Shell Scripting Basics: First Step to Automation

January 11, 2025 Reading Time: About 15 min Difficulty: Beginner~Intermediate

Shell scripting is a powerful tool for automating tasks in Linux systems. In this basics edition, you'll learn Bash script syntax from variables and conditionals to loops, with practical examples throughout.

Your First Shell Script
Variables and Input
Conditionals
Loops

1. Your First Shell Script

Basic Structure

First Script

#!/bin/bash
# This is a comment

echo "Hello, Shell Script!"
echo "Current date/time: $(date)"
echo "Username: $USER"

Creating and Running Scripts

Step 1: Create File

$ nano hello.sh

Enter the above code and save

Step 2: Add Execute Permission

$ chmod +x hello.sh

Step 3: Run Script

$ ./hello.sh

Hello, Shell Script!
Current date/time: Mon Jan 11 14:30:00 JST 2025
Username: user

About Shebang

#!/bin/bash is called a shebang and specifies the interpreter to execute this file.

#!/bin/bash - Use Bash
#!/bin/sh - Use POSIX shell
#!/usr/bin/env bash - Auto-detect from environment

2. Variables and Input

Variable Basics

Defining and Using Variables

#!/bin/bash

# Define variables (no spaces around =)
name="Linux User"
age=25
today=$(date +%Y-%m-%d)

# Use variables
echo "Name: $name"
echo "Age: ${age} years old"
echo "Today's date: $today"

Calculation Example

#!/bin/bash

num1=10
num2=3

# Numeric calculations
sum=$((num1 + num2))
diff=$((num1 - num2))
product=$((num1 * num2))
quotient=$((num1 / num2))

echo "Addition: $num1 + $num2 = $sum"
echo "Subtraction: $num1 - $num2 = $diff"
echo "Multiplication: $num1 × $num2 = $product"
echo "Division: $num1 ÷ $num2 = $quotient"

Special Variables

Variable	Description	Example
$0	Script name	./script.sh
$1, $2, ...	Command line arguments	1st arg, 2nd arg
$#	Number of arguments	3 for 3 arguments
$@	All arguments	"arg1" "arg2" "arg3"
$?	Exit status of last command	0 for success, non-0 for failure
$$	Current process ID	12345
$USER	Current username	user
$HOME	Home directory	/home/user
$PWD	Current directory	/home/user/scripts

User Input

Basic Input

#!/bin/bash

echo "Please enter your name:"
read name
echo "Hello, ${name}!"

Input with Prompt

#!/bin/bash

read -p "Enter your age: " age
read -s -p "Enter password: " password
echo
echo "Age: $age"
echo "Password entered silently"

Multiple Values at Once

#!/bin/bash

echo "Enter name and age (space-separated):"
read name age
echo "Name: $name, Age: $age"

3. Conditionals

if Statement

Basic if Statement

#!/bin/bash

read -p "Enter a number: " num

if [ $num -gt 0 ]; then
    echo "$num is positive"
elif [ $num -lt 0 ]; then
    echo "$num is negative"
else
    echo "$num is zero"
fi

File Existence Check

#!/bin/bash

filename="test.txt"

if [ -f "$filename" ]; then
    echo "$filename exists"
    echo "File size: $(wc -c < "$filename") bytes"
else
    echo "$filename does not exist"
    echo "Creating file..."
    touch "$filename"
fi

Conditional Operators

Numeric Comparison

Operator	Meaning	Example
-eq	Equal	[ $a -eq $b ]
-ne	Not equal	[ $a -ne $b ]
-gt	Greater than	[ $a -gt $b ]
-ge	Greater or equal	[ $a -ge $b ]
-lt	Less than	[ $a -lt $b ]
-le	Less or equal	[ $a -le $b ]

String Comparison

Operator	Meaning	Example
=	Equal	[ "$a" = "$b" ]
!=	Not equal	[ "$a" != "$b" ]
-z	Empty string	[ -z "$str" ]
-n	Not empty	[ -n "$str" ]

File Tests

Operator	Meaning	Example
-f	Regular file	[ -f file.txt ]
-d	Directory	[ -d /home ]
-e	Exists	[ -e path ]
-r	Readable	[ -r file ]
-w	Writable	[ -w file ]
-x	Executable	[ -x script ]

case Statement

Menu Selection Example

#!/bin/bash

echo "Select an option:"
echo "1) List files"
echo "2) Show current time"
echo "3) Show system info"
echo "4) Exit"

read -p "Choice (1-4): " choice

case $choice in
    1)
        echo "=== File List ==="
        ls -la
        ;;
    2)
        echo "=== Current Time ==="
        date
        ;;
    3)
        echo "=== System Info ==="
        uname -a
        ;;
    4)
        echo "Exiting."
        exit 0
        ;;
    *)
        echo "Invalid choice."
        ;;
esac

