40.1.4 Command-Line Arguments
You can use command-line arguments to request various actions when you start Emacs. Note that the recommended way of using Emacs is to start it just once, after logging in, and then do all editing in the same Emacs session (see Entering Emacs in The GNU Emacs Manual). For this reason, you might not use command-line arguments very often; nonetheless, they can be useful when invoking Emacs from session scripts or debugging Emacs. This section describes how Emacs processes command-line arguments.
function command-line​
This function parses the command line that Emacs was called with, processes it, and (amongst other things) 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 (see Building 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​
This variable is an alist of user-defined command-line options and associated handler functions. By default it is empty, but you can add elements if you wish.
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 (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 (see below). (The entire list of command-line arguments is in command-line-args.)
Note that the handling of command-switch-alist doesn’t treat equals signs in option specially. That is, if there’s an option like --name=value on the command line, then only a command-switch-alist member whose car is literally --name=value will match this option. If you want to parse such options, you need to use command-line-functions instead (see below).
The command-line arguments are parsed by the command-line-1 function in the startup.el file. See also Command Line Arguments for Emacs Invocation 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-args-left​
The value of this variable is the list of command-line arguments that have not yet been processed.
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 treated as a file name to visit.