `%D'.
This has a locale-specific meaning. In the default locale (named `C'), it is equivalent to `%T'.
This stands for the year without century (00-99).
This stands for the year with century.
This stands for the time zone abbreviation.

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 the middle of a `%'-sequences. If you start the field width with `0', it means to pad with zeros. If you start the field width with `_', it means to pad with spaces.

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 between `%' and one of the letters in the table above. `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.

If universal is non-nil, that means to describe the time as Universal Time; nil means describe it using what Emacs believes is the local time zone (see current-time-zone).

This function uses the C library function strftime to do most of the work. In order to communicate with that function, it first encodes its argument using the coding system specified by locale-coding-system (see section 33.12 Locales); after strftime returns the resulting string, format-time-string decodes the string using that same coding system.

Function: decode-time time
This function converts a time value into calendrical information. The return value is a list of nine elements, as follows:

(seconds minutes hour day month year dow dst zone)

Here is what the elements mean:

The number of seconds past the minute, as an integer between 0 and 59.
The number of minutes past the hour, as an integer between 0 and 59.
The hour of the day, as an integer between 0 and 23.
The day of the month, as an integer between 1 and 31.
The month of the year, as an integer between 1 and 12.
The year, an integer typically greater than 1900.
The day of week, as an integer between 0 and 6, where 0 stands for Sunday.
t if daylight savings time is effect, otherwise nil.
An integer indicating the time zone, as the number of seconds east of Greenwich.

Common Lisp Note: Common Lisp has different meanings for dow and zone.

Function: encode-time seconds minutes hour day month year &optional zone
This function is the inverse of decode-time. It converts seven items of calendrical data into a time value. For the meanings of the arguments, see the table above under decode-time.

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 optional argument zone defaults to the current time zone and its daylight savings time rules. If specified, it can be either a list (as you would get from current-time-zone), a string as in the TZ environment variable, or an integer (as you would get from decode-time). The specified zone is used without any further alteration for daylight savings time.

If you pass more than seven arguments to encode-time, the first six are used as seconds through year, the last argument is used as zone, and the arguments in between are ignored. This feature makes it possible to use the elements of a list returned by decode-time as the arguments to encode-time, like this:

(apply 'encode-time (decode-time ...))

You can perform simple date arithmetic by using out-of-range values for the seconds, minutes, hour, day, and month arguments; for example, day 0 means the day preceding the given month.

The operating system puts limits on the range of possible time values; if you try to encode a time that is out of range, an error results.

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40.7 Timers for Delayed Execution

You can set up a timer to call a function at a specified future time or after a certain length of idleness.

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.

Function: run-at-time time repeat function &rest args
This function arranges to call function with arguments args at time time. The argument function is a function to call later, and args are the arguments to give it when it is called. The time time is specified as a string.

Absolute times may be specified in a wide variety of formats; this function tries to accept all the commonly used date formats. Valid formats include these two,

year-month-day hour:min:sec timezone

hour:min:sec timezone month/day/year

where in both examples all fields are numbers; the format that current-time-string returns is also allowed, and many others as well.

To specify a relative time, 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.

If time is a number (integer or floating point), that specifies a relative time measured in seconds.

The argument repeat specifies how often to repeat the call. If repeat is nil, there are no repetitions; function is called just once, at time. If repeat is a number, it specifies a repetition period measured in seconds.

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.

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).

Macro: with-timeout (seconds timeout-forms...) body...
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. See section 20.6 Yes-or-No Queries.

Function: run-with-idle-timer secs repeat function &rest args
Set up a timer which runs when Emacs has been idle for secs seconds. The value of secs may be an integer or a floating point number.

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 (see below).

Emacs becomes "idle" when it starts waiting for user input, 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.

Function: cancel-timer timer
Cancel the requested action for timer, which should be a value previously returned by run-at-time or run-with-idle-timer. This cancels the effect of that call to run-at-time; the arrival of the specified time will not cause anything special to happen.

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40.8 Terminal Input

This section describes functions and variables for recording or manipulating terminal input. See 38. Emacs Display, for related functions.

