/etc/passwd in Linux is a file that stores the list of users on the system along with important information regarding these users. Identifying users uniquely is essential and necessary at the time of login. /etc/passwd is used by Linux system at the time of login. The file should be readable by all users but write permission should only belong to the root user.
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Understanding the /etc/passwd file
To view the contents of the file use cat command.
$ cat /etc/passwd
The output will look something like this. Here each line represents a user. Each user has a list of parameters separated by ‘:’
The seven fields are :
- Login name
- Encrypted Password
- UID number
- GID number
- GECOS
- Home directory
- Login shell
Let’s see what each of them mean :
- Login name: The unique name given to a user. It is case sensitive.
- Encrypted Password: This field is for the encrypted password. However, in most modern Linux systems, the password is stored in a shadow file named /etc/shadow. The ‘x’ in the second field represents that the password is stored in a shadow file.
- UID number: A unique ID assigned for each user. The system identifies a user by its UID number. The root user gets UID 0 which is reserved. UID 1-99 are reserved for predefined accounts. These are also called fake logins. In most Linux systems UID for new users starts from 1000.
- GID number: GID is used to identify the group user belongs to. Information about groups is stored in /etc/group file. In Linux, a user can belong in up to 16 groups.
- GECOS: stands for General Electric Comprehensive Operating Supervisor. This field allows the admin to store additional information about the user. This field can have multiple entries in a comma-delimited list. Information such as full name, phone number, the designation can be stored in this field.
- Home directory: This field contains the absolute path to the user’s home directory. When a user logs in, the system puts him/her directly in his/her home directory.
- Login shell: This field is for specifying the program that will run automatically whenever the user logs in. It’s typically a command interpreter (shell).
Create a user
We can use the useradd command to create a new user. Make sure to run the command with root access.
$ useradd Adam
This creates a user by the name Adam.
Modify GECOS field
To modify the GECOS field for Adam, use usermod command.
$ usermod -c "DevOp expert" Adam
Display entry for a particular user
From the etc/passwd file to get the entry for a single user use grep command.
$ grep Adam /etc/passwd
As we can see that GECOS field has been successfully updated for user Adam.
We break the values into their respective fields :
- Login name: Adam
- Encrypted Password: x (shadow file)
- UID number: 1001
- GID number: 1002
- GECOS: DevOp expert
- Home directory: /home/Adam
- Login shell: /bin/sh
Ending words
As a Linux administrator, it is important to know the /etc/passwd file at the back of your hand. Understand what each field means and keep a track of users on the system. To learn more about /etc/passwd file refer to this Wikipedia page for passwd command.
