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

Starting Up Emacs

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

Summary: Sequence of Actions at Start Up

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

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

  2. It loads `.emacs' unless `-q' was specified on command line. (This is not done in `-batch' mode.) `.emacs' is found in the user's home directory; the `-u' option can specify the user name whose home directory should be used.

  3. It loads `default.el' unless inhibit-default-init is non-nil. (This is not done in `-batch' mode or if `-q' was specified on command line.)

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

  5. It loads the terminal-specific Lisp file, if any, except when in batch mode.

  6. It runs term-setup-hook.

  7. It runs window-setup-hook. See section Window Systems.

  8. It displays copyleft and nonwarranty, plus basic use information, unless the value of the variable inhibit-startup-message is non-nil.

    This display is also inhibited in batch mode, and if the current buffer is not `*scratch*'.

  9. It processes any remaining command line arguments.

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.

The Init File: `.emacs'

When you start Emacs, it normally attempts to load the file `.emacs' from your home directory. This file, if it exists, must contain Lisp code. It is called your init file. The command line switches `-q' and `-u' can be used to control the use of the init file; `-q' says not to load an init file, and `-u' says to load a specified user's init file instead of yours. See section `Entering Emacs' in The GNU Emacs Manual.

Emacs may also 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 How Programs Do Loading). The Emacs distribution does not have any such file; you may create one at your site 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 will not subsequently load the `default.el' file.

If there is a great deal of code in your `.emacs' file, you should move it into another file named `something.el', byte-compile it (see section Byte Compilation), and make your `.emacs' file load the other file using load (see section Loading).

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

Variable: after-init-hook

These two normal hooks are run just before, and just after, loading of the user's init file or `default.el'.

Terminal-Specific Initialization

Each terminal type can have its own Lisp library that Emacs loads when run on that type of terminal. For a terminal type named termtype, the library is called `term/termtype'. Emacs finds the file by searching the load-path directories as it does for other files, and trying the `.elc' and `.el' suffixes. Normally, terminal-specific Lisp library is located in `emacs/lisp/term', a subdirectory of the `emacs/lisp' directory in which most Emacs Lisp libraries are kept.

The library's name is constructed by concatenating the value of the variable term-file-prefix and the terminal type. Normally, term-file-prefix has the value "term/"; changing this is not recommended.

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 fully explain what should go in it. See section 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 `.emacs' file can prevent the loading of the terminal-specific library by setting the variable term-file-prefix to nil. This feature is very 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 `.emacs' 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 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 `.emacs' file if you do not wish to load the terminal-initialization file. To do this, put the following in your `.emacs' file: (setq term-file-prefix nil).

Variable: term-setup-hook

This variable is a normal hook which Emacs runs after loading your `.emacs' file, the default initialization file (if any) and after loading terminal-specific Lisp files. arguments.

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

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

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.

Function: command-line

This function parses the command line which Emacs was called with, processes it, loads the user's `.emacs' file and displays the initial nonwarranty information, etc.

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 of the form:

-option

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

(option . handler-function)

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 arguments passed by the shell to Emacs, as a list of strings.

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.

Killing Emacs

Killing Emacs means ending the execution of the Emacs process. The parent process normally resumes control.

All the information in the Emacs process, aside from files that have been saved, is lost when the Emacs 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.

Function: kill-emacs &optional no-query

This function exits the Emacs process and kills it.

Normally, if there are modified files or if there are running processes, kill-emacs asks the user for confirmation before exiting. However, if no-query is supplied and non-nil, then Emacs exits without confirmation.

If no-query is an integer, then it is used as the exit status of the Emacs process. (This is useful primarily in batch operation; see section Batch Mode.)

If no-query 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.

Variable: kill-emacs-hook

This variable is a normal hook (a list of functions); the first thing kill-emacs does is to run this hook with run-hooks. That calls each of the functions in the list, with no arguments.

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 such as X, 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 an X client.

Function: suspend-emacs string

This function stops Emacs and returns to the superior process. It returns nil.

