Linux Shell Scripting Guide

#!/bin/bash
# This is a comment
# Shebang line tells system which interpreter to use

echo "Hello, World!"

# Create script
nano myscript.sh

# Make executable
chmod +x myscript.sh

# Run script
./myscript.sh

# Or
bash myscript.sh

#!/bin/bash

# Variable assignment (no spaces around =)
NAME="John"
AGE=25
TODAY=$(date +%Y-%m-%d)

# Using variables
echo "Name: $NAME"
echo "Age: ${AGE}"
echo "Today is $TODAY"

# Read-only variable
readonly PI=3.14159

# Unset variable
unset NAME

#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "All arguments: $@"
echo "Number of arguments: $#"
echo "Exit status of last command: $?"
echo "Process ID: $$"
echo "Current directory: $PWD"
echo "Home directory: $HOME"
echo "Current user: $USER"

#!/bin/bash

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

# One-line prompt
read -p "Enter your age: " age

# Silent input (passwords)
read -sp "Enter password: " password
echo

# Read into array
read -a colors
echo "First color: ${colors[0]}"

# Read with timeout
read -t 5 -p "Quick! Enter something: " input

#!/bin/bash

# Check number of arguments
if [ $# -eq 0 ]; then
    echo "Usage: $0 <filename>"
    exit 1
fi

# Use arguments
FILE=$1
OPTION=${2:-default}  # Default value if not provided

echo "Processing file: $FILE"
echo "Option: $OPTION"

#!/bin/bash

# Basic if
if [ $AGE -gt 18 ]; then
    echo "Adult"
fi

# If-else
if [ $AGE -ge 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi

# If-elif-else
if [ $AGE -lt 13 ]; then
    echo "Child"
elif [ $AGE -lt 18 ]; then
    echo "Teenager"
else
    echo "Adult"
fi

# Multiple conditions
if [ $AGE -gt 18 ] && [ "$NAME" != "" ]; then
    echo "Valid adult user"
fi

# Numeric comparisons
-eq    # Equal
-ne    # Not equal
-gt    # Greater than
-ge    # Greater than or equal
-lt    # Less than
-le    # Less than or equal

# String comparisons
=      # Equal
!=     # Not equal
-z     # Empty string
-n     # Not empty

# File tests
-e     # Exists
-f     # Is regular file
-d     # Is directory
-r     # Is readable
-w     # Is writable
-x     # Is executable
-s     # Not empty

#!/bin/bash

read -p "Enter fruit: " fruit

case $fruit in
    apple)
        echo "Red or green"
        ;;
    banana)
        echo "Yellow"
        ;;
    orange|lemon)
        echo "Citrus fruit"
        ;;
    *)
        echo "Unknown fruit"
        ;;
esac

#!/bin/bash

# Iterate over list
for item in apple banana orange; do
    echo "Fruit: $item"
done

# Iterate over files
for file in *.txt; do
    echo "Processing: $file"
done

# Range
for i in {1..10}; do
    echo "Number:$i"
done

# C-style for loop
for ((i=0; i<10; i++)); do
    echo "Count: $i"
done

# Array iteration
fruits=(apple banana orange)
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

#!/bin/bash

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

# Read file line by line
while IFS= read -r line; do
    echo "Line: $line"
done < file.txt

# Infinite loop
while true; do
    echo "Press Ctrl+C to stop"
    sleep 1
done

#!/bin/bash

count=0
until [ $count -ge 5 ]; do
    echo "Count: $count"
    ((count++))
done

#!/bin/bash

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

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

# Call functions
greet "John"
say_goodbye "Jane"

#!/bin/bash

add_numbers() {
    local num1=$1
    local num2=$2
    local sum=$((num1 + num2))
    echo $sum
}

result=$(add_numbers 5 3)
echo "Sum: $result"

# Return status code
check_file() {
    if [ -f "$1" ]; then
        return 0
    else
        return 1
    fi
}

if check_file "test.txt"; then
    echo "File exists"
else
    echo "File not found"
fi

#!/bin/bash

# Declare array
fruits=("apple" "banana" "orange")

# Access elements
echo "${fruits[0]}"    # First element
echo "${fruits[@]}"    # All elements
echo "${#fruits[@]}"   # Array length

# Add element
fruits+=("grape")

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

# Associative array (dictionary)
declare -A colors
colors[apple]="red"
colors[banana]="yellow"

echo "${colors[apple]}"    # red

# Iterate associative array
for key in "${!colors[@]}"; do
    echo "$key: ${colors[$key]}"
done

#!/bin/bash

text="Hello, World!"