4. Loops

for Loop

Basic for Loop

#!/bin/bash

# Numeric range
for i in {1..5}
do
    echo "Count: $i"
done

echo "---"

# File processing
for file in *.txt
do
    echo "Processing: $file"
    wc -l "$file"
done

Loop with Arrays

#!/bin/bash

# Define array
fruits=("apple" "banana" "orange" "grape")

echo "Fruit list:"
for fruit in "${fruits[@]}"
do
    echo "- $fruit"
done

C-style for Loop

#!/bin/bash

echo "Multiplication table (partial):"
for ((i=1; i<=5; i++))
do
    for ((j=1; j<=5; j++))
    do
        result=$((i * j))
        printf "%2d " $result
    done
    echo
done

while Loop

Basic while Loop

#!/bin/bash

count=1
while [ $count -le 5 ]
do
    echo "Loop iteration $count"
    count=$((count + 1))
done

Reading Files

#!/bin/bash

filename="data.txt"

if [ -f "$filename" ]; then
    while IFS= read -r line
    do
        echo "Read: $line"
    done < "$filename"
else
    echo "File $filename not found"
fi

until Loop

Repeat Until Condition is True

#!/bin/bash

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

Loop Control

break - Exit loop
continue - Skip to next iteration

#!/bin/bash

for i in {1..10}
do
    if [ $i -eq 3 ]; then
        echo "Skipping 3"
        continue
    fi

    if [ $i -eq 8 ]; then
        echo "Stopping at 8"
        break
    fi

    echo "Number: $i"
done

⚠️ Common Beginner Mistakes and Pitfalls

Detailed explanations of common problems beginners frequently encounter and how to avoid them.

🚫 Mistake 1: Spaces in Variable Assignment

❌ Error-causing Examples

name = "Taro"     # Space before/after =
age= 25          # Space after =
city ="Tokyo"     # Space before =

Results in "command not found" error.

✅ Correct Usage

name="Taro"      # No spaces around =
age=25          # No spaces around =
city="Tokyo"     # No spaces around =

Variable assignment must have no spaces around =.

🚫 Mistake 2: Missing Braces in Variable Reference

❌ Unexpected Results

filename="test"
echo "$filenameback.txt"    # Empty
echo "$filename_backup"     # Empty

Variable name boundary unclear, not expanded properly.

✅ Correct Usage

filename="test"
echo "${filename}back.txt"   # testback.txt
echo "${filename}_backup"    # test_backup

Use {} to clearly define variable name scope.

🚫 Mistake 3: Missing Spaces in if Statement Conditions

❌ Error-causing Examples

if [$num -gt 5]; then       # No space after [
if [ $num -gt 5]; then       # No space before ]
if [$num -gt 5 ]; then       # No space after [

Syntax errors occur.

✅ Correct Usage

if [ $num -gt 5 ]; then     # Spaces inside [ ] required
if [[ $num -gt 5 ]]; then    # Same for [[ ]]
if (( $num > 5 )); then      # Arithmetic expression

Always include spaces inside [ ].

🚫 Mistake 4: Missing Quotes in String Comparison

❌ Dangerous Examples

if [ $name = John Doe ]; then    # Issues with spaces
if [ $empty_var = "" ]; then     # Error when empty

Errors occur when variable is empty or contains spaces.

✅ Safe Usage

if [ "$name" = "John Doe" ]; then    # Quote both sides
if [ "$empty_var" = "" ]; then       # No error when empty
if [ -z "$var" ]; then               # Dedicated empty check option

Always quote variables for safety.

🚫 Mistake 5: Old Command Substitution Syntax

❌ Old Syntax (Not Recommended)

date=`date`                 # Backticks
files=`ls *.txt`            # Difficult to nest
result=`cat `which ls``     # Nesting causes error

Backticks are hard to nest and less readable.

✅ Modern Syntax (Recommended)

date=$(date)                # Use $()
files=$(ls *.txt)           # More readable
result=$(cat $(which ls))   # Easy to nest

Use $() notation for readability and easy nesting.

🚫 Mistake 6: Arithmetic Operation Errors

❌ Error-causing Examples

result = $num1 + $num2      # Treated as string in shell
sum="$a + $b"               # Not calculated

Shell treats everything as strings by default.

✅ Correct Calculation Methods

result=$((num1 + num2))     # Arithmetic expansion
sum=$(($a + $b))            # Calculate inside parentheses
let "result = num1 + num2"  # Use let command

Use $(()) or let for arithmetic operations.

