Shell Scripting Basics

What is Shell Scripting?

Shell scripting is writing a series of commands in a file to automate tasks. The shell interprets and executes these commands.

Shebang

The first line of a shell script: ```bash

!/bin/bash

Tells the system which interpreter to use.

## Variables

### Defining Variables
```bash
name="John"
age=25

Using Variables

echo "Hello, $name"
echo "You are $age years old"

Special Variables

$0: Script name
$1, $2, ...: Command line arguments
$#: Number of arguments
$@: All arguments
$?: Exit status of last command
$$: Process ID of script

Basic Commands

echo

echo "Hello World"
echo -n "No newline"  # Suppress newline

read

read name
echo "Hello, $name"

printf

printf "Name: %s, Age: %d\n" "$name" "$age"

Control Structures

if Statement

if [ condition ]; then
    commands
elif [ condition ]; then
    commands
else
    commands
fi

Conditions

[ -f file ]    # File exists
[ -d dir ]     # Directory exists
[ -z string ]  # String is empty
[ string1 = string2 ]  # Strings equal
[ num1 -eq num2 ]  # Numbers equal
[ num1 -gt num2 ]  # Greater than

for Loop

for item in list; do
    commands
done

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

while Loop

while [ condition ]; do
    commands
done

until Loop

until [ condition ]; do
    commands
done

case Statement

case $variable in
    pattern1)
        commands
        ;;
    pattern2)
        commands
        ;;
    *)
        default commands
        ;;
esac

Functions

function_name() {
    commands
}

# Call function
function_name

Input/Output Redirection

Standard Streams

stdin (0): Standard input
stdout (1): Standard output
stderr (2): Standard error

Redirection

command > file      # Redirect stdout to file
command >> file     # Append stdout to file
command 2> file     # Redirect stderr to file
command &> file     # Redirect both stdout and stderr
command < file      # Redirect stdin from file

Pipes

command1 | command2  # Send output of command1 to command2

Command Substitution

current_date=$(date)
files=$(ls)

Arithmetic

result=$((5 + 3))
echo $result  # 8

let result=5*3
echo $result  # 15

Debugging

bash -x script.sh  # Execute with tracing
set -x             # Enable tracing in script
set +x             # Disable tracing

Error Handling

set -e  # Exit on error
set -u  # Exit on undefined variable
trap 'echo "Error occurred"' ERR

Example Script: Backup

#!/bin/bash

# Simple backup script

SOURCE_DIR="/home/user/documents"
BACKUP_DIR="/home/user/backup"
DATE=$(date +%Y%m%d_%H%M%S)

if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
fi

tar -czf "$BACKUP_DIR/backup_$DATE.tar.gz" "$SOURCE_DIR"

echo "Backup completed: $BACKUP_DIR/backup_$DATE.tar.gz"

Best Practices

Use descriptive variable names
Add comments to explain complex parts
Use set -euo pipefail for robust scripts
Quote variables: "$variable"
Use functions for reusable code
Test scripts with different inputs

Practice Exercises

Write a script that greets the user by name.
Create a script that checks if a file exists and displays its size.
Write a backup script similar to the example.
Create a script that processes command line arguments.
Write a script that monitors disk usage and alerts if low.

Next, we'll cover networking basics.