40.8.1 Input Modes  Options for how input is processed.
40.8.2 Translating Input Events  Low level conversion of some characters or events into others.
40.8.3 Recording Input  Saving histories of recent or all input events.

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40.8.1 Input Modes

Function: set-input-mode interrupt flow meta quit-char
This function sets the mode for reading keyboard input. If interrupt is non-null, then Emacs uses input interrupts. If it is nil, then it uses CBREAK mode. The default setting is system-dependent. Some systems always use CBREAK mode regardless of what is specified.

When Emacs communicates directly with X, it ignores this argument and uses interrupts if that is the way it knows how to communicate.

If flow is non-nil, then Emacs uses XON/XOFF (C-q, C-s) flow control for output to the terminal. This has no effect except in CBREAK mode. See section 40.12 Flow Control.

The argument meta controls support for input character codes above 127. If meta is t, Emacs converts characters with the 8th bit set into Meta characters. If meta is nil, Emacs disregards the 8th bit; this is necessary when the terminal uses it as a parity bit. If meta is neither t nor nil, Emacs uses all 8 bits of input unchanged. This is good for terminals that use 8-bit character sets.

If quit-char is non-nil, it specifies the character to use for quitting. Normally this character is C-g. See section 21.10 Quitting.

The current-input-mode function returns the input mode settings Emacs is currently using.

Function: current-input-mode
This function returns the current mode for reading keyboard input. It returns a list, corresponding to the arguments of set-input-mode, of the form (interrupt flow meta quit) in which:
is non-nil when Emacs is using interrupt-driven input. If nil, Emacs is using CBREAK mode.
is non-nil if Emacs uses XON/XOFF (C-q, C-s) flow control for output to the terminal. This value is meaningful only when interrupt is nil.
is t if Emacs treats the eighth bit of input characters as the meta bit; nil means Emacs clears the eighth bit of every input character; any other value means Emacs uses all eight bits as the basic character code.
is the character Emacs currently uses for quitting, usually C-g.

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40.8.2 Translating Input Events

This section describes features for translating input events into other input events before they become part of key sequences. These features apply to each event in the order they are described here: each event is first modified according to extra-keyboard-modifiers, then translated through keyboard-translate-table (if applicable), and finally decoded with the specified keyboard coding system. If it is being read as part of a key sequence, it is then added to the sequence being read; then subsequences containing it are checked first with function-key-map and then with key-translation-map.

Variable: extra-keyboard-modifiers
This variable lets Lisp programs "press" the modifier keys on the keyboard. The value is a bit mask:

The SHIFT key.
The LOCK key.
The CTL key.
The META key.

Each time the user types a keyboard key, it is altered as if the modifier keys specified in the bit mask were held down.

When using a window system, the program can "press" any of the modifier keys in this way. Otherwise, only the CTL and META keys can be virtually pressed.

Variable: keyboard-translate-table
This variable is the translate table for keyboard characters. It lets you reshuffle the keys on the keyboard without changing any command bindings. Its value is normally a char-table, or else nil.

If keyboard-translate-table is a char-table (see section 6.6 Char-Tables), then each character read from the keyboard is looked up in this char-table. If the value found there is non-nil, then it is used instead of the actual input character.

In the example below, we set keyboard-translate-table to a char-table. Then we fill it in to swap the characters C-s and C-\ and the characters C-q and C-^. Subsequently, typing C-\ has all the usual effects of typing C-s, and vice versa. (See section 40.12 Flow Control, for more information on this subject.)

(defun evade-flow-control ()
  "Replace C-s with C-\ and C-q with C-^."
  (setq keyboard-translate-table
        (make-char-table 'keyboard-translate-table nil))
  ;; Swap C-s and C-\.
  (aset keyboard-translate-table ?\034 ?\^s)
  (aset keyboard-translate-table ?\^s ?\034)
  ;; Swap C-q and C-^.
  (aset keyboard-translate-table ?\036 ?\^q)
  (aset keyboard-translate-table ?\^q ?\036))

Note that this translation is the first thing that happens to a character after it is read from the terminal. Record-keeping features such as recent-keys and dribble files record the characters after translation.

