groups Command Cheat Sheet

The groups command prints the names of the primary and supplementary groups for each given username, or the current process if no names are given.

Synopsis

groups [OPTION]... [USERNAME]...

Basic Usage

Check Your Own Groups

groups

Output:

faruk sudo docker adm cdrom

Note: The first group listed is usually the effective primary group.

Check Another User

groups alice

Check Multiple Users

groups alice bob root

Output:

alice : alice sudo
bob : bob devs
root : root

How it Works

groups reads directly from /etc/group and potentially other databases (like LDAP/NIS via nsswitch).

Primary Group: Defined in /etc/passwd (the 4th field).
Supplementary Groups: Defined in /etc/group (users listed in the last field).

Comparison: groups vs id

groups is simple and human-readable. id provides numeric IDs and more detail.

Command	Output Style	Detail Level
`groups`	Names only	Low (Simple)
`id -Gn`	Names only	Low (Same as groups)
`id`	`uid=1000(u) gid=1000(g) groups=1000(g),27(sudo)`	High (IDs + Names)

Scripting & Automation

Check if User is in a Group

Since groups output is just text, use grep to check membership.

if groups $USER | grep -q "\bdocker\b"; then
    echo "User is in docker group"
else
    echo "User is NOT in docker group"
fi

Note: \b (word boundary) is important to avoid matching "dockers" or "dockeruser".

List All Groups in the System

groups only shows groups with members (or for specific users). To see every defined group:

cut -d: -f1 /etc/group | sort

or

getent group | cut -d: -f1

Common Issues

"I added myself to a group but `groups` doesn't show it!"

Group membership is applied at login time. If you add a user to a group (e.g., usermod -aG docker $USER), the change is NOT reflected in currently running shells.

Solutions: 1. Log out and log back in. (Best) 2. Use newgrp:

newgrp docker

(Starts a new subshell with the group active). 3. Run commands with sg:

sg docker -c "docker ps"

Exit Status

Code	Meaning
0	Success
1	Username not found

Notes

Obsolete systems might limit the number of groups a user can be in (historically 16 or 32). Modern Linux kernels support 65536+.
If groups shows cannot find name for group ID ..., it means you belong to a GID that has no corresponding entry in /etc/group.