User and permissions

Intended for Centos or Redhat.

Linux Users and groups

A user is a number uid. Information on the user is stored in /etc/passwd

#to add a user
useradd <username>

Use id to identify the user whom you are using right now.

A group is a number gid by default the group name is the userโ€™s name, the gid will be the next available. Information for groups are stored in /etc/group

#to add a group
groupadd <groupname>

uid and gid are two completely different number that both starts at 1000 (up to 2^16 for backward compatibility) and get incremented for each creation of group and user. They may not be in sync. Rootโ€™s uid is 0. If you need an uid for a system you would create it between 201 to 999. From 1 to 200 it is assigned by the operating system by default.



Password files are located in /etc/shadow and are only accessible by root. Type passwd to change the password.

There are three pieces of information stored in a modern password hash: $1$gCjLa2/Z$6Pu0EK0AzfCjxjv2hoLOB/

  1. 1: The hashing algorithm. The number 1 indicates an MD5 hash. The number 6 appears when a SHA-512 hash is used.
  2. gCjLa2/Z: The salt used to encrypt the hash. This is originally chosen at random. The salt, and the unencrypted password are combined and encrypted to create the encrypted password hash. The use of a salt prevents two users with the same password from having identical entries in the /etc/shadow file.
  3. 6Pu0EK0AzfCjxjv2hoLOB/: The encrypted hash

Password aging

To see the configuration of the password, account and group you can:

view /etc/login.defs

There you can edit the expiration date of newly created user password or change the uid and gid range.

To get the date in 90 days you can use:

 date -d "+90 days"


To see the log of the actions on the system you can go to /var/log then you can check secure file to see the last sudo commands that has been used. If you are root or if you use su or sudo -i the actions are not logged in.


The rights can be seen using ls -l:

 #The rights of a file
 -rwxrw-r-- <user> <group> <file>

Here we have the first:

  • - because it is a file (d for a directory).
  • The first set rwx of rights for the user
  • The second set rw- of rights for the group
  • The third set r-- of rights for all other users.

When there is a - in the set of rights, it means there are no rights set.

The rights on file are executed such as when requesting an action on it:

  • Is the file is owned by the user?
    • Yes -> Check the owner permission
  • Else: Is the user part of the group owning the file?
    • Yes -> Check the group permission
  • Else: Check the last set of permission (for other users)

To delete a file you would need to have write access to the directory it is situated in. To see the rights on the directory, you can do ls -ld.