The user ID
(or UID) is a unique number that differentiates a user from any other user. The
group ID (GID) determines the primary category of user that the user is. Groups
may be things like ‘staff’, ‘student’, ‘engineering’, ‘research’, or any other
descriptive term that can be used to partition users in a meaningful way.
/etc/passwd
One of the most critical of all files in Linux is /etc/passwd. All user accounts are stored here, with identifying information like so:
One of the most critical of all files in Linux is /etc/passwd. All user accounts are stored here, with identifying information like so:
username:password:userID:groupID:realname:homedirectory:shell
For example, a user “mybasicknowledge” who was added as user 300 and
group 200 has an entry that might look like this:
mybasicknowledge:x:300:200:Fred Smith:/home/fred:/bin/bash
To find a
specific user’s UID, goto the terminal and type the following (replace the
username with the actual username) -
id -u username
or
id
To find a
user’s GID, at the Unix prompt -
id -g username
If you wish
to find out all the groups a user belongs to, type in -
id -G username
If you wish
to see the UID and all groups associated with a user, enter id without any
options -
id username
Syntax
id [-a] [-G] [-g] [-u]
[user]
-a Reports user name, use ID and all the
groups to which the user belongs.
-G Output all different group IDs (effective,
real and supplementary) only, using the format "%u\n". If there is
more than one distinct group affiliation, output each such affiliation, using
the format " %u", before the new line character is output.
-g Output only the effective group ID, using
the format "%u\n".
-n Output the name in the format
"%s" instead of the numeric ID using the format "%u".
-r Output the real ID instead of the
effective ID.
-u Output only the effective user ID, using
the format "%u\n".
user The user (login) name for which information
is to be written.
