Group is a way to organize users in Linux where each user need to be assigned a primary group while alternatively can also be a member of other groups.

System permissions and privileges can be assigned to both user and group where members of a group will inherit the privileges of the group that they are a member of.

Local users and groups can be configured by manually by editing /etc/passwd and /etc/group files but a misconfiguration could easily cause the whole system inaccessible.

It is therefore recommended to add users to groups using dedicated tools such as usermod that would simplify the process while minimizing the risk.

Steps to add user to group in Linux:

  1. Launch terminal.
  2. Create a new group if required.
    $ sudo groupadd finance
    [sudo] password for user:
  3. Get the list of users for your system.
    $ getent passwd
    root:x:0:0:root:/root:/bin/bash
    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
    sync:x:4:65534:sync:/bin:/bin/sync
    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
    uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
    proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
    www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
    backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
    list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
    irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
    gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
    nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
    systemd-timesync:x:100:101:systemd Time Synchronization,,,:/run/systemd:/usr/sbin/nologin
    systemd-network:x:101:103:systemd Network Management,,,:/run/systemd:/usr/sbin/nologin
    systemd-resolve:x:102:104:systemd Resolver,,,:/run/systemd:/usr/sbin/nologin
    messagebus:x:103:106::/nonexistent:/usr/sbin/nologin
    syslog:x:104:110::/home/syslog:/usr/sbin/nologin
    _apt:x:105:65534::/nonexistent:/usr/sbin/nologin
    tss:x:106:111:TPM software stack,,,:/var/lib/tpm:/bin/false
    uuidd:x:107:114::/run/uuidd:/usr/sbin/nologin
    tcpdump:x:108:115::/nonexistent:/usr/sbin/nologin
    avahi-autoipd:x:109:117:Avahi autoip daemon,,,:/var/lib/avahi-autoipd:/usr/sbin/nologin
    usbmux:x:110:46:usbmux daemon,,,:/var/lib/usbmux:/usr/sbin/nologin
    rtkit:x:111:118:RealtimeKit,,,:/proc:/usr/sbin/nologin
    dnsmasq:x:112:65534:dnsmasq,,,:/var/lib/misc:/usr/sbin/nologin
    avahi:x:113:120:Avahi mDNS daemon,,,:/run/avahi-daemon:/usr/sbin/nologin
    cups-pk-helper:x:114:121:user for cups-pk-helper service,,,:/home/cups-pk-helper:/usr/sbin/nologin
    speech-dispatcher:x:115:29:Speech Dispatcher,,,:/run/speech-dispatcher:/bin/false
    kernoops:x:116:65534:Kernel Oops Tracking Daemon,,,:/:/usr/sbin/nologin
    saned:x:117:123::/var/lib/saned:/usr/sbin/nologin
    nm-openvpn:x:118:124:NetworkManager OpenVPN,,,:/var/lib/openvpn/chroot:/usr/sbin/nologin
    whoopsie:x:119:125::/nonexistent:/bin/false
    colord:x:120:126:colord colour management daemon,,,:/var/lib/colord:/usr/sbin/nologin
    sssd:x:121:127:SSSD system user,,,:/var/lib/sss:/usr/sbin/nologin
    geoclue:x:122:128::/var/lib/geoclue:/usr/sbin/nologin
    pulse:x:123:129:PulseAudio daemon,,,:/var/run/pulse:/usr/sbin/nologin
    hplip:x:124:7:HPLIP system user,,,:/run/hplip:/bin/false
    gnome-initial-setup:x:125:65534::/run/gnome-initial-setup/:/bin/false
    gdm:x:126:131:Gnome Display Manager:/var/lib/gdm3:/bin/false
    user:x:1000:1000:user,,,:/home/user:/bin/bash
    systemd-coredump:x:999:999:systemd Core Dumper:/:/usr/sbin/nologin
    shakir:x:1001:1002:shakir,,,:/home/shakir:/bin/bash
  4. Get the list of groups for your system.
    $ getent group 
    root:x:0:
    daemon:x:1:
    bin:x:2:
    sys:x:3:
    adm:x:4:syslog,user
    tty:x:5:
    disk:x:6:
    lp:x:7:
    mail:x:8:
    news:x:9:
    uucp:x:10:
    man:x:12:
    proxy:x:13:
    kmem:x:15:
    dialout:x:20:
    fax:x:21:
    voice:x:22:
    cdrom:x:24:user
    floppy:x:25:
    tape:x:26:
    sudo:x:27:user
    audio:x:29:pulse
    dip:x:30:user
    www-data:x:33:
    backup:x:34:
    operator:x:37:
    list:x:38:
    irc:x:39:
    src:x:40:
    gnats:x:41:
    shadow:x:42:
    utmp:x:43:
    video:x:44:
    sasl:x:45:
    plugdev:x:46:user
    staff:x:50:
    games:x:60:
    users:x:100:
    nogroup:x:65534:
    systemd-timesync:x:101:
    systemd-journal:x:102:
    systemd-network:x:103:
    systemd-resolve:x:104:
    crontab:x:105:
    messagebus:x:106:
    input:x:107:
    kvm:x:108:
    render:x:109:
    syslog:x:110:
    tss:x:111:
    bluetooth:x:112:
    ssl-cert:x:113:
    uuidd:x:114:
    tcpdump:x:115:
    ssh:x:116:
    avahi-autoipd:x:117:
    rtkit:x:118:
    netdev:x:119:
    avahi:x:120:
    lpadmin:x:121:user
    scanner:x:122:saned
    saned:x:123:
    nm-openvpn:x:124:
    whoopsie:x:125:
    colord:x:126:
    sssd:x:127:
    geoclue:x:128:
    pulse:x:129:
    pulse-access:x:130:
    gdm:x:131:
    lxd:x:132:user
    user:x:1000:
    sambashare:x:133:user
    systemd-coredump:x:999:
    finance:x:1001:
    shakir:x:1002:
  5. Change primary group for a user.
    $ sudo usermod -g finance shakir

  6. Add existing user secondary groups.
    $ sudo usermod -a -G sudo shakir
  7. Add existing user to multiple groups.
    $ sudo usermod -a -G sudo,user shakir
  8. View the groups that a user is assigned to.
    $ groups shakir
    shakir : finance sudo user
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