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linux : user management and permission

zakiarsyad profile image Zaki Arrozi Arsyad Updated on ・2 min read

This is one of the best part of linux, we can grant access for all services, directories, files, and even commands that can be executed for a specific user or group.


User management

  • Check who am i
$ whoami
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  • Check the user id
$ id USERNAME
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  • Check the group of user
$ groups USERNAME
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  • Check who is currently logged in
$ w
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  • Check the last logged in
$ last
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  • Change user
$ su
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  • Add a new user
$ useradd USERNAME
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  • Set password for a user
$ passwd PASSWORD
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  • Change user name
$ usermod -l USERNAME NEW_USERNAME
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  • Add a user to a group
$ usermod -a -G GROUPNAME USERNAME
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  • Change user primary group
$ usermod -g GROUPNAME USERNAME
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  • Remove user from a group
$ gpasswd -d USERNAME GROUPNAME
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  • Delete a user
$ userdel -r USERNAME
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  • Lock a user
$ passwd -l USERNAME
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  • Unlock a user
$ passwd -u USERNAME
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Group management

  • Add a new group
$ groupadd GROUPNAME
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  • Change group name
$ groupmod -n GROUPNAME NEW_GROUPNAME
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  • Delete a user
$ groupdel GROUPNAME
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Permission

After create a user or group, we can grant access for specific user or group.

Permission format :

rwxrwxrwx
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First 3 characters are the user permission, next 3 characters are the group permission, and the last 3 are other permission.

  • r is for read with value 4
  • w is for write with value 2
  • x is for execute with value 1
  • - is for no permission with value 0

We can also use the value of the characters to define the permission. Simply sum the values of the permissions for each role, and use only 3 numbers to define the permission.

# rwxrwxrwx
777

# rwxr-x-rx
755

# rwx------
700
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We can see the files or directories permission by run ls -l.

$ ls -la
drwxrwxrwx 1 root root  96 Jun 11 07:58 directory
-rwxr-xr-x 1 zaki zaki  96 Jun 11 07:58 file.txt
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The first column is the permission, the third column is the user, the fourth column is the group name.

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