Shell Scripting

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Shell Scripting

1. Shell Script Basics

A shell script is a program that collects commands in a file for automation.

First Script

#!/bin/bash
# This is a comment
echo "Hello, World!"

shebang (#!)

The first line of a script specifies the interpreter.

#!/bin/bash        # bash shell
#!/bin/sh          # standard shell
#!/usr/bin/env bash  # find bash in environment (more portable)

Running Scripts

# 1. Grant execute permission and run
chmod +x script.sh
./script.sh

# 2. Run directly with bash
bash script.sh

# 3. Run with source (in current shell)
source script.sh
. script.sh

2. Variables

Variable Declaration and Usage

#!/bin/bash

# Variable declaration (no spaces around =!)
name="John"
age=25
readonly PI=3.14159    # constant

# Variable usage
echo $name
echo ${name}           # recommended (clearer)
echo "Hello, ${name}!"
echo "Age: $age"

# Delete variable
unset name

Special Variables

Variable Description
$0 Script name
$1` ~ `$9 Positional parameters
$# Number of parameters
$@ All parameters (individual)
$* All parameters (string)
$? Exit status of last command
$$ Current process PID
$! Last background PID 
#!/bin/bash
echo "Script: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Argument count: $#"
echo "All arguments: $@"
echo "PID: $$"

Environment Variables

# View environment variables
env
printenv

# Key environment variables
echo $HOME       # Home directory
echo $USER       # Username
echo $PATH       # Execution path
echo $PWD        # Current directory
echo $SHELL      # Current shell

# Set environment variable
export MY_VAR="value"

# Set within script
#!/bin/bash
export PATH="$PATH:/opt/myapp/bin"

Variable Default Values

# Use default value if variable is unset
name=${name:-"default"}

# Assign default value if variable is unset
name=${name:="default"}

# Error if variable is unset
name=${name:?"Variable not set"}

3. Input

read Command

#!/bin/bash

# Basic input
echo -n "Enter your name: "
read name
echo "Hello, $name!"

# With prompt
read -p "Enter your age: " age
echo "You are ${age} years old."

# Password input (hidden)
read -sp "Password: " password
echo
echo "Password set."

# Timeout
read -t 5 -p "Enter within 5 seconds: " input

# Multiple variables
read -p "Name and age: " name age
echo "$name, $age"

Command-line Arguments

#!/bin/bash
# Usage: ./script.sh arg1 arg2

if [ $# -lt 2 ]; then
    echo "Usage: $0 <name> <age>"
    exit 1
fi

name=$1
age=$2
echo "$name is ${age} years old."

4. Conditionals

if Statement

#!/bin/bash

# Basic format
if [ condition ]; then
    command
fi

# if-else
if [ condition ]; then
    command1
else
    command2
fi

# if-elif-else
if [ condition1 ]; then
    command1
elif [ condition2 ]; then
    command2
else
    command3
fi

Comparison Operators

Numeric Comparison

Operator Description
-eq Equal
-ne Not equal
-gt Greater than
-ge Greater or equal
-lt Less than
-le Less or equal 
#!/bin/bash
num=10

if [ $num -gt 5 ]; then
    echo "$num is greater than 5."
fi

if [ $num -eq 10 ]; then
    echo "$num is 10."
fi

String Comparison

Operator Description
= Equal
!= Not equal
-z Empty string
-n Not empty 
#!/bin/bash
str="hello"

if [ "$str" = "hello" ]; then
    echo "String is hello."
fi

if [ -z "$empty" ]; then
    echo "Variable is empty."
fi

if [ -n "$str" ]; then
    echo "Variable has value."
fi

File Tests

Operator Description
-e File exists
-f Regular file
-d Directory
-r Readable
-w Writable
-x Executable
-s File size > 0 
#!/bin/bash
file="/etc/passwd"

if [ -e "$file" ]; then
    echo "File exists."
fi

if [ -f "$file" ]; then
    echo "Regular file."
fi

if [ -d "/home" ]; then
    echo "Directory."
fi

if [ -r "$file" ]; then
    echo "Readable."
fi

Logical Operators

#!/bin/bash

# AND
if [ $a -gt 0 ] && [ $b -gt 0 ]; then
    echo "Both positive"
fi

# OR
if [ $a -gt 0 ] || [ $b -gt 0 ]; then
    echo "At least one positive"
fi

# NOT
if [ ! -f "file.txt" ]; then
    echo "File doesn't exist."
fi

# Using [[ ]] (recommended)
if [[ $a -gt 0 && $b -gt 0 ]]; then
    echo "Both positive"
fi

case Statement

#!/bin/bash
read -p "Choose a fruit (apple/banana/orange): " fruit

case $fruit in
    apple)
        echo "You chose apple."
        ;;
    banana)
        echo "You chose banana."
        ;;
    orange)
        echo "You chose orange."
        ;;
    *)
        echo "Unknown fruit."
        ;;
esac

