Shell Scripting — Automate Everything with Bash

🎯 Learning Objectives

• Write and execute bash scripts
• Use variables, conditionals, and loops
• Handle command-line arguments
• Perform arithmetic operations
• Handle errors and exit codes

1. What is Shell Scripting?

• Backup automation
• Log rotation and cleanup
• Deployment scripts
• Data processing pipelines
• System monitoring

2. Your First Script

bash
#!/bin/bash
# This is a comment
echo "Hello, World!"
echo "Current directory: $(pwd)"
echo "Current user: $USER"

bash
chmod +x myscript.sh   # Make executable
./myscript.sh           # Run
bash myscript.sh        # Or run with bash

3. Variables

bash
#!/bin/bash
# Assigning variables (no spaces around =)
name="Alice"
age=25
greeting="Hello, $name!"  # Variable expansion with $
echo $name          # Alice
echo ${name}        # Alice (braces useful for disambiguation)
echo "Age: $age"    # Age: 25
echo "$greeting"    # Hello, Alice!
# Command substitution
current_dir=$(pwd)          # Preferred syntax
current_dir=`pwd`           # Old syntax
files_count=$(ls | wc -l)
# Read-only and unset
readonly PI=3.14159         # Cannot be changed
unset name                  # Remove variable
# Special variables
echo $0    # Script name
echo $1    # First argument
echo $2    # Second argument
echo $#    # Number of arguments
echo $@    # All arguments as separate words
echo $*    # All arguments as single string
echo $?    # Exit code of last command
echo $$    # Process ID of current script

4. Arrays

bash
#!/bin/bash
# Declare array
fruits=("apple" "banana" "cherry")
numbers=(1 2 3 4 5)
# Access elements
echo ${fruits[0]}      # apple
echo ${fruits[1]}      # banana
echo ${fruits[@]}      # All elements
echo ${#fruits[@]}     # Length (3)
# Add element
fruits+=("date")
# Loop through array
for fruit in "${fruits[@]}"; do
    echo "I like $fruit"
done

5. Conditionals

bash
#!/bin/bash
# if-then-else
if [ "$age" -ge 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi
# elif
if [ "$score" -ge 90 ]; then
    echo "A"
elif [ "$score" -ge 80 ]; then
    echo "B"
elif [ "$score" -ge 70 ]; then
    echo "C"
else
    echo "D"
fi
# File tests
if [ -f "$filename" ]; then    # File exists and is regular file
    echo "File exists"
fi
if [ -d "$dirname" ]; then     # Directory exists
    echo "Directory exists"
fi
if [ -x "$file" ]; then        # File is executable
    echo "File is executable"
fi
if [ -z "$str" ]; then         # String is empty
    echo "Empty string"
fi
if [ -n "$str" ]; then         # String is not empty
    echo "Non-empty string"
fi
# String comparison
if [ "$name" = "Alice" ]; then   # Equal (use single =)
    echo "Hello Alice"
fi
if [ "$name" != "Bob" ]; then    # Not equal
    echo "Not Bob"
fi
# Numeric comparison
if [ "$count" -eq 10 ]; then    # Equal
if [ "$count" -ne 10 ]; then    # Not equal
if [ "$count" -gt 10 ]; then    # Greater than
if [ "$count" -lt 10 ]; then    # Less than
if [ "$count" -ge 10 ]; then    # Greater or equal
if [ "$count" -le 10 ]; then    # Less or equal
# Logical operators
if [ "$a" -gt 0 ] && [ "$a" -lt 10 ]; then  # AND
    echo "Between 0 and 10"
fi
if [ "$a" -lt 0 ] || [ "$a" -gt 10 ]; then   # OR
    echo "Outside range"
fi
# Double brackets (bash-specific, more powerful)
if $name == A*; then       # Pattern matching
    echo "Name starts with A"
fi

6. Loops

bash
#!/bin/bash
# For loop (list)
for color in red green blue; do
    echo "Color: $color"
done
# For loop (C-style)
for ((i=0; i<5; i++)); do
    echo "Iteration $i"
done
# For loop (files)
for file in *.txt; do
    echo "Processing $file"
    wc -l "$file"
done
# While loop
count=0
while [ $count -lt 5 ]; do
    echo "Count: $count"
    ((count++))
done
# Until loop (while false)
count=0
until [ $count -ge 5 ]; do
    echo "Count: $count"
    ((count++))
done
# Break and continue
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        continue    # Skip 5
    fi
    if [ $i -eq 8 ]; then
        break       # Stop at 8
    fi
    echo $i
done
# Output: 1 2 3 4 6 7

7. Functions

bash
#!/bin/bash
# Define function
function greet() {
    local name=$1    # $1 is first argument to function
    echo "Hello, $name!"
}
say_hello() {        # Alternative syntax
    echo "Hello, $1!"
}
greet "Alice"        # Hello, Alice!
greet "Bob"          # Hello, Bob!
# Function with return value
add() {
    local sum=$(( $1 + $2 ))
    echo $sum        # Return via stdout
    return 0         # Return status
}
result=$(add 5 3)    # Capture output
echo $result         # 8
# Global vs local variables
count=10
increment() {
    local count=5    # