Explain role of /etc/passwd, /etc/shadow, /etc/groups files with respect to user administration.

Mumbai University > Information Technology > Sem 5 > Open Source Technology

Marks: 10M

Year: Dec 2015

1 Answer

Role of /etc/passwd file:

/etc/passwd file stores essential information, which is required during login i.e. user account information. /etc/passwd is a text file, that contains a list of the system’s accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser/root account.

Understanding fields in /etc/passwd

The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows. Generally, passwd file entry looks as follows:

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  1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
  2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
  3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
  4. Group ID (GID): The primary group ID (stored in /etc/group file)
  5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
  6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
  7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

Role of /etc/shadow file:

The /etc/shadow file stores actual password in encrypted format for user’s account with additional properties related to user password i.e. it stores secure user account information. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file. Generally, shadow file entry looks as follows:

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  1. Username : It is your login name.
  2. Password : It is your encrypted password. The password should be minimum 6-8 characters long including special characters/digits and more.
  3. Last password change (lastchanged) : Days since Jan 1, 1970 that password was last changed
  4. Minimum : The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password
  5. Maximum : The maximum number of days the password is valid (after that user is forced to change his/her password)
  6. Warn : The number of days before password is to expire that user is warned that his/her password must be changed
  7. Inactive : The number of days after password expires that account is disabled

Role of /etc/group file:

/etc/group is a text file which defines the groups to which users belong under Linux and UNIX operating system. Under Unix / Linux multiple users can be categorized into groups. Unix file system permissions are organized into three classes, user, group, and others. The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals. This method, amongst others, also enables the Superuser to delegate some administrative tasks to normal users.

/etc/group file

There is one entry per line, and each line has the following format (all fields are separated by a colon (:):

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  1. group_name: It is the name of group. If you run ls -l command, you will see this name printed in the group field.
  2. Password: Generally password is not used, hence it is empty/blank. It can store encrypted password. This is useful to implement privileged groups.
  3. Group ID (GID): Each user must be assigned a group ID. You can see this number in your /etc/passwd file.
  4. Group List: It is a list of user names of users who are members of the group. The user names, must be separated by commas.
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