How the Linux id Command Displays User and Group Info

The id command in Linux is a fundamental utility used to query and display real and effective user identity (UID) and group identity (GID) information. This guide explains the core functionality of the id command, breaks down its default output, covers essential command-line options for filtering details, and describes how the operating system retrieves identity data from system databases.

Basic Usage and Default Output

Running the id command without any arguments outputs the identity information for the currently logged-in user. You can also inspect another account by appending the target username:

id
id username

When executed, the utility produces a single line of structured key-value pairs formatted as follows:

uid=1000(john) gid=1000(john) groups=1000(john),4(adm),24(cdrom),27(sudo)

The output contains three primary fields:

• uid (User ID): Displays the numeric user identifier followed by the associated username in parentheses. A UID of 0 indicates the root user, while standard non-system accounts typically begin at 1000.
• gid (Group ID): Shows the numeric primary group identifier and the primary group name in parentheses. When a user creates a file, this group is typically assigned as the file's owning group by default.
• groups (Supplementary Groups): Lists a comma-separated inventory of all secondary or supplementary groups the user belongs to, including both the numeric GID and the name for each group. Supplementary groups grant access to system permissions, such as administrative access (sudo or wheel).

Common Options and Flags

By default, id outputs all identity data simultaneously. Specific flags allow administrators and shell scripts to isolate individual values.

• Print Only User ID (-u):

  • id -u outputs only the numeric UID (e.g., 1000).
  • Combining it with -n (id -un) outputs the resolved username instead of the number.

• Print Only Primary Group ID (-g):

  • id -g outputs only the primary numeric GID.
  • Combining it with -n (id -gn) returns only the primary group name.

• Print All Supplementary Groups (-G):

  • id -G lists all numeric group IDs the user belongs to, separated by spaces.
  • Combining it with -n (id -Gn) lists all group names instead of numeric IDs.

• Real vs. Effective IDs (-r):

  • In situations involving setuid or setgid executables, a process may possess both a "real" ID (the user who ran the command) and an "effective" ID (the identity determining current access permissions). Passing the -r flag forces id to print the real ID instead of the effective one.

• Security Context (-Z):

  • On systems with SELinux or AppArmor enabled, id -Z outputs the current process security context (e.g., unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023).

How Linux Resolves Identity Information

The id command does not store identity records internally. Instead, it interfaces with the Linux Name Service Switch (NSS) configuration defined in /etc/nsswitch.conf.

For standard standalone systems, id parses local configuration files:

• /etc/passwd to resolve UIDs and usernames.
• /etc/group to resolve primary and supplementary GIDs and group names.

In enterprise or networked environments, the id utility transparently queries network directories—such as LDAP, Active Directory, or FreeIPA—via system daemons like SSSD or Winbind, providing identical output regardless of where user credentials originate.