utility displays the user and group names and numeric IDs, of the calling process, to the standard output. If the real and effective IDs are different, both are displayed, otherwise only the real ID is displayed.
If a user (login name or user ID) is specified, the user and group IDs of that user are displayed. In this case, the real and effective IDs are assumed to be the same.
The options are as follows:
Display the different group IDs (effective, real and supplementary) as white-space separated numbers, in no particular order.
Display the effective group ID as a number.
Display the name of the user or group ID for the -G, -g and -u options instead of the number. If any of the ID numbers cannot be mapped into names, the number will be displayed as usual.
Make the output human-readable. If the user name returned by getlogin(2) is different from the login name referenced by the user ID, the name returned by getlogin(2) is displayed, preceded by the keyword ``login''. The user ID as a name is displayed, preceded by the keyword ``uid''. If the effective user ID is different from the real user ID, the real user ID is displayed as a name, preceded by the keyword ``euid''. If the effective group ID is different from the real group ID, the real group ID is displayed as a name, preceded by the keyword ``rgid''. The list of groups to which the user belongs is then displayed as names, preceded by the keyword ``groups''. Each display is on a separate line.
Display the real ID for the -g and -u options instead of the effective ID.
Display the effective user ID as a number.
The id utility exits 0 on success, and >0 if an error occurs.
The historic groups(1)
command is equivalent to “id -Gn
The historic whoami(1) command is equivalent to “id -un”.
The id command first appeared in 4.4BSD.