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Linux User Management

User management operation needs to be done by root user or super user.

Create User

adduser [username]

You get a new username applying system default configuration, to list\change this configuration use option -D

adduser -D
#list default configuration
adduser -D --expiredate=2020-12-22
#change Default EXPIRE date to Dec 22 2020
#alternativly you can edit defaults in file /etc/default/useradd

• To set password upon creation use option -p
• User by default is added on usergroup ‘named after him’, to prevent this use option -N
• To add user into primary group use option -g
• To add user into secondary group(s) use option -G
• Read more adduser -h

Delete User

userdel [username]
#delete user
userdel -r [username]
#delete user with home directory and mail spool

Add/Remove User Into Group

To list groups assigned to user use command groups

groups [username]

To add existing user to group, use usermod command

usermod -g [group] [username]
#adding user to group as new primary
usermod -G [new,list,of,groups] [username]
#new list of supplementary groups
usermod -a [more,groups] [username]
#appending user to supplementary groups

To remove user from group, use gpasswd

gpasswd -d [username] [group]

Some distros (like ubuntu) uses adduser to add existing user to an existing group, deluser to remove user from group and addgroup to create new user group.

adduser [username] [group]
deluser [username] [group]

Delete User Group

Using groupdel

groupdel [group]
#delete group

you may force it with -f option so it will be deleted even if its primary of a user

Change Passwords

passwd [username]
#change password of specific user
passwd
#change my password

Password Expiration and Aging

To list user account aging info use -l option with chage

chage -l [username]

for more about it use chage --help

File permissions and ownership

Linux grants ownership by default to the creator, this can be changed by command chown as the following:

chown [option] [owner] [: [group]] FILE(s)
#change the owner and/or group of each FILE to OWNER and/or GROUP.
chown [OPTION]... --reference=RFILE FILE(s)
#change the owner and group of each FILE to those of RFILE.

Command chmod to modify permissions, that can be noted with symbols: [rwx] or octals [0-7]. Permissions are given associated with:

• User (file owner User)
• Group (file owner Group)
• Others so you set different set of permissions for each, example:

  chmod u=rwx,g=rx,o=r myfile
  # file owner user can read,write and execute myfile
  # file owner group members can read and execute
  # others can only read
  

  The following is equivalent using octal notaion:

  chmod 754 myfile
  

  Where each digit is assigning permissions for user,group and others, each digit is sum of permissions supposed to be assigned where:

• Read = 4
• Write = 2
• Execute = 1
• No permission = 0 There for:
• rwx = 4 + 2 + 1 = 7
• rx = 4 + 1 = 5

more about chmod

to view files with owners and permissions use option -l with ls command.