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. In Emacs version 18, suspend-hook was not a normal hook; its value was a single function, and if its value was non-nil, then suspend-emacs returned immediately without actually suspending anything.

After the user resumes Emacs, it runs the normal hook suspend-resume-hook using run-hooks. See section Hooks.

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

(suspend-emacs)
     => 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 ----------
Resumed!

Variable: suspend-hook

This variable is a normal hook run before suspending.

Variable: suspend-resume-hook

This variable is a normal hook run after suspending.

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-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 symbols for the operating systems that Emacs can run on up to version 19.1.

aix-v3
AIX version 3.

berkeley-unix
Berkeley BSD 4.1, 4.2, or 4.3.

hpux
Hewlett-Packard operating system, version 5, 6, or 7.

irix
Silicon Graphics Irix system.

rtu
RTU 3.0, UCB universe.

unisoft-unix
UniSoft's UniPlus 5.0 or 5.2.

usg-unix-v
AT&T's System V.0, System V Release 2.0, 2.2, or 3.

vax-vms
VAX VMS version 4 or 5.

xenix
SCO Xenix 386 Release 2.2.

We do not wish to add new symbols to make finer distinctions unless it is absolutely necessary! In fact, it would be nice to eliminate some of these alternatives in the future.

Function: system-name

This function returns the name of the machine you are running on.

(system-name)
     => "prep.ai.mit.edu"

Function: getenv var

This function returns the value of the environment variable var, as a string. If the variable process-environment specifies a value for var, that overrides the actual environment.

(getenv "USER")
     => "lewis"

lewis@slug[10] % printenv
PATH=.:/user/lewis/bin:/usr/bin:/usr/local/bin
USER=lewis
TERM=ibmapa16
SHELL=/bin/csh
HOME=/user/lewis

Command: setenv variable value

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

Variable: process-environment

This variable is a list of strings to append to the environment of processes as they are created. Each string assigns a value to a shell environment variable. (This applies both to asynchronous and synchronous processes.) The function getenv also looks at this variable.

process-environment
=> ("l=/usr/stanford/lib/gnuemacs/lisp"
    "PATH=.:/user/lewis/bin:/usr/class:/nfsusr/local/bin"
    "USER=lewis" 
    "TERM=ibmapa16" 
    "SHELL=/bin/csh"
    "HOME=/user/lewis")

Function: load-average

This function returns the current 1 minute, 5 minute and 15 minute load averages in a list. The values are integers that are 100 times the system load averages. (The load averages indicate the number of processes trying to run.)

(load-average)
     => (169 48 36)

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

Function: setprv privilege-name &optional setp getprv

This function sets or resets a VMS privilege. (It does not exist on Unix.) The first arg 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 success, nil if not.

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.

User Identification

Function: user-login-name

This function returns the name under which the user is logged in. This is based on the effective UID, not the real UID.

(user-login-name)
     => "lewis"

Function: user-real-login-name

This function returns the name under which the user logged in. This is based on the real UID, not the effective UID. This differs from user-login-name only when running with the setuid bit.

Function: user-full-name

This function returns the full name of the user.

(user-full-name)
     => "Bil Lewis"

Function: user-real-uid

This function returns the real UID of the user.

(user-real-uid)
     => 19

Function: user-uid

This function returns the effective UID of the user.

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 humanly-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. However, it would be wise to count the characters from the beginning of the string rather than from the end, as additional information may 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 cons cell containing two integers, or 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 Other Information about Files).

(current-time-string)
     => "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, 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 only on the resolution of a second).

The first two elements can be compared with file time values such as you get with the function file-attributes. See section 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 below) and from file-attributes (see section Other Information about Files).

Timers

You can set up a timer to call a function at a specified future time.

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; The form `hour:min:sec timezone month/day/year', where all fields are numbers, works; the format that current-time-string returns is also allowed.

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.

If time is an integer, 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 an integer, it specifies a repetition period measured in seconds.

Function: cancel-timer timer

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

Terminal Input

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

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.

