Day 5: Bash Scripting – Loops, Functions, Exit Codes

Series: 30 Days DevOps Interview Preparation

Welcome to Day 5 of the 30 Days of DevOps Interview Preparation series. Today we dive into Bash scripting, a critical component of DevOps automation. Mastering Bash allows you to automate infrastructure tasks, system administration, and application deployments efficiently.

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🔁 Bash Loops

📘 What is a Loop?

A loop is a control flow statement used to repeat a block of code multiple times. Bash supports multiple types of loops:

➤ for Loop

for i in {1..5}; do
  echo "Iteration $i"
done

This iterates from 1 to 5, echoing each iteration. Useful for bulk tasks like iterating over files or users.

➤ while Loop

count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

Executes while the condition is true.

➤ until Loop

count=1
until [ $count -gt 5 ]; do
  echo "Count: $count"
  ((count++))
done

Runs until the condition becomes true.

🧠 Real-World Example:

Loop through all .log files and delete if older than 30 days:

for file in /var/log/*.log; do
  if [ $(find "$file" -mtime +30) ]; then
    rm "$file"
  fi
done

💡 Interview Insight:

Q: How would you use loops to monitor a service every 5 seconds?

while true; do
  systemctl is-active nginx
  sleep 5
done

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🔧 Bash Functions

📘 What is a Function?

A function is a reusable block of code. It helps modularize logic, improve readability, and avoid repetition.

➤ Defining and Calling Functions

say_hello() {
  echo "Hello, $1!"
}

say_hello "Tathagat"

➤ Returning Values

add() {
  local sum=$(( $1 + $2 ))
  echo $sum
}

result=$(add 10 20)
echo "Result: $result"

🧠 Real-World Example:

Define a logging function for reuse:

log_info() {
  echo "[INFO] $(date): $1"
}

log_info "Starting deployment..."

💡 Interview Insight:

Q: What is the difference between return and echo in a function?

• return exits the function with a status code (numeric)

• echo is used to output text, commonly used to return strings

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🚪 Exit Codes

📘 What is an Exit Code?

Every command in Bash returns an exit code:

• 0 = Success

• Non-zero = Failure

This is critical in scripting because it allows conditional branching and error handling.

➤ Checking Exit Code

ls /notfound
echo $?

Output will be non-zero because the directory doesn’t exist.

➤ Use in Conditionals

if cp file.txt /backup/; then
  echo "Backup successful"
else
  echo "Backup failed"
fi

🧠 Real-World Use:

Abort script on first error:

#!/bin/bash
set -e  # Exit if any command fails

💡 Interview Insight:

Q: How do you handle multiple command failures in a script? Use set -euo pipefail to:

• -e: Exit on error

• -u: Error on undefined variables

• -o pipefail: Catch errors in piped commands

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🧠 Interview Questions & Answers

1. What are the different types of loops in Bash?
   for, while, and until loops. Each suited for different scenarios.

2. How do you pass parameters to Bash functions?
   Via $1, $2, etc. Example: greet() { echo Hello, $1; }

3. How are exit codes used in scripting?
   To determine success/failure and conditionally proceed or halt scripts.

4. What is the purpose of set -euo pipefail?
   Enforces stricter error handling to avoid silent failures.

5. Can functions return values in Bash?
   Functions return numeric values using return, or strings using echo and capturing with $()

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📝 Summary Table

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🚀 DevOps Tips

• Use logging inside scripts for better debugging.

• Make your scripts idempotent (safe to re-run).

• Use shebang (#!/bin/bash) and make scripts executable (chmod +x script.sh).

• Store common functions in a shared .bashlib file.