This variable says which user’s init files should be used by Emacs—or nil if none. "" stands for the user who originally logged in. The value reflects command-line options such as ‘-q’ or ‘-u user’.
Lisp packages that load files of customizations, or any other sort of user profile, should obey this variable in deciding where to find it. They should load the profile of the user name found in this variable. If init-file-user is nil, meaning that the ‘-q’, ‘-Q’, or ‘-batch’ option was used, then Lisp packages should not load any customization files or user profile.
This holds the email address of the user who is using Emacs.
This function returns the name under which the user is logged in. It uses the environment variables LOGNAME or USER if either is set. Otherwise, the value is based on the effective UID, not the real UID.
If you specify uid (a number), the result is the user name that corresponds to uid, or nil if there is no such user.
This function returns the user name corresponding to Emacs’s real UID. This ignores the effective UID, and the environment variables LOGNAME and USER.
This function returns the full name of the logged-in user—or the value of the environment variable NAME, if that is set.
If the Emacs process’s user-id does not correspond to any known user (and provided NAME is not set), the result is "unknown".
If uid is non-nil, then it should be a number (a user-id) or a string (a login name). Then user-full-name returns the full name corresponding to that user-id or login name. If you specify a user-id or login name that isn’t defined, it returns nil.
The symbols user-login-name, user-real-login-name and user-full-name are variables as well as functions. The functions return the same values that the variables hold. These variables allow you to fake out Emacs by telling the functions what to return. The variables are also useful for constructing frame titles (see Frame Titles).
This function returns the real UID of the user.
This function returns the effective UID of the user.
This function returns the effective GID of the Emacs process.
This function returns the real GID of the Emacs process.
This function returns a list of strings, listing the user names on the system. If Emacs cannot retrieve this information, the return value is a list containing just the value of user-real-login-name.
This function returns a list of strings, listing the names of user groups on the system. If Emacs cannot retrieve this information, the return value is nil.
This function returns the group name that corresponds to the numeric group ID gid, or nil if there is no such group.
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Licensed under the GNU GPL license.
https://www.gnu.org/software/emacs/manual/html_node/elisp/User-Identification.html