id - print real and effective user and group IDs

id [OPTION]... [USER]...

Print user and group information for each specified USER, or (when USER omitted) for the current process.

-a

ignore, for compatibility with other versions

-Z, --context

print only the security context of the process

-g, --group

print only the effective group ID

-G, --groups

print all group IDs

-n, --name

print a name instead of a number, for -ugG

-r, --real

print the real ID instead of the effective ID, with -ugG

-u, --user

print only the effective user ID

-z, --zero

delimit entries with NUL characters, not whitespace;

not permitted in default format

--help

display this help and exit

--version

output version information and exit

Without any OPTION, print some useful set of identified information.

Full documentation <https://www.gnu.org/software/coreutils/id>
or available locally via: info '(coreutils) id invocation'