5. Loops

for Loop

#!/bin/bash

# List iteration
for name in Alice Bob Charlie; do
    echo "Hello, $name!"
done

# Range iteration
for i in {1..5}; do
    echo "Number: $i"
done

# With increment
for i in {0..10..2}; do
    echo "Even: $i"
done

# C-style
for ((i=0; i<5; i++)); do
    echo "Index: $i"
done

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

# Command output
for user in $(cat /etc/passwd | cut -d: -f1); do
    echo "User: $user"
done

while Loop

#!/bin/bash

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

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

# Infinite loop
while true; do
    echo "Running... (Ctrl+C to exit)"
    sleep 1
done

until Loop

#!/bin/bash

# Repeat until condition is true
count=1
until [ $count -gt 5 ]; do
    echo "Count: $count"
    ((count++))
done

break and continue

#!/bin/bash

# break - exit loop
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break
    fi
    echo $i
done

# continue - skip to next iteration
for i in {1..5}; do
    if [ $i -eq 3 ]; then
        continue
    fi
    echo $i
done

6. Functions

Function Definition

#!/bin/bash

# Method 1
function greet() {
    echo "Hello, $1!"
}

# Method 2
say_bye() {
    echo "Goodbye, $1!"
}

# Call
greet "World"
say_bye "World"

Function Parameters

#!/bin/bash

print_info() {
    echo "Name: $1"
    echo "Age: $2"
    echo "Argument count: $#"
}

print_info "John" 25

Return Values

#!/bin/bash

# return for exit status (0-255)
is_even() {
    if [ $(($1 % 2)) -eq 0 ]; then
        return 0    # true
    else
        return 1    # false
    fi
}

if is_even 4; then
    echo "Even number."
fi

# echo for value return
add() {
    echo $(($1 + $2))
}

result=$(add 5 3)
echo "5 + 3 = $result"

Local Variables

#!/bin/bash

my_func() {
    local local_var="I'm local"
    global_var="I'm global"
    echo "$local_var"
}

my_func
echo "$global_var"     # prints
echo "$local_var"      # doesn't print

7. Arrays

#!/bin/bash

# Array declaration
fruits=("apple" "banana" "orange")

# Access by index
echo ${fruits[0]}     # apple
echo ${fruits[1]}     # banana

# All elements
echo ${fruits[@]}

# Element count
echo ${#fruits[@]}

# Add element
fruits+=("grape")

# Iterate array
for fruit in "${fruits[@]}"; do
    echo $fruit
done

# With index
for i in "${!fruits[@]}"; do
    echo "$i: ${fruits[$i]}"
done

8. Practical Script Examples

Backup Script

#!/bin/bash
# backup.sh - Directory backup script

SOURCE_DIR="${1:-/var/www}"
BACKUP_DIR="${2:-/backup}"
DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_FILE="backup_${DATE}.tar.gz"

# Check backup directory
if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
fi

# Check source directory
if [ ! -d "$SOURCE_DIR" ]; then
    echo "Error: $SOURCE_DIR directory not found."
    exit 1
fi

# Execute backup
echo "Starting backup: $SOURCE_DIR -> $BACKUP_DIR/$BACKUP_FILE"
tar -czvf "$BACKUP_DIR/$BACKUP_FILE" -C "$(dirname $SOURCE_DIR)" "$(basename $SOURCE_DIR)"

if [ $? -eq 0 ]; then
    echo "Backup complete: $BACKUP_DIR/$BACKUP_FILE"

    # Delete backups older than 30 days
    find "$BACKUP_DIR" -name "backup_*.tar.gz" -mtime +30 -delete
    echo "Old backups cleaned"
else
    echo "Backup failed!"
    exit 1
fi

