Bash/Linux Interview Questions for DevOps Engineers

Bash scripting is a superpower for engineers. Whether automating repetitive tasks, gluing together tools, or managing systems, Bash is always there, simple yet powerful.

Bash

1. What is the purpose of the shebang (#!) line?

It defines the interpreter that should execute the script. Example:

#!/bin/bash

2. How do you read command-line arguments in a shell script?

#!/bin/bash
echo "Script Name: $0"
echo "First Argument: $1"
echo "Total Arguments: $#"
echo "All Arguments: $@"
echo "Process ID: $$"
echo "Exit Status: $?"

3. What are the standard streams in Linux?

• Standard Input (stdin)

• Standard Output (stdout)

• Standard Error (stderr)

4. What are the various types of variables used in shell scripting?

There are two types of variables used in shell scripting:

• System-generated variables: These are created by the OS and can be viewed using the set command.

• User-defined variables: Created by users and accessed using the echo command.

5. How do you check if a file exists using a conditional statement?

if [ -f /var/log/messages ]; then echo "File exists"; fi

6. How do you find and delete files older than 7 days?

find /path/to/directory -type f -mtime +7 -exec rm -f {} \;

7. What is crontab?

crontab (CRON Table) is used to schedule jobs to run at specific times. It allows automation of system tasks.

8. How do you debug a shell script?

• Use set -x to enable debugging.

• Use echo statements to track variable values and execution flow.

9. How do you rename all .txt files to .log in a directory?

for file in *.txt; do mv "$file" "${file%.txt}.log"; done

10. What are the default permissions of a file when it is created?

The default permissions are determined by the umask value. Typically, a new file has rw-rw-r-- (664) permissions.

11. What are the four stages of a Linux process?

• Waiting: Process is waiting for a resource.

• Running: Process is currently executing.

• Stopped: Process execution has been stopped.

• Zombie: Process has finished but still exists in the process table.

12. What is a metacharacter?

A metacharacter is a special character in the shell used for pattern matching. Example:

ls s*

Lists all files starting with ‘s’.

13. Difference between $@ and $*?

• $* treats all arguments as a single string.

• $@ treats each argument as a separate entity.

14. How would you check if a file named example exists in a specific path?

[ -e example ] && echo "File exists" || echo "File does not exist"

You can also use:

test -e example && echo "File exists" || echo "File does not exist"

15. How would you check disk usage using shell commands?

• df -h: Shows available disk space.

• du -sh /data01/*: Shows disk usage per directory.

• dfspace: Provides space usage in megabytes.

16. What is the special shell variable $??

It stores the exit status of the last executed command.

cp file1 file2
if [ $? -eq 0 ]; then echo "Copy successful"; else echo "Copy failed"; fi

17. How to extract a substring from a variable?

variable="Her name is Jen, and she is a developer."
echo ${variable:12:6}  # Output: Jen, a

18. Difference between [[ $string == "efg*" ]] and [[ $string == efg* ]]?

• [[ $string == efg* ]] checks if $string starts with "efg".

• [[ $string == "efg*" ]] checks if $string is exactly "efg*".

19. How to list only filenames in a directory?

ls -1

To store the output in a variable:

filelist=$(ls -1 /somedir/)

20. What is the difference between soft and hard links?

• Soft link (symbolic link): A pointer to the original file.

• Hard link: A duplicate entry pointing to the same inode as the original file.

ln -s original_file soft_link
ln original_file hard_link

21. How do you replace a string in multiple files using sed?

sed -i 's/old_string/new_string/g' *.txt

22. What does grep -oP '\\d+' do?

It extracts numeric values from a string.

echo "Version=RevNo.201963" | grep -oP '\\d+'

Output: 201963

These questions cover key Bash and Linux concepts that every DevOps engineer should be familiar with. Mastering these will help you ace your next interview!

Happy scripting! 🖥️