🐧 Top 20 Linux Interview Questions and Answers for Freshers & Experienced (2025)

✅ 1. How to Set a User Password to Never Expire in Linux?

Command:

chage -M -1 <username>

This sets the password to never expire, ensuring uninterrupted system access for service or system users.

✅ 2. Why Are `/etc/passwd` and `/etc/shadow` Kept Separate?

/etc/passwd: Stores non-sensitive user data, world-readable.
/etc/shadow: Stores encrypted passwords, root-only access.
👉 This enhances Linux user account security by isolating sensitive password hashes.

✅ 3. How to List All Open Files by a Process in Linux?

Command:

lsof -p <PID>

Use this to find open files and sockets used by a specific process. Useful in troubleshooting I/O and locking issues.

✅ 4. Why Can’t You Unmount a Filesystem?

Common reasons:

Directory is in use: pwd
Users are active: fuser -cu /mountpoint
Files are open: lsof /mountpoint

✅ 5. Why Does the Server Boot Slowly After a Reboot?

If the filesystem is ext2, it lacks journaling. After a crash, it triggers full fsck, causing delays.

✅ 6. Getting “Permission Denied” But Space and Permissions Are Fine?

Check for inode exhaustion:

df -i

Inodes are essential for file metadata; 100% inode usage can prevent file creation even if space is available.

✅ 7. How to Check Kernel Routing Table in Linux?

Commands:

route -n
netstat -rn
ip route

These display Linux routing table entries for network debugging.

✅ 8. What Is Sticky Bit? How to Set It?

Only file owners or root can delete files inside a Sticky Bit directory:

chmod +t /dir
# OR
chmod 1757 /dir

✅ 9. How to Extend a Logical Volume (LVM) and Filesystem?

lvextend -L +1G /dev/vgname/lvname
resize2fs /dev/vgname/lvname

Used to dynamically resize volumes in LVM-based Linux systems.

✅ 10. How to Roll Back a Yum/DNF Package After Patching?

yum history undo <ID>      # RHEL 7
dnf history undo <ID>      # RHEL 8/9

✅ 11. Difference Between `df` and `du` in Linux?

df: Shows disk usage from metadata (fast).
du: Walks the directory to sum file sizes (accurate).
👉 Use du for directory-level usage, df for overall partition space.

✅ 12. How to Scan for New Disks Without Rebooting?

echo "---" > /sys/class/scsi_host/host0/scan

✅ 13. How to Check Listening Ports in Linux?

netstat -anp | grep <port>
# Or
ss -tuln

✅ 14. How to Safely Reduce an LVM Volume?

Unmount the LV
Check FS: e2fsck -f /dev/vg/lv
Resize FS: resize2fs
Reduce: lvreduce -L 1G

✅ 15. How to Kill a Process from `top`?

While in top, press k, enter the PID and signal (default: 15, use 9 for force kill).

✅ 16. How to Check Who Rebooted the Server?

last reboot
# Check logs
cat /var/log/messages

✅ 17. What Are SUID, SGID, and Sticky Bit in Linux?

SUID: Run as file owner
SGID: Run as file group
Sticky Bit: Restricts delete access in shared directories

✅ 18. How to Grant Sudo Access to a User Temporarily?

visudo

Add:

username ALL=(ALL) ALL

👉 Grants temporary root access via sudo.

✅ 19. How to Migrate LVM Data from One Physical Volume to Another?

pvmove /dev/sdX /dev/sdY

Transfers data from old to new physical volume without downtime.

✅ 20. How to Troubleshoot a Full `/opt` Even After Deleting Files?

Check if any process is holding deleted files:

lsof | grep /opt

Kill the process using: kill -9 <PID>