ID(1) General Commands Manual ID(1)

id — get credentials

id [user]…
id -u|-g|-G [-nrz] [user]…
id -Z [-z]
groups [user]…
whoami

The flag-free form writes users' (each of which can either be a name or a numeric ID; if none specified — the process') credentials to the standard output stream:

uid=0(root) gid=0(root) groups=0(root(wheel),22 context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
uid=1000(cicada) gid=100(users) euid=0(root) egid=39(irc) groups=100(users),4(adm),27(sudo)
When an ID is missing from passwd(5) or group(5) its name and parentheses are omitted.

The IDs listed are real; and , containing the effective IDs, are only written if they differ.

If one of -ugGZ is specified, the IDs are effective by default, and only the specified field is output, numerically. For example, those very same invocations, but with -G and -u respectively:

0 10 22
0
And with -rn:
root wheel 22
cicada
In this mode, unknown groups are written as numbers and a diagnostic is issued.

The set of supplementary groups is sorted, but always starts with the primary group.

An unknown user is not a fatal error — a diagnostic is issued and processing continues.

The mandatory access control context is omitted from the default format if unavailable or unset, and -Z is forbidden if it's unavailable; additionally, as it's a property of the process, not user, it's only available if no user was specified.

, --user
Write just the user ID.
, --group
Write just the primary group ID.
, --groups
Write just the secondary group IDs.
, --context
Write just the security context.

, --name
Write the specified field as a human-readable name, rather than numerical ID.
, --real
Use the real user or primary group ID of the process, rather than the effective one. No effect with users.
, --zero
Terminate each entry with a NUL character, rather than a single space or newline.

Ignored for compatibility with the illumos gate, whose default id requires it to write the supplementary groups. (But the version from AT&T research dated 2004 also just ignores it.)

if at least one user or group wasn't on file.

who(1), getegid(2), getgroups(2), getuid(2), getgrouplist(3), getgwgid(3), getpwuid(3), selinux(8)

Conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

Mandatory access control behaviour (-Z, context=) is compatible with the GNU system, though the latter deviates from the standard if a context is present; this is not considered a problem, as -ugG is recommended by the standard for noninteractive use, and scripts relying on the default CB-UNIX-style output already filter it.

-za and multiple users are extensions, also present on the GNU system.

The ordering of supplementary groups is not guaranteed.

whoami first appeared in 2BSD as whoami(UCB), noting:

It works even if you are su'd, while 'who am i' does not since it uses /etc/utmp.

groups appeared in 4.1cBSD as groups(1):

groups - show group memberships
with no arguments and present-day output, which also described the system's general group handling.

4.2BSD allows one user, as present-day.

id first appeared in CB-UNIX at or before version 2.1, using the familiar

uid=1000(user) gid=100(users) euid=0(root) egid=0(wheel)
format, with the notable exception of auxilliary groups, which originate from the BSD. CB-UNIX was, among others, the basis for AT&T System III UNIX, where it first saw light outside AT&T.

4.4BSD was the first to include id, quoting conformance with IEEE Std 1003.2 (“POSIX.2”), including the

100(users),0(wheel)
entry and -p extension, deprecating groups and whoami, replacing them with shell wrappers.

Since coreutils 8.31 (2019-03-10), the GNU system allows multiple users for id, too.

January 27, 2023 voreutils pre-v0.0.0-latest