In Linux, a user is identified using a unique identifier called as user id (UID). Each user belongs to at least one group called the user’s primary group. Like user, a group is also identified with the help of a unique identifier called as group id (GID). UID and GID controls the accessibility of the files and folders in Linux.

Linux maintains the information of all its users in a world readable file /etc/passwd.

# ls -l /etc/passwd
-rw-r--r-- 1 root root 1589 Aug 30 16:45 /etc/passwd

This article explains the contents of /etc/passwd file in Linux.

Linux passwd File Entries

Following is typical entry from a /etc/passwd file,

# cat /etc/passwd | grep kam
kam:x:502:502:Kam Aggarwal:/home/kam:/bin/bash

Each entry in Linux passwd file corresponds to a user and contains seven fields as shown below.

Field 1: Username

• This contains the user’s login id. This is the username which is used for login.
• Username uniquely identifies a user on a Linux system. We can’t have multiple users with same username.

Field 2 : Password

• This field contains the encrypted password for the user.
• Value ‘x’ in this field indicates that the user password is stored in the /etc/shadow file.

# cat /etc/shadow | grep kam
kam:$1$8zowrENV$XL8nvq94dwqU4kTmDlBQk.:15595:0:99999:7:::

• The second field represents the encrypted password for the user ‘kam’

Field 3 : User-Id

• User-Id represents a unique number which is used by the Linux to identify the user.
• There is one to one mapping between the Username and User-Id.
• Every user must be assigned with a unique UID in a Linux system. This applies to all UIDs but ’0′.
• The superuser ‘root’ is assigned with a special UID ’0′. Any user having UID ’0′ has root privileges.

Field 4 : Group-Id

• Group-Id represents a unique number identifying the primary group id for the user.
• Linux maintains the group name and group id mapping in a separate world readable file /etc/group. Following is the entry for GID ’502′ in the group file.

# cat /etc/group | grep kam
kam:x:502:

Field 5 : User Information

• This field is used to store the general information about the user, like, user’s full name, contact information etc.
• This field can be left blank.

Field 6 : User’s Home Directory

• This field contains the location of user’s home directory.

# ls -l /home | grep kam
drwx------ 3 kam   kam   4096 Sep 12 18:20 kam

• User’s home directory contains all user specific configuration files.
• A user is provided full access to it’s home directory. Which means that the user is free to add, modify and delete any file or folder in its home directory.

Field 7 : User’s Shell

• Shell is the first program that a user encounters after logging into a Linux system. Linux comes with several shells.

# cat /etc/shells 
/bin/sh
/bin/bash
/sbin/nologin
/bin/tcsh
/bin/csh
/bin/ksh
/bin/zsh

• This field stores the user’s default shell.

Tagged as: /etc/passwd, Group Id, Linux passwd file, passwd, Unix passwd file, user database, User Home Directory, User Id