/etc/passwd and /etc/shadow in Linux CLI - Commands & Configuration

Introduction

Linux stores user account information in special files. The /etc/passwd file holds basic user details, while /etc/shadow stores encrypted passwords securely. Understanding these files helps manage user access safely.

When you want to see a list of all user accounts on a Linux system.

When you need to check user information like home directory or default shell.

When you want to verify if a user has a password set or locked.

When you need to troubleshoot login issues related to user accounts.

When you want to securely manage or audit user passwords.

Commands

This command shows all user account information stored in /etc/passwd, including username, user ID, group ID, home directory, and default shell.

Terminal

cat /etc/passwd

Expected Output

root:x:0:0:root:/root:/bin/bash

bin:x:1:1:bin:/bin:/sbin/nologin

daemon:x:2:2:daemon:/sbin:/sbin/nologin

user1:x:1000:1000:User One:/home/user1:/bin/bash

This command shows the encrypted passwords and password-related settings for users. It requires sudo because it contains sensitive information.

Terminal

sudo cat /etc/shadow

Expected Output

root:$6$randomsalt$encryptedpasswordhash:18500:0:99999:7:::

user1:$6$othersalt$anotherencryptedhash:18500:0:99999:7:::

This command lists only the usernames from /etc/passwd by cutting the first field separated by colons. Useful to see all user names quickly.

Terminal

cut -d: -f1 /etc/passwd

Expected Output

root

bin

daemon

user1

This command finds the password entry for user1 in /etc/shadow to check if the user has a password set or if it is locked.

Terminal

sudo grep '^user1:' /etc/shadow

Expected Output

user1:$6$othersalt$anotherencryptedhash:18500:0:99999:7:::

Key Concept

If you remember nothing else, remember: /etc/passwd lists user info but does not store passwords, which are securely kept in /etc/shadow.

Common Mistakes

Trying to read /etc/shadow without sudo or root privileges.

The file is protected for security, so the command will fail with a permission denied error.

Use sudo to read /etc/shadow, for example: sudo cat /etc/shadow.

Editing /etc/passwd or /etc/shadow files directly without proper tools.

Manual edits can corrupt these files and lock users out of the system.

Use user management commands like useradd, passwd, or vipw to safely edit user info.

Assuming /etc/passwd contains passwords.

Passwords are not stored there; they are in /etc/shadow for security reasons.

Check /etc/shadow for password hashes, not /etc/passwd.

Summary

Use 'cat /etc/passwd' to view basic user account information.

Use 'sudo cat /etc/shadow' to view encrypted passwords and password settings securely.

Always use sudo when accessing /etc/shadow and avoid manual edits to these files.

Practice

1. What is the primary purpose of the /etc/passwd file in Linux?

easy

A. To store encrypted user passwords securely

B. To configure network settings

C. To store user account information accessible to all users

D. To log system errors

Solution

1. Step 1: Understand the role of /etc/passwd

   The /etc/passwd file contains user account details like username, user ID, group ID, home directory, and shell. It is readable by all users.

2. Step 2: Differentiate from /etc/shadow

   The encrypted passwords are stored separately in /etc/shadow for security, not in /etc/passwd.

3. Final Answer:

   To store user account information accessible to all users -> Option C

4. Quick Check:

   /etc/passwd = user info file [OK]

Hint: Remember: passwd = public user info, shadow = private passwords [OK]

Common Mistakes:

• Confusing /etc/passwd with /etc/shadow
• Thinking passwords are stored in /etc/passwd
• Assuming /etc/passwd stores system logs

2. Which of the following lines correctly shows the format of a typical /etc/shadow entry?

easy

A. username:x:1000:1000:User Name:/home/user:/bin/bash

B. username:$6$randomsalt$encryptedpassword:18000:0:99999:7:::

C. username:password:1000:1000:User Name:/home/user:/bin/bash

D. username:encryptedpassword:1000:1000:User Name:/home/user:/bin/bash

Solution

1. Step 1: Identify /etc/shadow format

   The /etc/shadow file stores encrypted passwords and related info. The password field looks like $6$randomsalt$encryptedpassword for SHA-512 encryption.

2. Step 2: Compare options

   The line username:$6$randomsalt$encryptedpassword:18000:0:99999:7::: matches the shadow file format with encrypted password (indicated by $6$) and password aging fields separated by colons. The other options resemble /etc/passwd format or have incorrect password fields.

