Complete Guide to Creating Simple Shell Scripts

Table of Contents

Introduction to Shell Scripting {#introduction}

Shell scripting is a powerful way to automate tasks in Unix-like operating systems. A shell script is a text file containing a series of commands that are executed sequentially by the shell interpreter.

What is a Shell Script?

A shell script is essentially a program written for the Unix shell. It allows you to: - Automate repetitive tasks - Combine multiple commands into a single executable file - Create custom utilities and tools - Perform system administration tasks - Process files and data

Types of Shells

| Shell Type | Description | Command | Features | |------------|-------------|---------|----------| | Bash | Bourne Again Shell | /bin/bash | Most common, feature-rich | | Sh | Bourne Shell | /bin/sh | POSIX compliant, basic | | Zsh | Z Shell | /bin/zsh | Advanced features, customizable | | Fish | Friendly Interactive Shell | /bin/fish | User-friendly, syntax highlighting | | Dash | Debian Almquist Shell | /bin/dash | Lightweight, fast |

Shell Script Basics {#basics}

Script Structure

Every shell script follows a basic structure:

`bash #!/bin/bash

This is a comment

Script description and author information

Variable declarations

Main script logic

Exit status

Shebang Line

The shebang (#!) line tells the system which interpreter to use:

| Shebang | Interpreter | Usage | |---------|-------------|-------| | #!/bin/bash | Bash shell | Most common for Linux | | #!/bin/sh | Bourne shell | POSIX compatibility | | #!/usr/bin/env bash | Bash via env | Portable across systems | | #!/bin/zsh | Z shell | Advanced features |

Comments

Comments are essential for documentation:

`bash

Single line comment

Multi-line comment block

Creating Your First Script {#first-script}

Step-by-Step Process

1. Create the script file: `bash touch myscript.sh `

2. Edit the script: `bash nano myscript.sh

or

3. Add content: `bash #!/bin/bash echo "Hello, World!" echo "This is my first shell script" `

4. Make it executable: `bash chmod +x myscript.sh `

5. Run the script: `bash ./myscript.sh

or

File Permissions

Understanding file permissions is crucial:

| Permission | Numeric | Symbolic | Description | |------------|---------|----------|-------------| | Read | 4 | r | Can read file content | | Write | 2 | w | Can modify file | | Execute | 1 | x | Can run as program |

Common permission combinations: - 755 (rwxr-xr-x): Owner can read/write/execute, others can read/execute - 644 (rw-r--r--): Owner can read/write, others can read only - 700 (rwx------): Only owner has full access

Variables and Data Types {#variables}

Variable Declaration and Usage

`bash #!/bin/bash

Variable assignment (no spaces around =)

Using variables

Read-only variables

Environment variables

Variable Types

| Type | Declaration | Example | Description | |------|-------------|---------|-------------| | String | var="text" | name="Alice" | Text data | | Integer | var=123 | count=10 | Numeric data | | Array | var=(a b c) | fruits=(apple banana) | Multiple values | | Readonly | readonly var=value | readonly PI=3.14 | Immutable | | Environment | export var=value | export PATH=/usr/bin | Global scope |

Special Variables

`bash #!/bin/bash

echo "Script name: $0" echo "First argument: $1" echo "Second argument: $2" echo "All arguments: $@" echo "Number of arguments: $#" echo "Process ID: $" echo "Exit status of last command: $?" `

| Variable | Description | Example | |----------|-------------|---------| | $0 | Script name | ./script.sh | | $1, $2, ... | Positional parameters | First, second argument | | $@ | All arguments as separate words | "arg1" "arg2" "arg3" | | $* | All arguments as single word | "arg1 arg2 arg3" | | $# | Number of arguments | 3 | | $ | Current process ID | 1234 | | $? | Exit status of last command | 0 for success | | $! | Process ID of last background job | 5678 |

Input/Output Operations {#input-output}

Reading User Input

`bash #!/bin/bash

Simple input

Input with prompt

Silent input (for passwords)

Reading multiple values

Reading with timeout

Output Commands

| Command | Purpose | Example | Notes | |---------|---------|---------|-------| | echo | Display text | echo "Hello" | Adds newline by default | | printf | Formatted output | printf "%s\n" "Hello" | More control over formatting | | cat | Display file content | cat file.txt | Concatenate and display | | tee | Write to file and stdout | echo "text" \| tee file.txt | Useful for logging |

Printf Formatting

`bash #!/bin/bash

name="Alice" age=25 height=5.8

String formatting

Integer formatting

Float formatting

Padding and alignment

Multiple values

Control Structures {#control-structures}

Conditional Statements

#### If-Then-Else

`bash #!/bin/bash

age=18

if [ $age -ge 18 ]; then echo "You are an adult" elif [ $age -ge 13 ]; then echo "You are a teenager" else echo "You are a child" fi

File testing

String comparison

#### Test Conditions

| Test | Description | Example | |------|-------------|---------| | -eq | Equal (numeric) | [ $a -eq $b ] | | -ne | Not equal (numeric) | [ $a -ne $b ] | | -lt | Less than | [ $a -lt $b ] | | -le | Less than or equal | [ $a -le $b ] | | -gt | Greater than | [ $a -gt $b ] | | -ge | Greater than or equal | [ $a -ge $b ] | | = | String equal | [ "$a" = "$b" ] | | != | String not equal | [ "$a" != "$b" ] | | -z | String is empty | [ -z "$string" ] | | -n | String is not empty | [ -n "$string" ] |

#### File Test Operators

| Test | Description | Example | |------|-------------|---------| | -f | Regular file | [ -f "file.txt" ] | | -d | Directory | [ -d "mydir" ] | | -e | File exists | [ -e "path" ] | | -r | Readable | [ -r "file.txt" ] | | -w | Writable | [ -w "file.txt" ] | | -x | Executable | [ -x "script.sh" ] | | -s | File not empty | [ -s "file.txt" ] |

Case Statements

`bash #!/bin/bash

read -p "Enter a grade (A-F): " grade

case $grade in A|a) echo "Excellent!" ;; B|b) echo "Good job!" ;; C|c) echo "Average" ;; D|d) echo "Below average" ;; F|f) echo "Failed" ;; *) echo "Invalid grade" ;; esac `

Loops

#### For Loops

`bash #!/bin/bash

Simple for loop

Range-based for loop

Step increment

C-style for loop

Iterating over files

Iterating over arrays

#### While Loops

`bash #!/bin/bash

Simple while loop

Reading file line by line

Infinite loop with break

#### Until Loops

`bash #!/bin/bash

counter=1 until [ $counter -gt 5 ]; do echo "Counter: $counter" ((counter++)) done `

Functions {#functions}

Function Declaration and Usage

`bash #!/bin/bash

Function definition

Function with multiple parameters

Function with return value

Using functions

if is_even 4; then echo "4 is even" fi `

Function Features

| Feature | Syntax | Example | Description | |---------|--------|---------|-------------| | Local variables | local var=value | local name="John" | Variable scope within function | | Parameters | $1, $2, ... | function_name param1 param2 | Access function arguments | | Return value | return n | return 0 | Exit status (0-255) | | Function call | function_name args | greet "Alice" | Execute function |

Advanced Function Example

`bash #!/bin/bash

Function with error handling and validation