Bash Scripting - Automate Everything with Shell Scripts

// Lesson 01: Your First Script

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The Shebang

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

# This tells Linux to use bash to run this script #!/bin/bash

Creating Your First Script

Let's create a simple script that greets you:

$ cat > hello.sh << 'EOF' #!/bin/bash # My first bash script echo "Hello, rebel! Welcome to bash scripting." echo "Today is $(date)" EOF

Now make it executable and run it:

$ chmod +x hello.sh $ ./hello.sh Hello, rebel! Welcome to bash scripting. Today is Tue Feb 24 12:00:00 EST 2026

Comments

Lines starting with # are comments. Use them liberally—your future self will thank you:

# This script backs up my documents folder # It creates a timestamped archive # Usage: ./backup.sh

                    Pro Tip: The shebang line #!/bin/bash must be the very first line of your script. No blank lines before it!
                

Quiz

1. What does the shebang line do?

Hint: It tells Linux how to run the script.

2. What command makes a script executable?

Hint: change mode + execute

Show Answers

Answers

tells the system which interpreter to use
chmod +x

// Lesson 02: Variables and Parameters

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Creating Variables

Variables store data for later use. No dollar sign when assigning, but use $ when reading:

# Assign a value (no $) name="Rebel" count=42 # Read the value (with $) echo $name echo $count

                    Critical: There must be NO spaces around the = sign. name="Rebel" works, but name = "Rebel" does not!
                

Command Substitution

Capture command output into variables using $(command) or backticks:

# Get current date today=$(date +%Y-%m-%d) echo $today # Count files in current directory file_count=$(ls -1 | wc -l) echo $file_count files found

Command-Line Arguments

Scripts can accept arguments passed from the command line:

# script.sh arg1 arg2 arg3 $0 # script.sh (the script name) $1 # arg1 (first argument) $2 # arg2 (second argument) $@ # all arguments as a list $# # number of arguments

Example script:

#!/bin/bash echo "Script: $0" echo "First arg: $1" echo "Second arg: $2" echo "Total args: $#"

Environment Variables

Access system information through environment variables:

echo $HOME # Your home directory echo $USER # Your username echo $PATH # Executable search path echo $PWD # Current directory echo $HOSTNAME # Computer name

Quiz

1. How do you read a variable value in bash?

Hint: It starts with a dollar sign.

2. What variable holds all command-line arguments?

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Answers

$
$@

// Lesson 03: Conditionals

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The if Statement

Make decisions based on conditions:

if [ $age -ge 18 ]; then echo "You are an adult" fi

Comparison Operators

Numbers use these operators:

-eq # equal -ne # not equal -lt # less than -le # less than or equal -gt # greater than -ge # greater than or equal

Strings use these operators:

= # equal (single =) != # not equal -z # is empty -n # is not empty

if/else

if [ $1 = "admin" ]; then echo "Welcome, Administrator" else echo "Welcome, $1" fi

elif (Else If)

if [ $score -ge 90 ]; then echo "Grade: A" elif [ $score -ge 80 ]; then echo "Grade: B" elif [ $score -ge 70 ]; then echo "Grade: C" else echo "Grade: F" fi

File Tests

Check file properties:

-e # file exists -f # regular file exists -d # directory exists -r # file is readable -w # file is writable -x # file is executable

Example: Check if a file exists before deleting:

if [ -f "$filename" ]; then
    rm "$filename"
    echo "Deleted $filename"
else
    echo "File not found"
fi

Quiz

1. What operator checks if two strings are equal?

2. What flag checks if a file is a directory?

Show Answers

Answers

=
-d

// Lesson 04: Loops

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For Loops

Iterate over a list of items:

for color in red green blue; do echo "Color: $color" done

Output:

Color: red Color: green Color: blue

Loop Over Files

Process all files in a directory:

for file in *.txt; do echo "Processing: $file" done

For Loop with Sequence

Use brace expansion for number ranges:

# Count from 1 to 5 for i in {1..5}; do echo "Number: $i" done

While Loops

Loop while a condition is true:

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

Reading Input

Read file line by line:

while read line; do echo "Line: $line" done < file.txt

Loop Control

break # exit the loop entirely continue # skip to next iteration

Example: Find first file containing "error":

for file in *.log; do
    if grep -q "error" "$file"; then
        echo "Found error in: $file"
        break
    fi
done

Quiz

1. What command skips to the next iteration of a loop?

2. What syntax loops from 1 to 10?

Show Answers

Answers

continue
{1..10}

// Lesson 05: Functions

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Defining Functions

Two ways to define functions in bash:

# Method 1: name() syntax greet() { echo "Hello!" } # Method 2: function keyword function farewell { echo "Goodbye!" }

Calling Functions

greet # calls the greet function farewell # calls the farewell function

Function Parameters

Functions use $1, $2, etc. just like scripts:

greet_user() { echo "Hello, $1!" } greet_user Alice # Output: Hello, Alice! greet_user Bob # Output: Hello, Bob!

