/etc/passwd and /etc/shadow in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The /etc/passwd file stores basic information about user accounts, such as username, user ID, group ID, home directory, and default shell. It is readable by all users.

beginner

Why is the /etc/shadow file separated from /etc/passwd?

The /etc/shadow file stores encrypted user passwords and related security information. It is kept separate and readable only by the root user to protect password security.

intermediate

What kind of information does each line in /etc/passwd contain?

Each line has 7 fields separated by colons: username, password placeholder (usually 'x'), user ID (UID), group ID (GID), user info (comment), home directory, and default shell.

intermediate

Who can read the /etc/shadow file and why?

Only the root user or processes with root privileges can read /etc/shadow because it contains sensitive encrypted password data that must be protected from unauthorized access.

intermediate

How does Linux use /etc/passwd and /etc/shadow together during login?

Linux uses /etc/passwd to find user details and /etc/shadow to verify the encrypted password during login. This separation improves security by limiting password exposure.

Which file contains the encrypted passwords in a Linux system?

A /etc/shadow

B /etc/passwd

C /etc/group

D /etc/login

Encrypted passwords are stored in /etc/shadow to keep them secure and restrict access.

What does the 'x' in the password field of /etc/passwd indicate?

A User has no password

B Password is stored in /etc/shadow

C User account is locked

D Password is empty

The 'x' means the actual password is stored securely in /etc/shadow.

Who can read the /etc/passwd file by default?

A No one

B Only users in the admin group

C Only root user

D All users

/etc/passwd is world-readable because it contains non-sensitive user info.

What is the main reason for separating password data into /etc/shadow?

A To protect password security by restricting access

B To speed up login

C To allow users to edit their passwords

D To store user groups

Separating passwords into /etc/shadow restricts access to sensitive data.

Which field in /etc/passwd specifies the user's default shell?

A First field

B Third field

C Seventh field

D Sixth field

The seventh field in /etc/passwd is the user's default shell.

Explain the roles of /etc/passwd and /etc/shadow files in Linux user management.

Think about what info is public and what needs protection.

Describe the typical fields found in a line of /etc/passwd and their meanings.

There are seven fields separated by colons.

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