Server Health Check Script

#!/bin/bash
# health_check.sh - Server health check

LOG_FILE="/var/log/health_check.log"

log_message() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a "$LOG_FILE"
}

check_disk() {
    local usage=$(df -h / | awk 'NR==2 {print $5}' | tr -d '%')
    if [ "$usage" -gt 80 ]; then
        log_message "Warning: Disk usage ${usage}%"
        return 1
    fi
    log_message "Disk: ${usage}% used"
    return 0
}

check_memory() {
    local usage=$(free | awk '/Mem:/ {printf("%.0f", $3/$2 * 100)}')
    if [ "$usage" -gt 80 ]; then
        log_message "Warning: Memory usage ${usage}%"
        return 1
    fi
    log_message "Memory: ${usage}% used"
    return 0
}

check_service() {
    local service=$1
    if systemctl is-active --quiet "$service"; then
        log_message "Service $service: running"
        return 0
    else
        log_message "Warning: Service $service stopped"
        return 1
    fi
}

# Main
log_message "===== Health check started ====="
check_disk
check_memory
check_service "sshd"
check_service "nginx" 2>/dev/null || true
log_message "===== Health check completed ====="

Log Analysis Script

#!/bin/bash
# log_analyzer.sh - Log analysis

LOG_FILE="${1:-/var/log/syslog}"

if [ ! -f "$LOG_FILE" ]; then
    echo "File not found: $LOG_FILE"
    exit 1
fi

echo "=== Log Analysis: $LOG_FILE ==="
echo

echo "=== Error Count ==="
grep -ci "error" "$LOG_FILE"

echo
echo "=== Recent 10 Errors ==="
grep -i "error" "$LOG_FILE" | tail -10

echo
echo "=== Log Count by Hour ==="
awk '{print $3}' "$LOG_FILE" | cut -d: -f1 | sort | uniq -c | sort -k2n

User Creation Script

#!/bin/bash
# create_users.sh - Batch user creation from file
# Usage: sudo ./create_users.sh users.txt
# users.txt format: username:password:groupname

USER_FILE="$1"

if [ -z "$USER_FILE" ] || [ ! -f "$USER_FILE" ]; then
    echo "Usage: $0 <userfile>"
    exit 1
fi

while IFS=: read -r username password groupname; do
    # Skip empty lines or comments
    [[ -z "$username" || "$username" =~ ^# ]] && continue

    # Check if user exists
    if id "$username" &>/dev/null; then
        echo "User exists: $username (skipped)"
        continue
    fi

    # Create group if it doesn't exist
    if ! getent group "$groupname" &>/dev/null; then
        groupadd "$groupname"
        echo "Group created: $groupname"
    fi

    # Create user
    useradd -m -s /bin/bash -g "$groupname" "$username"
    echo "$username:$password" | chpasswd
    echo "User created: $username (group: $groupname)"

done < "$USER_FILE"

echo "Done!"

9. Debugging

Debug Mode

# Trace script execution
bash -x script.sh

# Enable within script
#!/bin/bash
set -x    # Start debug
# commands...
set +x    # Stop debug

# Exit on error
set -e

# Error on undefined variable
set -u

# Detect pipe errors
set -o pipefail

# Best practice (script start)
#!/bin/bash
set -euo pipefail

10. Practice Exercises

Exercise 1: Simple Calculator

#!/bin/bash
# calculator.sh

read -p "First number: " num1
read -p "Operator (+, -, *, /): " op
read -p "Second number: " num2

case $op in
    +) result=$((num1 + num2)) ;;
    -) result=$((num1 - num2)) ;;
    \*) result=$((num1 * num2)) ;;
    /) result=$((num1 / num2)) ;;
    *) echo "Invalid operator"; exit 1 ;;
esac

echo "Result: $num1 $op $num2 = $result"

Exercise 2: File Organization Script

#!/bin/bash
# organize.sh - Organize files by extension

DIR="${1:-.}"

for file in "$DIR"/*; do
    [ -f "$file" ] || continue

    ext="${file##*.}"
    mkdir -p "$DIR/$ext"
    mv "$file" "$DIR/$ext/"
    echo "Moved: $file -> $DIR/$ext/"
done

Next Steps

Let's learn about network management in 10_Network_Basics.md!

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