Function: keyboard-translate from to
This function modifies keyboard-translate-table to translate character code from into character code to. It creates the keyboard translate table if necessary.

The remaining translation features translate subsequences of key sequences being read. They are implemented in read-key-sequence and have no effect on input read with read-event.

Variable: function-key-map
This variable holds a keymap that describes the character sequences sent by function keys on an ordinary character terminal. This keymap has the same structure as other keymaps, but is used differently: it specifies translations to make while reading key sequences, rather than bindings for key sequences.

If function-key-map "binds" a key sequence k to a vector v, then when k appears as a subsequence anywhere in a key sequence, it is replaced with the events in v.

For example, VT100 terminals send ESC O P when the keypad PF1 key is pressed. Therefore, we want Emacs to translate that sequence of events into the single event pf1. We accomplish this by "binding" ESC O P to [pf1] in function-key-map, when using a VT100.

Thus, typing C-c PF1 sends the character sequence C-c ESC O P; later the function read-key-sequence translates this back into C-c PF1, which it returns as the vector [?\C-c pf1].

Entries in function-key-map are ignored if they conflict with bindings made in the minor mode, local, or global keymaps. The intent is that the character sequences that function keys send should not have command bindings in their own right--but if they do, the ordinary bindings take priority.

The value of function-key-map is usually set up automatically according to the terminal's Terminfo or Termcap entry, but sometimes those need help from terminal-specific Lisp files. Emacs comes with terminal-specific files for many common terminals; their main purpose is to make entries in function-key-map beyond those that can be deduced from Termcap and Terminfo. See section 40.1.3 Terminal-Specific Initialization.

Variable: key-translation-map
This variable is another keymap used just like function-key-map to translate input events into other events. It differs from function-key-map in two ways:

The intent of key-translation-map is for users to map one character set to another, including ordinary characters normally bound to self-insert-command.

You can use function-key-map or key-translation-map for more than simple aliases, by using a function, instead of a key sequence, as the "translation" of a key. Then this function is called to compute the translation of that key.

The key translation function receives one argument, which is the prompt that was specified in read-key-sequence---or nil if the key sequence is being read by the editor command loop. In most cases you can ignore the prompt value.

If the function reads input itself, it can have the effect of altering the event that follows. For example, here's how to define C-c h to turn the character that follows into a Hyper character:

(defun hyperify (prompt)
  (let ((e (read-event)))
    (vector (if (numberp e)
                (logior (lsh 1 24) e)
              (if (memq 'hyper (event-modifiers e))
                (add-event-modifier "H-" e))))))

(defun add-event-modifier (string e)
  (let ((symbol (if (symbolp e) e (car e))))
    (setq symbol (intern (concat string
                                 (symbol-name symbol))))
    (if (symbolp e)
      (cons symbol (cdr e)))))

(define-key function-key-map "\C-ch" 'hyperify)

Finally, if you have enabled keyboard character set decoding using set-keyboard-coding-system, decoding is done after the translations listed above. See section 33.10.6 Specifying a Coding System for One Operation. In future Emacs versions, character set decoding may be done before the other translations.

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40.8.3 Recording Input

Function: recent-keys
This function returns a vector containing the last 100 input events from the keyboard or mouse. All input events are included, whether or not they were used as parts of key sequences. Thus, you always get the last 100 input events, not counting events generated by keyboard macros. (These are excluded because they are less interesting for debugging; it should be enough to see the events that invoked the macros.)

A call to clear-this-command-keys (see section 21.4 Information from the Command Loop) causes this function to return an empty vector immediately afterward.

Command: open-dribble-file filename
This function opens a dribble file named filename. When a dribble file is open, each input event from the keyboard or mouse (but not those from keyboard macros) is written in that file. A non-character event is expressed using its printed representation surrounded by `<...>'.

You close the dribble file by calling this function with an argument of nil.

This function is normally used to record the input necessary to trigger an Emacs bug, for the sake of a bug report.

(open-dribble-file "~/dribble")
     => nil

See also the open-termscript function (see section 40.9 Terminal Output).

