Reverse id command, I want the User/Group Name from the uid or gid

Question

I know some uid and gids that I don't know who they belong to and they are not in /etc/passwd (could be LDAP or from another system). How do I get Id info from only uid or gid preferably without using ldapsearch or any LDAP commands outside of getent or id, or something easy. Linux 2.6 Kernel, Red Hat variants.

Solution 1

You can use the getent command, which will perform the lookup based on /etc/nsswitch.conf. This means that if ldap is configured on the system, getent will attempt to lookup the user in ldap. Here is an example:

$ getent passwd 33
www-data:x:33:33:www-data:/var/www:/bin/sh

Solution 2

To get the username associated with a UID, use the -n option of id:

$ id
uid=1000(myself) gid=1000(myself) groups=1000(myself), 0(wheel), 9(wsrc)
$ id -n -u 1000
myself

This would not work on GIDs though as id expects usernames or UIDs:

$ id -n -g 9
id: 9: No such user

You can query both the passwd database and the group database for UIDs and GIDs with getent:

$ getent passwd 1000
myself:*:1000:1000:Just me,,,:/home/myself:/bin/sh

$ getent group 9
wsrc:*:9:myself

The actual username or group name may be parsed out of that by piping the output through cut -d : -f 1.

---

id is a POSIX utility, while getent is not.