3. Final Answer:

   username:$6$randomsalt$encryptedpassword:18000:0:99999:7::: -> Option B

4. Quick Check:

   /etc/shadow entries have encrypted passwords with $ signs [OK]

Hint: Look for $ signs and many colon-separated fields in shadow entries [OK]

Common Mistakes:

• Confusing /etc/passwd format with /etc/shadow
• Expecting plain text passwords in /etc/shadow
• Missing fields after encrypted password

3. Given the following command:

grep '^root:' /etc/passwd | cut -d':' -f1,3,7

What will be the output format?

medium

A. root:/bin/bash

B. root:x:0:0:root:/root:/bin/bash

C. root:0:0

D. root:0:/bin/bash

Solution

1. Step 1: Understand the command parts

   grep '^root:' /etc/passwd finds the line starting with 'root:'. cut -d':' -f1,3,7 extracts fields 1 (username), 3 (UID), and 7 (shell) separated by colons.

2. Step 2: Identify root line fields

   Typical root line: root:x:0:0:root:/root:/bin/bash. Fields: 1=root, 2=x, 3=0, 4=0, 5=root, 6=/root, 7=/bin/bash.

3. Step 3: Extract fields 1,3,7

   Fields 1=root, 3=0, 7=/bin/bash. Joined by colons: root:0:/bin/bash.

4. Final Answer:

   root:0:/bin/bash -> Option D

5. Quick Check:

   Fields 1,3,7 from /etc/passwd line = root:0:/bin/bash [OK]

Hint: Count fields carefully; cut -f1,3,7 picks username, UID, shell [OK]

Common Mistakes:

• Mixing up field numbers in cut command
• Expecting full /etc/passwd line output
• Confusing shadow file format with passwd

4. You run this command to check password expiry info:

sudo chage -l username

But it shows an error: chage: user 'username' does not exist. What is the most likely cause?

medium

A. The username is missing in /etc/passwd

B. The /etc/shadow file is empty

C. The chage command is not installed

D. The user has no password set

Solution

1. Step 1: Understand chage user check

   The chage command checks user password expiry info by looking up the username in /etc/passwd.

2. Step 2: Analyze error cause

   Error 'user does not exist' means the username is not found in /etc/passwd. If the user was missing there, chage cannot proceed.

3. Final Answer:

   The username is missing in /etc/passwd -> Option A

4. Quick Check:

   User must exist in /etc/passwd for chage to work [OK]

Hint: Check /etc/passwd for username before using chage [OK]

Common Mistakes:

• Assuming /etc/shadow emptiness causes this error
• Thinking chage command missing causes user error
• Believing no password set triggers 'user does not exist'

5. You want to write a script that lists all users who have a valid login shell (not /usr/sbin/nologin or /bin/false) by reading /etc/passwd. Which command snippet correctly filters these users?

hard

A. awk -F':' '$7 != "/usr/sbin/nologin" && $7 != "/bin/false" {print $1}' /etc/passwd

B. grep -v '/usr/sbin/nologin' /etc/shadow | cut -d':' -f1

C. cat /etc/passwd | grep '/bin/bash' | awk '{print $1}'

D. sed '/nologin/d' /etc/passwd | cut -d':' -f7

Solution

1. Step 1: Identify shell field in /etc/passwd

   The 7th field in /etc/passwd is the user's login shell. We want to exclude /usr/sbin/nologin and /bin/false.

2. Step 2: Analyze each option

   awk -F':' '$7 != "/usr/sbin/nologin" && $7 != "/bin/false" {print $1}' /etc/passwd uses awk with field separator ':' and filters out unwanted shells, printing usernames (field 1). grep -v '/usr/sbin/nologin' /etc/shadow | cut -d':' -f1 wrongly reads /etc/shadow and uses grep -v incorrectly. cat /etc/passwd | grep '/bin/bash' | awk '{print $1}' filters only /bin/bash but misses other valid shells. sed '/nologin/d' /etc/passwd | cut -d':' -f7 deletes lines with 'nologin' but then prints shell field, not usernames.

3. Final Answer:

   awk -F':' '$7 != "/usr/sbin/nologin" && $7 != "/bin/false" {print $1}' /etc/passwd -> Option A

4. Quick Check:

   Use awk on /etc/passwd to filter shell field and print usernames [OK]

Hint: Use awk with FS=':' and check field 7 for valid shells [OK]

Common Mistakes:

• Using /etc/shadow instead of /etc/passwd for shell info
• Printing wrong fields (shell instead of username)
• Using grep without proper field filtering