Return Values

Bash functions return exit codes (0 = success, non-zero = error):

is_admin() { if [ $1 = "admin" ]; then return 0 # success else return 1 # failure fi } is_admin $user if [ $? -eq 0 ]; then echo "User is admin" fi

Scope: Local Variables

Use local to keep variables from leaking:

calculate() { local result=$(( $1 * $2 )) echo $result } result=100 # global variable calculate 5 10 # echoes 50, doesn't change global echo $result # still 100

                    Best Practice: Always use local for variables inside functions unless you intentionally want them to be global.
                

Quiz

1. How do you keep a variable local inside a function?

2. What does $? contain after a function returns?

Show Answers

Answers

local
exit code

// Lesson 06: Arrays and Strings

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Arrays

Store multiple values in a single variable:

# Create an array fruits=("apple" "banana" "cherry" "date") # Access elements (0-indexed) echo ${fruits[0]} # apple echo ${fruits[2]} # cherry # Get all elements echo ${fruits[@]} # all fruits # Get array length echo ${#fruits[@]} # 4

Array Operations

# Add element fruits+=("elderberry") # Remove element (by index) unset fruits[1] # Slice (elements 1-3) echo ${fruits[@]:1:3}

String Operations

str="Hello, Rebel!" # String length echo ${#str} # 13 # Substring echo ${str:0:5} # Hello # Replace echo ${str/Rebel/World} # Hello, World!

Pattern Matching

file="document.pdf" # Remove shortest match from beginning echo ${file#*.} # pdf # Remove longest match from beginning echo ${file##*.} # pdf # Remove shortest match from end echo ${file%.*} # document # Remove longest match from end echo ${file%%.*} # document

Practical use: Get file extensions and names:

filename="report.2024.pdf"
ext="${filename##*.}"     # pdf
base="${filename%.*}"     # report.2024

Quiz

1. How do you get the length of an array?

2. What removes the shortest match from the end of a string?

Show Answers

Answers

${#array[@]}
%

// Lesson 07: Exit Codes and Error Handling

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Exit Codes

Every command returns an exit code: 0 = success, non-zero = failure:

$ ls /valid/path $ echo $? 0 $ ls /invalid/path $ echo $? 2

Checking Exit Codes

if $command; then echo "Command succeeded" else echo "Command failed with code $?" fi

set Options

Automatically handle errors in scripts:

set -e # Exit on any error set -u # Exit on undefined variable set -o pipefail # Catch errors in pipelines

Add at the top of your script:

#!/bin/bash set -euo pipefail

Custom Exit Codes

Return specific codes for different errors:

#!/bin/bash if [ $# -lt 1 ]; then echo "Usage: $0 " >&2 exit 1 # Usage error fi if [ ! -f $1 ]; then echo "Error: File not found" >&2 exit 2 # File not found fi exit 0 # Success

Trap Errors

Execute cleanup code when script exits:

cleanup() { echo "Cleaning up..." rm -f /tmp/tempfile } trap cleanup EXIT

                    Standard Exit Codes:

                    0 = success

                    1 = general error

                    2 = misuse of command

                    126 = command not executable

                    127 = command not found

Quiz

1. What exit code means success?

2. What option makes the script exit on any command failure?

Show Answers

Answers

0
set -e

// Lesson 08: Real-World Automation

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Automated Backup Script

#!/bin/bash set -euo pipefail SOURCE="/home/user/documents" BACKUP_DIR="/backup" DATE=$(date +%Y-%m-%d) mkdir -p $BACKUP_DIR echo "Starting backup of $SOURCE..." if tar -czf $BACKUP_DIR/backup-$DATE.tar.gz $SOURCE; then echo "Backup completed successfully" exit 0 else echo "Backup failed!" >&2 exit 1 fi

