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:

-option

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.