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Linux:User

lastmod 2025-12-04created 2015-01-1629 edits

Notes on how to manage users in Linux.

Communication between users

Users can communicate with each other using the following commands.

  • write: Sends a message to another logged-in user.
  • wall
  • mesg
  • mail
  • biff
  • who - Information about currently logged-in users.
  • w - Detailed information about currently logged-in users.

Creating users

  • adduser: Handles the user's home directory and user group password interactively. (Creates default account information along with it)
  • useradd: The user environment must be specified with options when running the command. (Only creates the account; other information must be created and configured manually)

Creating user passwords

  • passwd: Manages your own password.

Deleting users

  • deluser: Deletes interactively
  • userdel: Must be handled with options when running the command

Changing user information

  • chfn: Changes user information interactively
  • chsh: Changes the user's shell interactively
  • usermod: Non-interactive changes

Checking users/groups

  • cat /etc/passwd: Shows the list of users.
  • cat /etc/group:Shows the list of groups.

Modifying a user's groups

  • gpasswd -a [USERNAME] [GROUPNAME]: Adds a specific user to a group.
  • gpasswd -d [USERNAME] [GROUPNAME]: Removes a specific user from a group.

Creating groups

  • addgroup: Creates interactively
  • groupadd: Non-interactive, works with various options

Changing groups

Deleting groups

  • delgroup: Deletes a group interactively
  • groupdel: Deletes a group non-interactively

How to change a username

To change a username, you must terminate all processes running as that user. Then log in as an administrator. 

sudo usermod -l newUsername oldUsername

Some linux distributions, such as ubuntu, use a group name identical to the username. Change this group name too: 

sudo groupmod -n newUsername oldUsername

Finally, also change the home directory path that matches the username. 

sudo usermod -d /home/newHomeDir -m newUsername

Example Script

Checks whether a user exists and adds it if not. 

function install_user
{
    local username=$1
    local homedir=$2

    local exists_user
    exists_user=$(awk -F: '{print $1}' /etc/passwd | grep "$username")

    if [[ -n $exists_user ]]; then
        echo "Exists '$username' user"
    else
        echo "Create '$username' user"
        useradd -l -M -N -d "$homedir" -s /bin/bash "$username"
    fi
}

Checks whether a group exists and adds it if not. 

function install_group
{
    local username=$1
    local groupname=$1

    local exists_group
    exists_group=$(awk -F: '{print $1}' /etc/group | grep "$groupname")

    if [[ -n $exists_group ]]; then
        echo "Exists '$groupname' group"
    else
        echo "Create '$groupname' group"
        groupadd "$groupname"
        gpasswd -a "$username" "$groupname"
        usermod -aG docker "$username"
    fi
}

Linux password & shadow file formats

Shadow 

root:$6$5H0QpwprRiJQR19Y$bXGOh7dIfOWpUb/Tuqr7yQVCqL3UkrJns9.7msfvMg4ZO/PsFC5Tbt32PXAw9qRFEBs1254aLimFeNM8YsYOv. : 16431 : 0 : 99999 : 7 : : :
|-1–|———————————–2——————————————————————————————————————————————-|—3—|-4-|—5–|-6-|-7-|-8-|
  1. Login Name : User account. The fields are separated by : (colon), and each field between colons is structured as follows.
  2. Encrypted : The encrypted password value
  3. Last Changed : The date the password was last changed, counted in days since January 1, 1970
  4. Minimum : Minimum number of days the password must be used before it can be changed
  5. Maximum : Maximum number of days the password can be used before it must be changed
  6. Warn : Number of days before password expiration to show a warning message
  7. Inactive : Number of days until login access is blocked
  8. Expire : Number of days login use is prohibited (month/day/year)
  9. Reserved : Not used

If you look closely at the front part of field 2, it is separated by $. These separators hold the following values. 

[$Hashid $Salt $Hash vlaue]
  • Hashid: Shows which scheme was used for hashing. The id value differs depending on the scheme used, and the most commonly used ones are $1, $5, and $6.
  • Salt: A hash is a one-way function. In other words, A->B is possible but B->A is not.
  • Hash value: The hash value is the result of running the hash function with the hashing method given by the hashid and the salt value.

Passwd 

root:x:0:0:root:/root:/bin/bash
  1. Username
  2. Password (encrypted in the /etc/shadow file)
  3. User account uid
  4. User account gid
  5. User account name (information)
  6. User account home directory
  7. User account login shell

Group

See also

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