💡 Basic Rules to Prevent Mistakes

📝 Writing Basics

Variable assignment: No spaces around =
Variable usage: Always quote "$var"
Complex variables: Use braces "${var}"

🔍 Conditional Basics

Using [ ]: Always include internal spaces
String comparison: Quote both sides
Numeric comparison: Use -eq, -gt, -lt, etc.

⚙️ Execution and Testing

Incremental development: Add features gradually and test
Error checking: set -e to stop on error
Debugging: set -x to display command execution

🐛 Debugging and Troubleshooting

Basic Debugging Techniques

#!/bin/bash

# Enable debug mode
set -x              # Display command execution
set -e              # Stop on error
set -u              # Error on undefined variables

# Check variable contents
echo "DEBUG: var = [$var]"

# Check file existence
if [ ! -f "$filename" ]; then
    echo "ERROR: File $filename not found" >&2
    exit 1
fi

Common Error Messages and Solutions

"command not found"

Unnecessary spaces in variable assignment
Missing execute permission → chmod +x script.sh
Incorrect shebang

"syntax error"

Missing spaces inside [ ] in if statement
Quote mismatch
Missing keywords (then, fi, done, etc.)

"Permission denied"

No execute permission → chmod +x script.sh
No write permission to file
No execute permission on directory

🚀 Next Steps

After mastering shell script basics, learn more advanced techniques in the practical edition:

📋 What You'll Learn in Practical Edition

Functions - Code reuse and structure
Arrays - Efficient handling of multiple data
File Operations - Advanced read/write and file handling
Error Handling - Creating robust scripts
Practical Examples - Scripts for real work scenarios

Continue to Practical Edition →

📋 Complete Shell Scripting Series

Basics (This Article) - Variables, conditionals, loops, functions
Practical Edition - Real-world scenarios, error handling, optimization

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Target Level: Beginner~Intermediate

Comprehensive Linux textbook including shell scripting chapters. Learn progressively from basic commands through variables, conditionals, and loops. Ideal for those wanting systematic understanding of shell script fundamentals.

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Target Level: Beginner~Intermediate (Specialized Reference)

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Target Level: Intermediate~Advanced

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🎉 Master Shell Scripting Through Practice

After gaining knowledge from books, practice hands-on with Penguin Gym Linux challenges to solidify your learning. Actually using variables, conditionals, and loops ensures skill development.

🐧 Practice with Penguin Gym Linux 📚 Next Article: Shell Scripting Practical

Shell Scripting Basics: First Step to Automation

Table of Contents

1. Your First Shell Script

Basic Structure

First Script

Creating and Running Scripts

Step 1: Create File

Step 2: Add Execute Permission

Step 3: Run Script

About Shebang

2. Variables and Input

Variable Basics

Defining and Using Variables

Calculation Example

Special Variables

User Input

Basic Input

Input with Prompt

Multiple Values at Once

3. Conditionals

if Statement

Basic if Statement

File Existence Check

Conditional Operators

Numeric Comparison

String Comparison

File Tests

case Statement

Menu Selection Example

4. Loops

for Loop

Basic for Loop

Loop with Arrays

C-style for Loop

while Loop

Basic while Loop

Reading Files

until Loop

Repeat Until Condition is True

Loop Control

⚠️ Common Beginner Mistakes and Pitfalls

🚫 Mistake 1: Spaces in Variable Assignment

❌ Error-causing Examples

✅ Correct Usage

🚫 Mistake 2: Missing Braces in Variable Reference

❌ Unexpected Results

✅ Correct Usage

🚫 Mistake 3: Missing Spaces in if Statement Conditions

❌ Error-causing Examples

✅ Correct Usage

🚫 Mistake 4: Missing Quotes in String Comparison

❌ Dangerous Examples

✅ Safe Usage

🚫 Mistake 5: Old Command Substitution Syntax

❌ Old Syntax (Not Recommended)

✅ Modern Syntax (Recommended)

🚫 Mistake 6: Arithmetic Operation Errors

❌ Error-causing Examples

✅ Correct Calculation Methods

💡 Basic Rules to Prevent Mistakes

📝 Writing Basics

🔍 Conditional Basics

⚙️ Execution and Testing

🐛 Debugging and Troubleshooting

Basic Debugging Techniques

Common Error Messages and Solutions

"command not found"

"syntax error"

"Permission denied"

🚀 Next Steps

📋 What You'll Learn in Practical Edition

📋 Complete Shell Scripting Series

🔗 Related Articles

📚 Recommended Books for Shell Scripting

📚 New Linux Textbook (Japanese)

📚 Shell Script Basic Reference (Japanese)

📚 Linux for Beginners - Learning While Resolving Questions (Japanese)

📚 Linux Server Construction & Operations Guide from Scratch 2nd Ed (Japanese)

🎉 Master Shell Scripting Through Practice