Linux Interview Questions India 2026: File Permissions, Process Management, and Shell Scripting

Linux file permissions control who can read, write, and execute files. Every file has three permission sets: owner (user), group, and others. Each set has three bits: read (r=4), write (w=2), execute (x=1).

Key commands:

1. chmod 755 file — sets owner to rwx (7), group to r-x (5), others to r-x (5)
2. chmod u+x file — adds execute permission for the file owner
3. chmod -R 644 directory/ — recursively sets permissions on all files
4. chown user:group file — changes the owner and group of a file
5. chown -R www-data:www-data /var/www/ — recursively change ownership
6. ls -la — list files with permissions, owner, group, and size
7. umask — controls the default permissions for newly created files

User management:

1. useradd -m username — create a new user with a home directory
2. usermod -aG sudo username — add a user to the sudo group
3. passwd username — set or change a user's password
4. cat /etc/passwd — list all users on the system
5. id username — show the user ID (UID) and group memberships

SUID, SGID, and Sticky Bit:

1. SUID (chmod u+s): when set on an executable, it runs with the owner's privileges (e.g., /usr/bin/passwd is SUID root so any user can change their own password)
2. SGID (chmod g+s): on a directory, new files inherit the group of the directory
3. Sticky bit (chmod +t): on a directory, only the file owner can delete their own files (used on /tmp)

Process management is heavily tested in Linux interviews. Understanding how to inspect, control, and troubleshoot running processes is essential.

Viewing processes:

1. ps aux — list all running processes with CPU and memory usage
2. ps -ef — full-format listing with parent process IDs (PPID)
3. top — real-time process monitor; press 'k' to kill a process, 'r' to renice
4. htop — improved top with a colour interface (may need installation)
5. pgrep nginx — find the PID of a process by name
6. pstree — display processes as a tree showing parent-child relationships

Controlling processes:

1. kill PID — send SIGTERM (graceful termination) to a process
2. kill -9 PID — send SIGKILL (forceful termination; the process cannot catch or ignore this)
3. killall nginx — terminate all processes named nginx
4. pkill -f 'python script.py' — kill processes matching a pattern
5. nohup command & — run a command that persists after you log out
6. bg and fg — send a process to the background or bring it back to the foreground

System resource monitoring:

1. free -h — display memory usage in human-readable units
2. df -h — disk space usage per filesystem
3. du -sh /var/log/ — size of a specific directory
4. vmstat 1 5 — CPU, memory, I/O, and process statistics (1-second intervals, 5 samples)
5. iostat — CPU and I/O statistics for block devices
6. lsof -p PID — list files opened by a specific process
7. lsof -i :8080 — find the process listening on port 8080

systemd service management:

1. systemctl start nginx — start a service
2. systemctl stop nginx — stop a service
3. systemctl restart nginx — restart a service
4. systemctl status nginx — check the status and recent logs of a service
5. systemctl enable nginx — enable a service to start on boot
6. journalctl -u nginx -f — follow the live logs for a specific service

Shell scripting and networking commands are tested for backend, DevOps, and SRE roles. Expect to write or debug a bash script and troubleshoot network connectivity.

Bash scripting essentials:

1. Variables: NAME='value'; echo $NAME (no spaces around = in assignment)
2. Conditionals: if [ -f /etc/nginx.conf ]; then echo 'exists'; fi
3. Loops: for i in $(seq 1 10); do echo $i; done
4. Functions: function greet() { echo "Hello, $1"; }; greet World
5. Read input: read -p 'Enter name: ' NAME
6. Exit codes: every command returns 0 (success) or non-zero (failure); check with $?
7. Redirecting output: command > file (overwrite), command >> file (append), command 2>&1 (stderr to stdout)
8. Pipes: ps aux | grep nginx | awk '{print $2}' — combine commands
9. Cron job syntax: (minute hour day-of-month month day-of-week); 0 2 /scripts/backup.sh runs at 2 AM daily

Networking commands:

1. ifconfig / ip addr — view network interfaces and IP addresses
2. ip route — view the routing table
3. ping 8.8.8.8 — test connectivity to a host
4. traceroute google.com — trace the path packets take to a host
5. netstat -tlnp — list TCP listening ports with the process name (may need -sudo)
6. ss -tlnp — modern replacement for netstat; faster
7. curl -I https://example.com — fetch HTTP headers only
8. wget https://example.com/file.tar.gz — download a file
9. dig google.com — DNS lookup; shows A records, TTL
10. nslookup google.com — another DNS query tool
11. iptables -L — list firewall rules

Common interview scenario questions:

1. 'A process is consuming 100% CPU. How do you find and fix it?' — top to identify the PID, strace -p PID to trace system calls, kill or restart the process
2. 'The disk is full. How do you find what is consuming space?' — df -h to see which filesystem, du -sh /* to drill down, find / -size +1G to find large files
3. 'How do you check which process is listening on port 80?' — ss -tlnp | grep :80 or lsof -i :80