Bash Scripting Fundamentals

Your First Script

Every bash script starts with a shebang line that tells the system which interpreter to use:

#!/bin/bash

# My first script

echo "Hello, World!"

Save this as hello.sh, make it executable, and run it:

$ chmod +x hello.sh
$ ./hello.sh
Hello, World!

Variables

#!/bin/bash

# Assign variables (no spaces around =)

name="Pranav"

age=25

# Use variables with $

echo "Name: $name"

echo "Age: ${age} years" # Curly braces for clarity

# Command substitution

current_date=$(date +%Y-%m-%d)

echo "Today is: $current_date"

Conditionals

#!/bin/bash

if [ -f "/etc/passwd" ]; then

echo "File exists"

elif [ -d "/etc" ]; then

echo "Directory exists"

else

echo "Not found"

Common Test Operators

Operator	Description
-f file	File exists and is regular file
-d dir	Directory exists
-z string	String is empty
-n string	String is not empty
$a -eq $b	Numbers are equal
$a -gt $b	a is greater than b

Loops

# For loop

for file in *.txt; do

echo "Processing: $file"

done

# While loop

count=1

while [ $count -le 5 ]; do

echo "Count: $count"

((count++))

done

Functions

#!/bin/bash

greet() {

local name="$1" # First argument

echo "Hello, $name!"

}

greet "World"

greet "Pranav"

Quoting (the #1 source of bugs)

Bash splits words on spaces and expands globs (*) unless you quote variables. If you remember one rule: quote your variables.

# Bad: breaks on spaces

file="My File.txt"

rm $file

# Good: always quote

file="My File.txt"

rm -- "$file"

Exit Codes and Error Handling

In Linux, “success” is exit code 0 and “failure” is anything non-zero. You can check the last exit code with $?.

curl -fsSL https://example.com

echo "Exit code: $?"

# Use || to handle failures

command || echo "Command failed"

Arguments and Flags (getopts)

Scripts become useful when you can pass inputs. Use positional parameters ($1, $2, …) and getopts for flags.

#!/bin/bash

set -e

while getopts ":f:v" opt; do

case "$opt" in

f) file="$OPTARG" ;;

v) verbose=1 ;;

*) echo "Usage: $0 -f file [-v]" ; exit 2 ;;

esac

done

shift $((OPTIND-1))

echo "File: $file"

A Safer Script Template

For production scripts, use a safer default mode and add a cleanup trap.

#!/usr/bin/env bash

set -euo pipefail

IFS=$'\n\t'

cleanup() {

rm -f "$tmp_file"

}

trap cleanup EXIT

✅ Practice (20 minutes)

Write a script that takes -f (file) and prints the number of lines via wc -l.
Break it by passing a filename with spaces, then fix it using proper quoting.
Add set -euo pipefail and intentionally trigger an error to see how it fails fast.

💡 Best Practice

For production scripts, prefer set -euo pipefail (fail fast, catch undefined vars, and detect pipeline failures) and always quote variables. This prevents cascading and hard-to-debug errors.