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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.

tshepang
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    Looking at the man page, whoami reports the name associated with your effective user ID. Which means it will return something different if you're using sudo or running a setuid executable. If you have sudo set up, try sudo whoami for example. – Joseph R. May 19 '13 at 11:44
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    USER and USERNAME are ordinary environment variables, which means that, if you want, you can set them to arbitrary values. Just type USER=xyz. In other words, even if those variables exist, there is no guarantee that their values match the currently logged-in username. – Uwe May 19 '13 at 12:03
  • @Uwe By guarantee, I meant by default (i.e. assuming user did not change them). – tshepang May 19 '13 at 12:22
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    @Tshepang As a follow up to my first comment: compare the results of sudo whoami and sudo echo $USER – Joseph R. May 19 '13 at 16:58
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    @JosephR. For sudo echo $USER, the shell expands $USER, then calls sudo. So of course it doesn't produce the same output as whoami. Like sudo whoami, sudo sh -c 'echo $USER' does (typically) output root. Regarding your comment about whoami using the EUID, note that sudo whoami would output root even if whoami used the UID. sudo sets both EUID and UID for the command it runs (except in the very unusual situation that you explicitly configure it to behave otherwise). Compare sudo id -u to sudo id -ru. – Eliah Kagan Oct 07 '14 at 16:55

4 Answers4

46

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);
Cristian Ciupitu
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poige
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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).

Stéphane Chazelas
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8

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.

Community
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nealmcb
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5

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. ;)

Jesse Chisholm
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  • All of these 3 variables are unset in a Docker container running centos:7 and Host network. – Dima Korobskiy Feb 26 '21 at 23:09
  • Interesting. My centos7 docker image sets USER. It would seem that your centos7 image never simulates running login to set those up. Hmm. – Jesse Chisholm Feb 28 '21 at 02:04
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    The Dockerfile does this: 
    ENV user=foo
USER ${user}
WORKDIR /home/${user}

    It works fine: "The USER instruction sets the user name (or UID) and optionally the user group (or GID) to use when running the image and for any RUN, CMD and ENTRYPOINT instructions that follow it in the Dockerfile."

     – Dima Korobskiy Feb 28 '21 at 15:28
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    If I want to use USER inside ENTRYPOINT, I can modify Dockerfile, I guess, to do ENV USER=foo. Just something to keep in mind... – Dima Korobskiy Feb 28 '21 at 15:41