System Monitor Script

#!/bin/bash set -euo pipefail CPU=$(top -bn1 | grep "Cpu(s)" | awk '{print $2}' | cut -d'%' -f1) MEM=$(free | grep Mem | awk '{printf "%.1f", $3/$2 * 100}') DISK=$(df -h / | tail -1 | awk '{print $5}' | cut -d'%' -f1) echo "=== System Status ===" echo "CPU: $CPU%" echo "Memory: $MEM%" echo "Disk: $DISK%" # Alert if usage is high if [ $CPU -gt 90 ] || [ $MEM -gt 90 ]; then echo "WARNING: High resource usage!" >&2 fi

Batch File Processor

#!/bin/bash set -euo pipefail PROCESSED=0 for file in *.csv; do echo "Processing $file..." # Skip if not a file [ -f $file ] || continue # Process the file tail -n +2 $file | sort > processed/$file PROCESSED=$((PROCESSED + 1)) done echo "Processed $PROCESSED files"

Scheduling with Cron

Make scripts run automatically:

# Edit crontab $ crontab -e # Add lines like: # Run backup every day at 2am 0 2 * * * /home/user/scripts/backup.sh # Run every hour 0 * * * * /home/user/scripts/monitor.sh # Run every Monday at 9am 0 9 * * 1 /home/user/scripts/weekly-report.sh

                    Crontab Format:

                    ┌───────────── minute (0-59)

                    │ ┌─────────── hour (0-23)

                    │ │ ┌───────── day of month (1-31)

                    │ │ │ ┌─────── month (1-12)

                    │ │ │ │ ┌───── day of week (0-6, Sunday=0)

                    │ │ │ │ │

                    * * * * * command

Quiz

1. What does "0 2 * * *" mean in crontab?

Hint: minute hour day month weekday

2. What does set -euo pipefail do?

Hint: It makes the script stop on errors and undefined variables.

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Answers

2am
exit on error

// Lesson 09: Input/Output Redirection

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Standard Streams

Every process has three standard streams:

stdin (0) # Standard input - keyboard stdout (1) # Standard output - screen stderr (2) # Standard error - screen

Output Redirection

# Redirect stdout to file (overwrite) echo "hello" > output.txt # Redirect stdout to file (append) echo "world" >> output.txt # Redirect stderr to file ls /invalid 2> errors.txt # Redirect both stdout and stderr command > all.txt 2>&1

Input Redirection

# Read from file instead of keyboard while read line; do echo "Line: $line" done < input.txt # Here document (inline input) cat << 'EOF' This is line 1 This is line 2 This is line 3 EOF

Pipes

Connect commands together - output of one becomes input of another:

Process Substitution

# Compare two file listings diff <(ls dir1) <(ls dir2) # Read from multiple sources cat <(echo "header") file.txt <(echo "footer")

Quiz

1. What file descriptor is stderr?

2. What operator appends output to a file?

Show Answers

Answers

2
>>

// Lesson 10: Text Processing Tools

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grep - Pattern Matching

Search for patterns in text:

# Basic search grep "error" logfile.txt # Case-insensitive grep -i "error" logfile.txt # Show line numbers grep -n "error" logfile.txt # Invert match (lines WITHOUT error) grep -v "error" logfile.txt # Recursive search grep -r "function" src/

sed - Stream Editor

Find and replace text:

# Replace first occurrence per line sed 's/old/new/' file.txt # Replace all occurrences sed 's/old/new/g' file.txt # Replace on specific line number sed '5s/old/new/' file.txt # Delete lines matching pattern sed '/^$/d' file.txt # Edit file in place sed -i 's/old/new/g' file.txt

awk - Text Processing

Powerful column-based processing:

# Print first column awk '{print $1}' data.txt # Print specific columns awk -F: '{print $1, $3}' /etc/passwd # Print lines longer than 80 chars awk 'length > 80' file.txt # Sum column awk '{sum+=$1} END {print sum}' numbers.txt # Conditional printing awk '$3 > 50 {print $1, $3}' data.txt

Combining Tools

# Real-world: Find errors, show context grep -n "ERROR" app.log | awk -F: '{print "Line " $1 ": " $2}' # Process CSV data awk -F, '$3 > 1000 {print $1 " owes $" $3}' customers.csv

                    Pro Tip: These tools are designed to work together. Pipe them to build powerful data processing pipelines.
                