# Length
echo ${#text}

# Substring
echo ${text:7:5}    # "World"

# Replace
echo ${text/World/Bash}    # "Hello, Bash!"

# Uppercase/Lowercase
echo ${text^^}      # HELLO, WORLD!
echo ${text,,}      # hello, world!

# Remove prefix/suffix
file="document.txt"
echo ${file%.txt}   # "document"
echo ${file#doc}    # "ument.txt"

#!/bin/bash

# Read entire file
content=$(cat file.txt)

# Read line by line
while IFS= read -r line; do
    echo "$line"
done < file.txt

# Read into array
mapfile -t lines < file.txt

#!/bin/bash

# Overwrite file
echo "New content" > file.txt

# Append to file
echo "Additional content" >> file.txt

# Here document
cat > config.txt << EOF
Setting1=value1
Setting2=value2
EOF

#!/bin/bash

# Check last command status
ls /nonexistent 2>/dev/null
if [ $? -ne 0 ]; then
    echo "Command failed"
fi

# Exit with status code
if [ ! -f "$FILE" ]; then
    echo "Error: File not found"
    exit 1
fi

#!/bin/bash

# Exit on error
set -e

# Exit on undefined variable
set -u

# Exit on pipe failure
set -o pipefail

# Trap errors
trap  'echo "Error on line $LINENO"' ERR

# Cleanup on exit
trap 'rm -f /tmp/tempfile' EXIT

#!/bin/bash

# Backup script
SOURCE="/var/www"
DEST="/backup"
DATE=$(date +%Y%m%d-%H%M%S)
FILENAME="backup-$DATE.tar.gz"

# Create backup
tar -czf "$DEST/$FILENAME" "$SOURCE"

# Check if successful
if [ $? -eq 0 ]; then
    echo "Backup successful: $FILENAME"

    # Clean old backups (keep last 7 days)
    find "$DEST" -name "backup-*.tar.gz" -mtime +7 -delete
else
    echo "Backup failed!" >&2
    exit 1
fi

#!/bin/bash

# System monitoring script
while true; do
    clear
    echo "=== System Monitor ==="
    echo "Time: $(date)"
    echo ""

    echo "CPU Usage:"
    top -bn1 | grep "Cpu(s)" | awk '{print $2 "%"}'

    echo "Memory Usage:"
    free -h | awk '/^Mem:/ {print $3 "/" $2}'

    echo "Disk Usage:"
    df -h | grep "^/dev" | awk '{print $1 ": " $5}'

    sleep 5
done

#!/bin/bash

# User management script
create_user() {
    local username=$1
    local fullname=$2

    # Check if user exists
    if id "$username" &>/dev/null; then
        echo "User $username already exists"
        return 1
    fi

    # Create user
    sudo useradd -m -c "$fullname" "$username"

    # Set password
    echo "$username:temp123" | sudo chpasswd

    # Force password change
    sudo chage -d 0 "$username"

    echo "User $username created successfully"
}

# Usage
create_user "jdoe" "John Doe"

#!/bin/bash
#
# Script Name:     myscript.sh
# Description:     Brief description of what the script does
# Author:          Your Name
# Date:            2025-02-07
# Version:         1.0
#

# Strict mode
set -euo pipefail

# Constants
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "$0")"

# Functions
usage() {
    cat << EOF
Usage: $SCRIPT_NAME [OPTIONS] <argument>

Description of the script.

OPTIONS:
    -h, --help      Show this help message
    -v, --verbose   Enable verbose mode
    -d, --debug     Enable debug mode

EXAMPLES:
    $SCRIPT_NAME file.txt
    $SCRIPT_NAME -v file.txt
EOF
}

main() {
    # Main script logic here
    echo "Script running..."
}

# Run main function
main "$@"

# Use meaningful variable names
user_name="john"  # Good
x="john"          # Bad

# Quote variables
rm "$file"        # Good
rm $file          # Bad (fails with spaces)

# Use [[ ]] instead of [ ]
if [[ "$var" == "value" ]]; then  # Good
    echo "Match"
fi

# Use lowercase for local variables
local_var="value"

# Use UPPERCASE for constants  
readonly MAX_USERS=100

# Check if variable is set
if [[ -n "${VAR:-}" ]]; then
    echo "VAR is set"
fi

# Debug mode (print each command)
bash -x script.sh

# Or in script
set -x

# Specific section
set -x
# Debug this section
set +x

# Verbose mode
set -v

# Check syntax without executing
bash -n script.sh

# Print to stderr
echo "Error message" >&2

# Custom debug function
DEBUG=true
debug() {
    if [[ "$DEBUG" == "true" ]]; then
        echo "[DEBUG] $*" >&2
    fi
}

debug "This is a debug message"

# Variables
var="value"                  # Assign
echo "$var"                  # Use variable
readonly var="value"         # Constant

# Conditionals
if [[ condition ]]; then
    # commands
fi

# Loops
for item in list; do
    # commands
done

while [[ condition ]]; do
    # commands
done

# Functions
function_name() {
    # commands
}

# Arrays
arr=(item1 item2)
echo "${arr[0]}"            # Access
echo "${arr[@]}"            # All items
echo "${#arr[@]}"           # Length

# String operations
${#var}                     # Length
${var:offset:length}        # Substring
${var//find/replace}        # Replace

# File tests
-e file                     # Exists
-f file                     # Is file
-d file                     # Is directory
-r file                     # Readable