Basics Questions

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Level: Basics

🟢 Foundational interview questions.

Focus on core concepts, definitions, and building blocks.

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1. What is a Shell Script?

A text file containing a sequence of Linux commands.

It allows you to automate repetitive tasks by executing commands sequentially, just as you would type them in the terminal.

2. What is a Shebang (#!)?

The first line of a script that tells the OS which interpreter to use.

Example: #!/bin/bash tells the system to use the Bash shell to execute the script.

3. How do you execute a shell script?

./script.sh or bash script.sh.

To run with ./, the file must have execute permissions (chmod +x script.sh).

4. How do you make a script executable?

chmod +x script.sh.

This grants execute permission to the file owner, group, and others (depending on umask).

5. How do you define a variable in Bash?

VAR_NAME=value.

Note: There should be no spaces around the = sign. Example: name="DevOps"

6. How do you access the value of a variable?

$VAR_NAME or ${VAR_NAME}.

Example: echo $name.

7. What is the difference between $* and $@?

They both represent all command-line arguments.

• $*: Treats all arguments as a single string ("arg1 arg2 arg3").
• $@: Treats each argument as a separate quoted string ("arg1" "arg2" "arg3").

8. What does $? represent?

The exit status of the last executed command.

• 0: Success.
• Non-zero (1-255): Failure.

9. How do you stop a script immediately if a command fails?

Add set -e at the beginning of the script.

This tells bash to exit immediately if any command returns a non-zero exit status.

10. What is the difference between single quotes '' and double quotes ""?

Double quotes allow variable expansion. Single quotes treat everything literally.

• echo "Hello $name" -> Hello DevOps
• echo 'Hello $name' -> Hello $name

11. How do you comment in a shell script?

Using the # symbol.

Everything after # on that line is ignored by the interpreter (except for the shebang on line 1).

12. How do you check if a file exists in a script?

Using if [ -f "filename" ]; then ....

The -f flag checks for the existence of a regular file.

13. How do you check if a directory exists?

Using if [ -d "dirname" ]; then ....

The -d flag checks for the existence of a directory.

14. What command is used to read user input?

read.

Example: read -p "Enter name: " name stores the input in the $name variable.

15. How do you define a constant (readonly variable)?

readonly VAR_NAME=value.

Once defined, its value cannot be changed later in the script.

16. What is the purpose of echo command?

To print text or variable values to the standard output (screen).

17. How do you debug a shell script?

Run with bash -x script.sh.

This prints each command and its arguments to the terminal before executing it.

18. How do you redirect output to a file?

Using > (overwrite) or >> (append).

• echo "log" > file.txt: Overwrites file.
• echo "log" >> file.txt: Appends to file.

19. What is $#?

The number of arguments passed to the script.

20. What is $0?

The name of the script itself.

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