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Create, Delete and Modify Local Groups

Create, Delete and Modify Local User Accounts

  • /etc/passwd– This file contains the list of users on the system.
  • Format:
  • username:password:UID:PrimaryGUID:comment:homedir:defaultshell
  • Example: linuxacademy:x:539:100:linux academy act:/home/linuxacademy:/bin/bash
  • User IDs under 100 are reserved for system users
  • Normal user accounts have IDs between 500-1000
  • Can directly edit the passwd file to add, remove or modify users
  • Changing the shell section of the format to /bin/false will prevent the user shell login access to the system
  • /etc/passwd permissions must be readable by all, but /etc/shadow only should be readable by superusers
  • pwck : Verifies the integrity of the users and authentication information. Checks entries for /etc/passwdand /etc/shadow for proper format
  • /etc/skel : Skeleton file used when creating new users; allows you to set what files and settings need to be configured as default for every user added
  • useradd/adduser : Creates a user on the system. Some distributions such as Slackware use adduser instead of useradd . Note: Both username and password are case-sensitive.
  • -c – Comment; can be used as a comment, but is currently used for user’s full name
  • -d – Sets the user home directory; by default it is /home/user, but can be set to anything -e– Expire-date; sets the user’s expiration date; on this date the account password will “expire” and the user will no longer have access; format:YYYY-MM-DD
  • -p – Sets an encrypted password; the pre-encrypted password is added as-is to your /etc/passwdand /etc/shadow files; this is not the advised method of setting the password
  • -M– Does not create the home directory, even if the /etc/login.defs has the default set to yes --m– Creates a home directory at /home/username if it does not exist; files contained in the /etc/ skel directory will be copied into the new user’s home directory; useradd creates a user’s home directory by default
  • -G– Defines all the other groups that the member belongs to; separate each group by a comma
  • -g – Sets the default group for the user; this is the user’s group when the user first logs in -f– Defines the number of days after a password expires before an account is permanently disabled; a value of 0 immediately disables the account after password expiration, whereas -1 disables the entire feature
  • -k – Defines which directory skeleton to use when creating a user; allows you to have different default settings for different users; if the option is not set, it uses the /etc/skel format by default
  • /etc/default/useradd – Location of default settings for the useradd command

  • /etc/shadow – Contains the encrypted passwords for the user accounts on the system; this and the /etc/passwd file can be directly modified; the useradd and usermod commands are an interface to automatically modify these files.

  • Format: -username:password:days_until_change_allowed:days_before_change_required:days_of_warning_before_expiration:days_between_expiration_activation:expiration_date:special_flag

  • Flag names are self-explanatory; however, a value of -1 or 99999 will indicate that the feature is disabled for that user

  • chage – Changes and manages user expiry information; changes the number of days between required password change, and forces password changes for users after x number days

  • -E – Sets the date that the user’s password will expire
  • -I– Sets the number of days of inactivity after a password has expired before locking account
  • –m – Sets minimum number of days between password changes

  • -M – Sets maximum number of days which a password is valid

  • userdel– Deletes a user account and associated files.

  • -f– Forces the removal of the user account even if the user is still logged in; also deletes the user’s home directory and mail; not typically recommended — you can use kill to boot a user from your system and then remove the user account

  • -r – Removes the user’s home directory, files located in the user’s home directory and the user’s mail; does not remove files owned by the user outside of their home directory — use the find command to find files based off owner.

  • usermod– Modifies a user account

  • -d – Sets the user’s home directory to a new directory
  • -e– Sets date for when the user account will expire; use YYYY-MM-DD
  • -f– Number of days after a password expires until account is permanently disabled
  • -g – Group ID/name of the user’s new default login group
  • -G – List of extra groups the user is a member of
  • -l – Changes the login name of the user
  • -L – Locks the user’s account

#### Create, Delete and Modify Local Groups

  • groupdel – Deleted group; if any user has this group as their primary group, then the group cannot be removed until it is removed as the primary group
  • groupmod– Modify group name or group ID --g – Specify a new group ID; returns error if group already exists

  • -o – When used with -g , allows two groups to share the same group ID --n – Specifies a new group name

  • /etc/group – This file contains a list of groups and all the members associated with the groups

  • Example of /etc/group :
  • groupName:Password:GUID:userlist
  • groupadd – Adds a group to the system --g – Specifies a group idea; if not specified it will auto-select one for you
  • -r – Instructs groupadd to pick a group ID; less than 500 used for system groups --f– Forces group creation even if another group already exists