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40.9 Terminal Output

The terminal output functions send output to the 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.

Variable: 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--even when using a window system. (We designed it this way despite the fact that a window system has no true "output speed", to give you a way to tune these decisions.)

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.

Function: baud-rate
This obsolete function returns the value of the variable baud-rate.

Function: send-string-to-terminal string
This function sends string to the terminal without alteration. Control characters in string have terminal-dependent effects.

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 filename
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.

See also open-dribble-file in 40.8 Terminal Input.

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

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40.10 Sound Output

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. Emacs version 20 and earlier did not support sound at all.

The sound must be stored as a file in RIFF-WAVE format (`.wav') or Sun Audio format (`.au').

Function: play-sound 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.

Function: play-sound-file file &optional volume device
This function is an alternative interface to playing a sound file specifying an optional volume and device.

Variable: 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.

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40.11 System-Specific X11 Keysyms

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

Variable: 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. See section 29.2 Multiple Displays.

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40.12 Flow Control

This section attempts to answer the question "Why does Emacs use flow-control characters in its command character set?" For a second view on this issue, read the comments on flow control in the `emacs/INSTALL' file from the distribution; for help with Termcap entries and DEC terminal concentrators, see `emacs/etc/TERMS'.

At one time, most terminals did not need flow control, and none used C-s and C-q for flow control. Therefore, the choice of C-s and C-q as command characters for searching and quoting was natural and uncontroversial. With so many commands needing key assignments, of course we assigned meanings to nearly all ASCII control characters.

Later, some terminals were introduced which required these characters for flow control. They were not very good terminals for full-screen editing, so Emacs maintainers ignored them. In later years, flow control with C-s and C-q became widespread among terminals, but by this time it was usually an option. And the majority of Emacs users, who can turn flow control off, did not want to switch to less mnemonic key bindings for the sake of flow control.

So which usage is "right"---Emacs's or that of some terminal and concentrator manufacturers? This question has no simple answer.

One reason why we are reluctant to cater to the problems caused by C-s and C-q is that they are gratuitous. There are other techniques (albeit less common in practice) for flow control that preserve transparency of the character stream. Note also that their use for flow control is not an official standard. Interestingly, on the model 33 teletype with a paper tape punch (around 1970), C-s and C-q were sent by the computer to turn the punch on and off!

As window systems and PC terminal emulators replace character-only terminals, the flow control problem is gradually disappearing. For the mean time, Emacs provides a convenient way of enabling flow control if you want it: call the function enable-flow-control.

Command: enable-flow-control
This function enables use of C-s and C-q for output flow control, and provides the characters C-\ and C-^ as aliases for them using keyboard-translate-table (see section 40.8.2 Translating Input Events).

You can use the function enable-flow-control-on in your init file to enable flow control automatically on certain terminal types.

Function: enable-flow-control-on &rest termtypes
This function enables flow control, and the aliases C-\ and C-^, if the terminal type is one of termtypes. For example:

(enable-flow-control-on "vt200" "vt300" "vt101" "vt131")

Here is how enable-flow-control does its job:

  1. It sets CBREAK mode for terminal input, and tells the operating system to handle flow control, with (set-input-mode nil t).

  2. It sets up keyboard-translate-table to translate C-\ and C-^ into C-s and C-q. Except at its very lowest level, Emacs never knows that the characters typed were anything but C-s and C-q, so you can in effect type them as C-\ and C-^ even when they are input for other commands. See section 40.8.2 Translating Input Events.

If the terminal is the source of the flow control characters, then once you enable kernel flow control handling, you probably can make do with less padding than normal for that terminal. You can reduce the amount of padding by customizing the Termcap entry. You can also reduce it by setting baud-rate to a smaller value so that Emacs uses a smaller speed when calculating the padding needed. See section 40.9 Terminal Output.

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40.13 Batch Mode

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, and `-f function', which calls function with no arguments.

Any Lisp program output that would normally go to the echo area, either using message, or using prin1, etc., with t as the stream, goes instead to Emacs's standard error descriptor when in batch mode. 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.)

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

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