Shell Scripting

Script Basics

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

Variables

 
# Variable assignment
name="John"
age=25
 
# Using variables
echo "Name: $name"
echo "Age: ${age}"
 
# Command substitution
current_date=$(date)
backtick_date=`date`
 
# Read input
read -p "Enter name: " username
echo "Hello, $username"
 

Output & Redirection

 
# echo vs printf
echo "Hello"             # Automatic newline
printf "Hello\n"         # Manual newline control
 
# Redirection
command > file           # Overwrite output
command >> file          # Append output
command < file           # Input from file
command 2> error.log     # Redirect stderr
command > output.log 2>&1  # Redirect both stdout & stderr
command &> file          # Redirect both (bash)
 
# Pipes
command1 | command2      # Pipe output
ls -la | grep "file" | wc -l
 

Conditional Statements

 
# if-elif-else
if [ condition ]; then
    commands
elif [ condition ]; then
    commands
else
    commands
fi
 
# Comparison operators
# Numeric: -eq, -ne, -lt, -gt, -le, -ge
# String: =, !=, -z (empty), -n (not empty)
# File: -f (file exists), -d (directory exists), -r (readable)
 
# Example
if [ $num -eq 10 ]; then
    echo "Number is 10"
fi
 
if [ -f "/path/to/file" ]; then
    echo "File exists"
fi

Loops

 
# For loop
for i in 1 2 3 4 5; do
    echo "Number: $i"
done
 
for file in *.txt; do
    echo "Processing $file"
done
 
for ((i=0; i<10; i++)); do
    echo "Count: $i"
done
 
# While loop
counter=1
while [ $counter -le 5 ]; do
    echo "Counter: $counter"
    ((counter++))
done
 
# Reading file line by line
while read line; do
    echo "Line: $line"
done < file.txt
 
# Until loop
counter=1
until [ $counter -gt 5 ]; do
    echo "Counter: $counter"
    ((counter++))
done
 
# Loop control
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break           # Exit loop
    fi
    if [ $i -eq 3 ]; then
        continue        # Skip iteration
    fi
    echo "Number: $i"
done
 

Functions

 
# Define function
function_name() {
    commands
    return value
}
 
# Or
function function_name {
    commands
}
 
# Example
greet() {
    echo "Hello, $1!"
    return 0
}
 
# Call function
greet "Alice"
 
# Capture return value
greet "Bob"
echo "Exit status: $?"

Advanced Scripting

 
# Arrays
fruits=("apple" "banana" "cherry")
echo ${fruits[0]}
echo ${fruits[@]}        # All elements
echo ${#fruits[@]}       # Number of elements
 
# Positional parameters
echo "Script name: $0"
echo "First arg: $1"
echo "All args: $@"
echo "Number of args: $#"
 
# Exit codes
exit 0                   # Success
exit 1                   # General error
exit 2                   # Misuse of shell builtins
 
# Debugging
#!/bin/bash -x           # Debug mode
set -x                   # Enable debugging
set +x                   # Disable debugging
 

Practical Exercises

Daily Tasks Script

 
#!/bin/bash
# system_report.sh
 
echo "=== System Report ==="
echo "Date: $(date)"
echo "Uptime: $(uptime -p)"
echo "Memory: $(free -h | awk '/Mem:/ {print $3 "/" $2}')"
echo "Disk: $(df -h / | awk 'NR==2 {print $3 "/" $2}')"
echo "Top processes:"
ps aux --sort=-%mem | head -5
 

Backup Script

 
#!/bin/bash
# backup.sh
 
BACKUP_DIR="/backups"
SOURCE_DIR="/home/user/documents"
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"
 
if [ $? -eq 0 ]; then
    echo "Backup completed: backup_$DATE.tar.gz"
else
    echo "Backup failed!"
    exit 1
fi
 
# Cleanup old backups (keep last 7 days)
find "$BACKUP_DIR" -name "backup_*.tar.gz" -mtime +7 -delete

User Management Script

 
#!/bin/bash
# create_users.sh
 
if [ "$(id -u)" -ne 0 ]; then
    echo "Please run as root"
    exit 1
fi
 
while read -r username; do
    if id "$username" &>/dev/null; then
        echo "User $username already exists"
    else
        useradd -m -s /bin/bash "$username"
        echo "User $username created"
        echo "$username:default123" | chpasswd
        echo "Default password set for $username"
    fi
done < users.txt