Intro To 'id' Command In Linux

2023-05-22 - By Robert Elder

     I use the 'id' command to list all of the id values that represent users and groups on my system.  By default, the 'id' command will show the ID information associated with the current user and all of the groups that the current user belongs to:

id

uid=1000(robert) gid=1000(robert) groups=1000(robert),4(adm),20(dialout),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),131(lxd),132(sambashare),1001(tomcat)

Origin Of Group And User Ids

     These ID values can also be found in the file located at '/etc/passwd':

cat /etc/passwd

daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
robert:x:1000:1000:robert,,,:/home/robert:/bin/bash
tomcat:x:1001:1001::/opt/tomcat:/bin/false
...

     and the group 'id's can be found in the '/etc/group' file:

cat /etc/group

root:x:0:
daemon:x:1:
bin:x:2:
sys:x:3:
adm:x:4:syslog,robert
tty:x:5:syslog
disk:x:6:
lp:x:7:
mail:x:8:
news:x:9:
...

Show ID Information For A Specific User

     You can also specify a user to the 'id' command to see information for only that user:

id postgres

uid=126(postgres) gid=133(postgres) groups=133(postgres),113(ssl-cert)

Effective User ID Using '-u' Flag

     The '-u' flag restricts the output to show only the effective user ID:

id -u

1000

Effective Group ID Using '-g' Flag

     The '-g' flag shows the effective group ID:

id -g

1000

Group ID Memberships Using '-G' Flag

     and the '-G' flag shows the IDs of groups that the user belongs to:

id -G

1000 4 20 24 27 30 46 120 131 132 1001

Effective User ID Versus Real User ID

     If you make a copy of the 'id' command executable:

cp /usr/bin/id ./id-copy

     and then enable the setuid bit, while also changing the file ownership:

sudo chown postgres:postgres id-copy
sudo chmod u+s id-copy

     you'll be able to observe a difference between the effect of user ID and the real user ID:

./id-copy -u

126

./id-copy -ru

1000

     And that's why the 'id' command is my favourite Linux command.

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