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

The normal setting is system dependent. Some systems always use CBREAK mode regardless of what is specified.

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 using European 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 Quitting.

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

Function: current-input-mode

This function returns 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:

INTERRUPT
is non-nil when Emacs is using interrupt-driven input. If nil, Emacs is using CBREAK mode.
FLOW
is non-nil if Emacs uses XON/XOFF (C-q, C-s) flow control for output to the terminal. This value has no effect unless INTERRUPT is non-nil.
META
is non-nil if Emacs is paying attention to the eighth bit of input characters; if nil, Emacs clears the eighth bit of every input character.
QUIT
is the character Emacs currently uses for quitting, usually C-g.

Variable: meta-flag

This variable used to control whether to treat the 0200 bit in keyboard input as the Meta bit. nil meant no, and anything else meant yes. This variable existed in Emacs versions 18 and earlier but no longer exists in Emacs 19; use set-input-mode instead.

Translating Input Events

Variable: extra-keyboard-modifiers

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

1
The SHIFT key.
2
The LOCK key.
4
The CTL key.
8
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 you use X windows, 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 must be a string or nil.

If keyboard-translate-table is a string, then each character read from the keyboard is looked up in this string and the character in the string is used instead. If the string is of length n, character codes n and up are untranslated.

In the example below, we set keyboard-translate-table to a string of 128 characters. 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 Flow Control for more information on this subject.)

(defun evade-flow-control ()
  "Replace C-s with C-\ and C-q with C-^."
  (interactive)
  (let ((the-table (make-string 128 0)))
    (let ((i 0))
      (while (< i 128)
        (aset the-table i i)
        (setq i (1+ i))))

    ;; Swap C-s and C-\.
    (aset the-table ?\034 ?\^s)
    (aset the-table ?\^s ?\034)
    ;; Swap C-q and C-^.
    (aset the-table ?\036 ?\^q)
    (aset the-table ?\^q ?\036)

    (setq keyboard-translate-table the-table)))

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 or enlarges the translate table if necessary.

Variable: function-key-map

This variable holds a keymap which describes the character sequences sent by function keys on an ordinary character terminal. This keymap uses the data structure as other keymaps, but is used differently: it specifies translations to make while reading events.

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.

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 a number of 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 Terminal-Specific Initialization.

Emacs versions 18 and earlier used totally different means of detecting the character sequences that represent function keys.

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.

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 inputs, not counting keyboard macros. (Events from keyboard macros are excluded because they are less interesting for debugging; it should be enough to see the events which invoked the macros.)

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) are 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. The function always returns 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 Terminal Output).

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 function returns the value of the variable baud-rate. In Emacs versions 18 and earlier, this was the only way to find out the terminal speed.

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 which are due to incorrect Termcap entries or to undesirable settings of terminal options more often than 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 section Terminal Input.

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

Flow Control

This section attempts to answer the question "Why does Emacs choose to 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 was unobjectionable. Emacs, for economy of keystrokes and portability, used nearly all the ASCII control characters, with mnemonic meanings when possible; thus, C-s for search and C-q for quote.

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 did not pay attention. 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 users, who can turn flow control off, were unwilling 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 is a rhetorical (or religious) question; it 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 (which is very old), C-s and C-q were sent by the computer to turn the punch on and off!

GNU Emacs version 19 provides a convenient way of enabling flow control if you want it: call the function enable-flow-control.

Function: 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 Translating Input Events).

You can use the function enable-flow-control-on in your `.emacs' 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 kernel 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 were typed. 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. For example:

    (setq keyboard-translate-table (make-string 128 0))
    (let ((i 0))
      ;; Map most characters into themselves.
      (while (< i 128)
        (aset keyboard-translate-table i i)
        (setq i (1+ i))))
      ;; Map C-\ to C-s.
      (aset the-table ?\034 ?\^s)
      ;; Map C-^ to C-q.
      (aset the-table ?\036 ?\^q)))
    

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 Terminal Output.

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 output descriptor when in batch mode. 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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