Linux and Shell Scripting Interview Questions for DevOps
25 Linux and Shell Scripting questions with answers for devops roles — 10 easy, 11 medium, 4 hard.
About Linux and Shell Scripting
Linux runs almost every server and container in production, so it is the base of all DevOps work. This topic covers file permissions, processes, systemd services, logs, networking commands and bash scripting with cron, plus the troubleshooting questions interviewers love: full disks, high CPU, OOM kills and SSH failures.
Sheet progressStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
✓ 3 questions free • Unlock the rest in seconds
01
What is the difference between a process and a thread in Linux?
A process is a running program with its own address space, file descriptors and PID. A thread is a unit of execution inside a process that shares the same memory and open files with other threads. Creating threads is cheaper and they communicate easily, but a bug in one thread can crash the whole process, while separate processes are isolated from each other.
02
Explain Linux file permissions and what chmod 755 means.
Every file has read (4), write (2) and execute (1) bits for three classes: owner, group and others. chmod 755 gives the owner 7 (read, write, execute) and gives group and others 5 (read, execute). It is typical for scripts and directories that everyone may run or enter but only the owner may change. Use ls -l to see permissions.
03
What is the difference between a hard link and a soft link?
A hard link is another directory entry pointing to the same inode, so the data survives until every hard link is deleted; it cannot cross file systems or point to directories. A soft (symbolic) link is a small file that stores a path to the target; it can cross file systems and point to directories, but it breaks if the target is moved or deleted.
systemctl status nginx
sudo systemctl start nginx
sudo systemctl restart nginx
sudo systemctl enable nginx # start at boot
sudo systemctl disable nginx
journalctl -u nginx -f # follow service logs
sudo systemctl daemon-reload # after editing a unit file
Unit files live in /etc/systemd/system. enable creates the symlink for the boot target, while start only runs it now.
05
What is the difference between kill, kill -15 and kill -9?
kill sends SIGTERM (15) by default, which asks the process to stop so it can close files, finish requests and clean up. kill -9 sends SIGKILL, which the kernel enforces immediately; the process cannot catch it, so data may be lost and temp files left behind. Always try SIGTERM first and use SIGKILL only when the process does not respond.
06
Explain stdin, stdout, stderr and what 2>&1 means.
Every process gets three file descriptors: 0 is stdin, 1 is stdout and 2 is stderr. > redirects stdout to a file, 2> redirects errors, and 2>&1 sends stderr to wherever stdout currently points. So command > out.log 2>&1 captures both normal output and errors in one file. Order matters: 2>&1 placed before > out.log would still print errors to the terminal.
07
How do you schedule a cron job to run every day at 2:30 AM?
crontab -e
# min hour day month weekday command
30 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1
crontab -l # list jobs
Use absolute paths because cron runs with a minimal PATH and environment. Redirect output to a log, otherwise errors are mailed or lost. On systemd hosts, timers are an alternative with better logging.
08
Your app fails to start with 'address already in use' on port 8080. What do you do?
Find what is holding the port with ss -ltnp | grep 8080 or lsof -i :8080. If it is an old instance of the same app that did not shut down, stop it gracefully or fix the service manager so only one runs. If it is another service, change one app's port through configuration rather than hard coding. Then restart and confirm with curl localhost:8080.
su switches to another user, usually root, and needs that user's password, giving a full shell as them. sudo runs a single command with elevated rights using your own password, and is controlled by rules in /etc/sudoers. sudo is preferred in teams because each action is logged against the real user and access can be limited to specific commands.
I enjoy both writing code and understanding how systems run in production, and DevOps sits between the two. In my projects I set up a CI pipeline and Docker deployment and liked seeing a change go from commit to live in minutes. I want to keep learning cloud, Kubernetes and automation, and help teams release faster with fewer failures.
An inode is a data structure that stores a file's metadata: owner, permissions, size, timestamps, link count and pointers to data blocks, but not the file name. Names live in directory entries that map to inode numbers. A file system has a fixed number of inodes, so millions of tiny files can exhaust them while disk space is still free; check with df -i.
12
What is load average in Linux and how do you read it?
Load average is the average number of processes running or waiting for CPU (and uninterruptible I/O) over 1, 5 and 15 minutes, shown by uptime or top. Compare it with the number of CPU cores: a load of 4 on a 4-core machine means full use, while 12 means work is queuing. A high load with low CPU use often points to disk or network I/O waits.
A zombie is a process that has exited but whose parent has not yet read its exit status with wait(), so it still holds a PID entry and shows state Z. An orphan is a running process whose parent died; it is adopted by PID 1, which reaps it later. Many zombies indicate a buggy parent; fixing or restarting the parent clears them.
set -e exits the script when any command fails, set -u treats unset variables as errors, and set -o pipefail makes a pipeline fail if any command in it fails, not just the last. Together they stop a script from silently continuing after an error, which matters in CI and deployment scripts. Commands that may fail on purpose need explicit handling such as || true.
