Shell Scripting Interview Questions, Explained: Bash Fundamentals, UNIX Processes, and Practical Commands

This revised guide turns the original long Q&A into a practical reference for engineers preparing for Bash and UNIX interviews. It focuses on topics that show up in real DevOps work: scripting, command execution, redirection, permissions, and process control. Original post: Shell Bash Scripting Interview Questions and Answer – Part 1.

Originally published on 2010-04-16; fully revised and updated.

What shell scripting is and why it matters

Shell scripting means writing command-line instructions in a text file and running them through a shell interpreter such as bash, sh, or ksh. It is primarily used for automation on UNIX-like systems, and it often acts as glue between OS tools, CI jobs, and deployment scripts.

• Automates repetitive admin tasks.
• Combines existing tools quickly without compiling code.
• Works well for bootstrapping, CI/CD steps, and operational runbooks.
• Can be risky when scripts are unquoted or run with destructive commands.

How a shell command executes under the hood

Many interview questions around fork, exec, return codes, and zombies become easier once you understand the command lifecycle.

flowchart TD
    A[User enters command] --> B[Shell parses tokens and expansions]
    B --> C{Builtin}
    C -- yes --> D[Run builtin in current shell]
    C -- no --> E[Fork child process]
    E --> F[Exec program in child]
    F --> G[Program exits with status]
    D --> H[Shell records exit code]
    G --> H
    H --> I[Prompt returns]

If the command is an external program, the shell usually creates a child process and then executes that program in the child. The exit code is returned to the shell, which makes it available as $?.

Essential interview topics with practical command examples

Exit codes and command chaining

By convention, exit code 0 means success and non-zero means failure. Operators like && and || chain commands based on that status.

#!/usr/bin/env bash
grep -q "ready" app.log
echo "Last exit code: $?"

grep -q "ready" app.log && echo "Service is ready" || echo "Service is not ready"Code language: PHP (php)

Stdin, stdout, and stderr (file descriptors 0, 1, 2)

Standard input, output, and error are core to shell pipelines and logging strategy.

my_command >stdout.log 2>stderr.log
my_command >combined.log 2>&1
my_command <input.txtCode language: CSS (css)

Executing a script vs sourcing a script

# env-demo.sh
export DEMO_VALUE="set-by-script"

./env-demo.sh
echo "$DEMO_VALUE"    # usually empty in parent shell

source ./env-demo.sh
echo "$DEMO_VALUE"    # now visible in current shellCode language: PHP (php)

Shebang and interpreter selection

The shebang is the first line of an executable script and selects the interpreter.

#!/usr/bin/env bash
set -euo pipefail
echo "Running with Bash $BASH_VERSION"Code language: PHP (php)

Permissions, links, and file access fundamentals

Interviewers frequently ask how to set permissions and how links behave in UNIX filesystems.

chmod 664 myfile
chmod u+rwx,g+rx,o-rwx deploy.sh
ls -l deploy.shCode language: CSS (css)

Command comparisons often asked in interviews

Process management, signals, daemons, and IPC

Core process questions usually center on process IDs, states, and signaling. A daemon is a background service process detached from an interactive terminal session.

sleep 300 &
BG_PID=$!

ps -p "$BG_PID" -o pid,ppid,stat,cmd
kill "$BG_PID"      # graceful request
kill -9 "$BG_PID"   # force kill if neededCode language: PHP (php)

Common inter-process communication (IPC) mechanisms include unnamed pipes, named pipes (FIFO), sockets, message queues, and shared memory.

mkfifo /tmp/demo.fifo

# terminal 1
cat /tmp/demo.fifo

# terminal 2
echo "hello via fifo" > /tmp/demo.fifoCode language: PHP (php)

Login shells, non-login shells, and environment variables

Startup files differ by shell implementation. The table below reflects common Bash behavior on Linux and UNIX-like systems.

echo "Login shell from env: $SHELL"
ps -p $$ -o comm=
shopt -q login_shell && echo "login shell" || echo "non-login shell"

export APP_ENV=prod
echo "$APP_ENV"
unset APP_ENVCode language: PHP (php)

Rapid-fire interview questions and concise answers

• What does $? store? The exit status of the most recently executed command.
• What are file descriptors 0, 1, and 2? Standard input, standard output, and standard error.
• Difference between "$*" and "$@"? "$*" joins all positional parameters into one word; "$@" preserves each parameter as a separate word.
• What is IFS? The internal field separator used during word splitting.
• What is a zombie process? A terminated child process whose parent has not yet collected its exit status with wait.
• What does exec do in shell? Replaces the current shell process with the specified program.
• How do you make a script executable? Add execute permission, for example chmod +x script.sh.

Best practices and common pitfalls

• Quote variable expansions (for example "$var") to avoid word-splitting bugs.
• Check exit codes and fail fast for automation workflows.
• Use set -euo pipefail in Bash scripts where appropriate.
• Be careful with destructive commands like rm, redirection, and wildcards.
• Prefer explicit paths in cron/CI contexts where environment variables differ.
• Lint scripts with tools such as shellcheck before production use.

A strong interview answer is not just a definition; it shows that you understand safe operational behavior, process control, and debugging in real shell automation.

Rajesh Kumar

I’m Rajesh Kumar, a DevOps, SRE, DevSecOps, Cloud, and Platform Engineering expert passionate about sharing practical knowledge, real-world experiences, and industry best practices. I have worked at Cotocus and regularly write about technology, travel, investing, health, product reviews, and digital marketing through my various platforms.

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