Quiz

1. Which flag makes grep case-insensitive?

2. What sed command replaces all occurrences?

Show Answers

Answers

-i
s/old/new/g

// Lesson 11: Advanced Arguments

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The shift Command

Process arguments one by one:

#!/bin/bash while [ $# -gt 0 ]; do echo "Processing: $1" shift done

getopts - Option Parsing

Handle command-line flags properly:

#!/bin/bash while getopts ":vhf:o:" opt; do case $opt in v) VERBOSE=1 ;; f) FILE=$OPTARG ;; o) OUTPUT=$OPTARG ;; \?) echo "Invalid option: -$OPTARG" >&2 ;; :) echo "Option -$OPTARG requires an argument." >&2 ;; esac done echo "File: $FILE, Output: $OUTPUT, Verbose: $VERBOSE"

Usage:

./script.sh -f input.txt -o result.txt -v # -f requires argument, -v is a flag

Argument Validation

if [ $# -lt 2 ]; then echo "Usage: $0 " >&2 exit 1 fi if [ ! -f $1 ]; then echo "Error: Input file '$1' not found" >&2 exit 2 fi

Default Values

OUTPUT=${OUTPUT:-"output.txt"} # Uses "output.txt" if OUTPUT is empty MAX=${MAX:-100} # Uses 100 if MAX is empty

Quiz

1. What command moves to the next argument?

2. In getopts, what stores the option letter?

Show Answers

Answers

shift
opt

// Lesson 12: Debugging & Best Practices

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Debug Mode

Use -x to trace execution:

# Run script in debug mode bash -x script.sh # Add to script for debugging section set -x # your code here set +x

Debug Options

set -x # Print commands before execution set -v # Print input lines as read set -e # Exit on error set -u # Exit on undefined variable set -o pipefail # Exit if any command in pipe fails set -n # Syntax check only (no execution)

Common Pitfalls

# WRONG: Spaces around = name = "value" # RIGHT: No spaces name="value" # WRONG: Glob expands ls *.txt # Fails if no txt files # RIGHT: Quote variables ls "*.txt" ls "$file"

Best Practices

# Always use these at the top #!/bin/bash set -euo pipefail IFS=$'\n\t' # Always quote variables echo "$var" # Safe echo $var # Dangerous # Use descriptive names BACKUP_SOURCE="/home/data" # Clear bs="/home/data" # Unclear # Use functions for organization main() { parse_args $@ validate run_backup }

Testing Scripts

# Syntax check bash -n script.sh # Dry run (echo commands) bash -n script.sh && echo "OK" # Check for unset variables bash -u script.sh

Production Script Template:

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

log() { echo "[$(date)] $*"; }
error() { echo "[ERROR] $*" >&2; exit 1; }

main() {
    log "Starting"
    # your code here
    log "Done"
}

main "$@"

Quiz

1. What flag enables command tracing?

2. Why quote variables?

Show Answers

Answers

-x
prevent word splitting

BASH
SCRIPTING

// Your Training Path

Your First Script

Variables and Parameters

Conditionals

Loops

Functions

Arrays and Strings

Exit Codes and Error Handling

Real-World Automation

Input/Output Redirection

Text Processing Tools

Advanced Arguments

Debugging & Best Practices

// Tools & References

GNU Bash Reference

ShellCheck

Advanced Bash Scripting

Bash Hackers Wiki

explainshell

TLDP Bash Guide

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BASHSCRIPTING

// Your Training Path

Your First Script

Variables and Parameters

Conditionals

Loops

Functions

Arrays and Strings

Exit Codes and Error Handling

Real-World Automation

Input/Output Redirection

Text Processing Tools

Advanced Arguments

Debugging & Best Practices

// Tools & References

GNU Bash Reference

ShellCheck

Advanced Bash Scripting

Bash Hackers Wiki

explainshell

TLDP Bash Guide

// Lesson 01: Your First Script

The Shebang

Creating Your First Script

Comments

Quiz

Answers

// Lesson 02: Variables and Parameters

Creating Variables

Command Substitution

Command-Line Arguments

Environment Variables

Quiz

Answers

// Lesson 03: Conditionals

The if Statement

Comparison Operators

if/else

elif (Else If)

File Tests

Quiz

Answers

// Lesson 04: Loops

For Loops

Loop Over Files

For Loop with Sequence

While Loops

Reading Input

Loop Control

Quiz

Answers

// Lesson 05: Functions

Defining Functions

Calling Functions

Function Parameters

Return Values

Scope: Local Variables

Quiz

Answers

// Lesson 06: Arrays and Strings

Arrays

Array Operations

String Operations

Pattern Matching

Quiz

Answers

// Lesson 07: Exit Codes and Error Handling

Exit Codes

Checking Exit Codes

set Options

Custom Exit Codes

Trap Errors

Quiz

Answers

// Lesson 08: Real-World Automation

Automated Backup Script

System Monitor Script

Batch File Processor

Scheduling with Cron

Quiz

BASH
SCRIPTING