Linux user Command

useradd
1. Purpose
  The useradd command is used to create user accounts and create the user’s home directory, with superuser privileges required.
2. Format
  useradd [-d home] [-s shell] [-c comment] [-m [-k template]] [-f inactive] [-e expire] [-p passwd] [-r] name
3. Main Parameters
  -c: Adds a comment text, which is saved in the comment field of passwd.
  -d: Specifies the home directory the user logs into.
  -D: Changes default values.
  -e: Specifies the account expiration date; default means permanently valid.
  -f: Specifies the number of days after password expiration before the account is disabled.
  -g: Specifies the primary group the user belongs to.
  -G: Specifies supplementary groups the user belongs to.
  -m  [-k] : Forces creation of the user home directory. (The user home directory will be created if it does not exist. If the -k option is used, files contained in skeleton_dir will be copied to the home directory, without copying files from /etc/skel. If the -k option is not used, files under /etc/skel will be copied to the user home directory. The -k option only applies when used together with the -m option. By default, the directory is not created and no files are copied.)
  -M: Do not automatically create the user’s login directory.
  -n: Cancel creating a group named after the username.
  -r: Create a system account.
  -s: Specifies the shell to be used after the user logs in.
  -u: Specifies the user ID number.
4. Description
  useradd can be used to create user accounts; it is identical to the adduser command. After the account is created, use passwd to set the account password. Accounts created using the useradd command are actually stored in the /etc/passwd text file.

usermod

1. Purpose

  Modify user accounts.

2. Format
  usermod [-c comment] [-d home_dir [-m]] [-e expire_date] [-f inactive_time] [-g initial_group] [-G group [,…]] [-l login_name][-p passwd] [-s shell] [-u uid [-o]] [-L|-U] login

3. Main Parameters
  -c: Modifies the comment text of the user account.
  -d: Modifies the directory used upon login.
  -e: Modifies the account expiration date.
  -f: Modifies the number of days after password expiration before the account is disabled.
  -g: Modifies the primary group the user belongs to.
  -G: Modifies supplementary groups the user belongs to.
  -l: Modifies the user account name.
  -L: Locks the user password, rendering it invalid.
  -s: Modifies the shell used after login.
  -u: Modifies the user ID.
  -U: Unlocks the password lock.

Additional Notes:

  usermod can be used to modify various settings of user accounts.

passwd

1. Purpose

  The passwd command is used to change account login passwords; it can be used by all users.

2. Format
  passwd [-k] [-l] [-u [-f]] [-d] [-n mindays] [-x maxdays] [-w warndays] [-i inactivedays] [-S] [–stdin] [username]
3. Main Parameters
  -k: Sets that the password can only be updated after it has expired.

  -l: Locks the named account; only users with superuser privileges can use this.
  -u [-f]: Unlocks the account lock status; [-f forces execution]. Only users with superuser privileges can use this.
  -d: Deletes the user’s password; only users with superuser privileges can use this.
  -n: Minimum password usage time (in days); only users with superuser privileges can use this.
  -x: Maximum password usage time (in days); only users with superuser privileges can use this.
  -w: Number of days in advance to warn the user before password expiration; only users with superuser privileges can use this.
  -i: User expiration date. This parameter specifies the number of days since the user is deactivated (days since January 1, 1970). Only users with superuser privileges can use this.
  -S: Checks the password authentication type of the specified user; only users with superuser privileges can use this.