`id`

Return user and group IDs (POSIX)

Syntax:

id [username]

id -G [-n] [username]

id -g [-nr] [username]

id -u [-nr] [username]

Options:

-G: Write the real (and effective, if different) group ID only.
-g: Write the effective group ID only.
-u: Write the effective user ID only.
-n: In combination with -G, -g, or -u, write the ID as its name instead of as an unsigned integer.
-r: In combination with -g or -u, write the real ID instead of the effective ID.

Description:

The id utility writes the current real and/or effective user ID and group ID. When no options are specified, output is as follows:

uid=nnn(username) gid=nnn(groupname)

If the effective user ID is different from the real user ID, the effective user ID also appears:

... euid=nnn(effective_username)

Likewise, if the effective group ID is different from the real group ID, the effective group ID appears as well:

... egid=nnn(effective_groupname)

If no entry exists for an ID in the /etc/passwd or /etc/group files, the output lacks the (name) after the numerical value. No error is generated.

If a username is supplied as an operand, the effective uid and gid don't appear since there's no process associated with it — the information is simply looked up in the /etc/passwd and /etc/group files.

When options are specified, the requested data is written as an unsigned integer, unless option -n is specified, in which case the data is written as the corresponding user or group name. With option -G, the id utility might produce two lines of output (one value per line) if the real and effective IDs differ. In all other cases, id always produces one line of output.

Examples:

Write information on current IDs (real and effective):

$ id
uid=109(eric) gid=120(techies)

Write the effective group ID as a number:

$ id -g
120

Write the effective group ID as a name:

$ id -gn
techies

Files:

/etc/passwd: Password file; defines user IDs, home directories, etc.
/etc/group: Group file; defines the valid group IDs for the system, also lists user IDs who may change to each group.

Exit status:

0: Successful completion.
>0: An error occurred.