15
How do you find files larger than 100 MB and older than 7 days under /var/log?
find /var/log -type f -size +100M -mtime +7 -exec ls -lh {} +
# compress them instead of just listing
find /var/log -type f -size +100M -mtime +7 -exec gzip {} +
-size +100M matches files above 100 MB, -mtime +7 means modified more than 7 days ago, and -exec with + passes many files to one command call, which is faster than running it once per file.
16
Write a bash script that checks whether a service is running and restarts it if not.
#!/bin/bash
set -u
SERVICE="nginx"
if ! systemctl is-active --quiet "$SERVICE"; then
echo "$(date) $SERVICE down, restarting" >> /var/log/svc-check.log
systemctl restart "$SERVICE"
if ! systemctl is-active --quiet "$SERVICE"; then
echo "$SERVICE failed to start" | mail -s "ALERT" ops@example.com
fi
fi
Run it from cron every few minutes; a real setup would rely on systemd Restart= plus monitoring.
17
How do you count unique client IP addresses in an nginx access log?
awk '{print $1}' /var/log/nginx/access.log | sort | uniq -c | sort -rn | head -10
# total number of unique IPs
awk '{print $1}' access.log | sort -u | wc -l
The first field in the default log format is the client IP. uniq -c only collapses adjacent lines, so sort must run before it; the final sort -rn shows the busiest IPs first.
18
Write a script that alerts when any disk partition is above 80 percent used.
#!/bin/bash
THRESHOLD=80
df -hP | awk 'NR>1 {gsub("%","",$5); print $5, $6}' | while read -r used mount; do
if [ "$used" -ge "$THRESHOLD" ]; then
echo "WARNING: $mount is ${used}% full on $(hostname)"
fi
done
df -P keeps each entry on one line, awk strips the percent sign, and the loop compares each value with the threshold.
19
A production server is very slow. How do you troubleshoot high CPU usage?
Start with uptime and top or htop to see load and which processes use CPU. Check whether it is user, system or iowait time, and use pidstat or ps -eo pid,pcpu,cmd --sort=-pcpu for detail. For a Java process, take thread dumps; for others, use perf or strace. Correlate with recent deployments, cron jobs or traffic spikes before deciding to scale, roll back or fix code.
20
You suddenly cannot SSH into a Linux server. What do you check?
Check reachability with ping and whether port 22 is open with nc -zv host 22. Then look at security groups, firewall rules and whether the sshd service is running, using the cloud console or serial access. Run ssh -vvv to see whether it fails at key exchange or authentication. Common causes are wrong key permissions, a full disk, a changed IP, or fail2ban blocking you.
21
Tell me about a time you automated a repetitive manual task.
Situation: our team manually cleaned old build artefacts on shared servers every week, which took two hours and sometimes deleted the wrong files. Task: I offered to automate it. Action: I wrote a bash script with dry-run mode, logging and a retention rule, tested it on staging and scheduled it with cron. Result: zero manual effort and no more full-disk alerts for six months.
The firmware (BIOS or UEFI) runs hardware checks and loads the bootloader, usually GRUB. GRUB loads the kernel and the initramfs, which contains drivers needed to mount the real root file system. The kernel then starts PID 1, which is systemd on most modern distributions. systemd mounts file systems and starts services in parallel according to unit dependencies and the default target.
23
df shows the disk is 100 percent full but du does not find large files. What is happening?
Usually a large file was deleted while a process still holds it open, so the space is not released. Run lsof +L1 or lsof | grep deleted to find the process and file. Restarting or reloading that process frees the space; if you cannot restart, truncate the file through /proc/<pid>/fd/<n>. The fix is proper log rotation with copytruncate or a reopen signal.
24
The kernel OOM killer keeps killing your application. How do you handle it?
Confirm with dmesg or journalctl -k, which shows the OOM message and the killed process. Check whether memory grows over time (a leak) or spikes with load. Set correct heap limits, for example JVM -Xmx below container memory, add swap only as a buffer, and set memory limits so one service cannot starve others. Then profile the app and right-size the instance.
25
An application logs 'Too many open files'. How do you fix it?
The process has hit its file descriptor limit. Check usage with ls /proc/<pid>/fd | wc -l and the limit in /proc/<pid>/limits. Raise it through LimitNOFILE in the systemd unit or /etc/security/limits.conf, then restart. But also look for a leak: sockets or files not closed in code keep growing and will hit any limit. Raising the limit alone only buys time.
Frequently asked questions
How important is Linux and Shell Scripting in DevOps interviews?
Linux runs almost every server and container in production, so it is the base of all DevOps work. This topic covers file permissions, processes, systemd services, logs, networking commands and bash scripting with cron, plus the troubleshooting questions interviewers love: full disks, high CPU, OOM kills and SSH failures.
How should I use the Linux and Shell Scripting sheet?
Go top to bottom — easy to hard. Answer before opening each answer, tick what you solved and save misses to Last-Minute Revision.