Who sets $USER and $USERNAME environment variables?

Question

Also, will these variables always match currently logged-in username (they do on my Debian system)? Can I assume their availability in other Unix(-like) systems?

I'm also curious why one would use whoami instead of just reading any of these variables.

Answer 1

It's login.

The Linux login(1) man page says:

The value for $HOME, $USER, $SHELL, $PATH, $LOGNAME, and $MAIL are set according to the appropriate fields in the password entry.

The FreeBSD login(1) man page says:

The login utility enters information into the environment (see environ(7)) specifying the user's home directory (HOME), command interpreter (SHELL), search path (PATH), terminal type (TERM) and user name (both LOGNAME and USER).

The NetBSD, OpenBSD and OS X man pages say the same thing.

Here's the source code from the util-linux login:

setenv("HOME", pwd->pw_dir, 0); /* legal to override */
setenv("USER", pwd->pw_name, 1);
setenv("SHELL", pwd->pw_shell, 1);
/* ... */
setenv("LOGNAME", pwd->pw_name, 1);

Here's the source code from the FreeBSD login:

(void)setenv("LOGNAME", username, 1);
(void)setenv("USER", username, 1);
(void)setenv("PATH", rootlogin ? _PATH_STDPATH : _PATH_DEFPATH, 0);

Answer 2

There's no rule. Some shells like tcsh or zsh set $LOGNAME. zsh sets $USERNAME (you can even assign a value to the variable to change uids/gids to those of that user there if permitted to).

It may be set by some things that log you in like login (as invoked by getty when login on a terminal and sometimes by other things like in.rlogind), cron, su, sudo, sshd, rshd, graphical login managers or may not.

If there's been a login though, in my experience, $USER is generally set (but it may not be updated after a change of user id (via setuid commands) within that login session. POSIX requires that $LOGNAME be set upon login (and cron).

To get the login name portably, best is to use the logname command (if there's not been any login, it may return nothing). To get the user id, use id -u. To get one username corresponding to the current effective user id: id -un. To get all of them (most of the time, there's only one user name per user id, but that's not guaranteed):

perl -le 'while ($n = getpwent()) {print $n if getpwnam($n) == $>}'

Though that may not work on systems where the user database cannot be enumerated (as happens sometimes with networked user databases for instance).

Answer 3

You probably want to rely on the POSIX standard here, since at some point in time you will probably care about not just user login (managed by the login program) but also cron jobs and the like.

Therefore, you should know that POSIX requires $LOGNAME but not $USER. E.g. $USER may not be set by cron, as pointed out in an answer by Keith Thompson, which also references some history on how this relates to the history of System-V vs BSD:

... at least on my system (Ubuntu 14.04) the $USER environment variable is not set for cron jobs. Instead, you can use $LOGNAME, which is part of the environment for cron jobs.

According to the environ(7) man page (type man environ to read it), $USER is used by BSD-derived programs and $LOGNAME is used by System-V-derived programs.

Answer 4

If you want to use the environment variables (instead of whoami or getpwent and getpwnam) and you are unsure if they are always set the same way on all *NIX systems, then try this in bash:

THIS_USER=${USER:-${USERNAME:-${LOGNAME}}}
echo ${THIS_USER}

If it is still empty after all that, then you are on a rather esoteric system. ;)