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Users Management

adduser

  • add a new user, automatically creating a

    • home directory
    • choosing login shell
    • creating a password

  • id user - show all the groups user is in and its uid and pid

  • su - user - switch user to the user, usually used for users with login disabled

useradd

  • usually used to create account for services like mysql systemd
  • adds new user but with no home directory
    • useradd -m username - adds new user and also creates a home directory

/etc/passwd file

  • contains information about all the users on system
  • format - user_name:password:UID:GID:other_information:home_directory:login_shell
    • sample - totoro:x:1000:1001:totoro:/home/totoro:/usr/bin/zsh
      • user_name - totoro
      • password - x means encrypted
      • UID - 1001
      • GID - 1001
      • other_information - totoro
      • home_directory - /home/totoro
      • login_shell - /usr/bin/zsh
  • other_information - this usually contains the description about the user. On older machine it contained contact info, room number, which we are asked when we use adduser
  • login_shell - for user account which is unusable it is set to /usr/sbin/nologin or /bin/false
  • password - x in password indicates encrypted password, which is present in the shadow file

/etc/shadow file

  • contains the information about the password used by the users
  • format - user:$encryption$salt$hash:lastPasswordChange:min:max:warning:disable:expire:reserved_field
    • user - name of the user
    • password - compromise of $encryption$salt$hash
      • * or ! indicates that we cannot login in the system with that user
      • encryption - type of encryption used
        • $1 - md5
        • $2 blowfish
        • $2a eksBlowfish
        • $5 sha-256
        • $6 SHA-512
      • $salt - salt value added while encryption
      • $hash - the encrypted password
    • lastPasswordChange - date in unix format (no of days since Jan 1, 1970) of last password change
    • min - min number of days before you can change your password, 0 means can be changed now
    • max - max number of days till which your password is valid, 9999 means will never expire
    • warning - no of days before expiration to show the password expiration warning
    • disable - no of days after expiration that the account will be disabled in, nothing means never disable
    • expire - date when account will expire
    • reserved_field
    • sample
      • totoro:$6$g3NynZLzI5A.7UcE$2vSxbUvSasdfsG4:18898:0:99999:7:::
        • totoro - user
        • $6 - indicates sha512 encryption
        • $g3NynZLzI5A.7UcE - salt
        • $2vSxbUvSasdfsG4 - hash, will be longer, here used as example
        • 18898 - last date when account was changed
        • 0 - password can be changed now
        • 9999 - password will never expire
        • 7 - expiration warning will appear 7 days before expiration

passwd

  • change password for the current user
  • passwd username - change password for the username

chage

  • change the account expiration date, and other expiration dates mentioned in the /etc/shadow file

getent

  • get entries from Name Service Switch libraries
  • config file in /etc/nsswitch.conf

usermod

  • modify the entires of /etc/passwd file
  • change the home directory, login shell, UID, etc.

finger

  • show the description of the user form the /etc/passwd file

chfn

  • change finger
  • changes the description of the user form /etc/passwd file

how to force user to change password when the login next time

  • passwd --expire [uid]
  • sudo chage --lastday 1970-01-01 [uid]
  • sudo chage --lastday 0 [uid]

[uid] user will asked to change their password next time they login.

Lock a user account

  • usermod -L [uid] - lock, place a ! in the password field of the uid in /etc/passwd file
  • usermod -L [uid] - unlock
  • passwd -l [uid]
  • chage -E0 [uid]

last

  • prints the last time the user logged in the system

deluser

  • deluser user - delete the user
  • deluser --remove-home user - delete user and remove the home directory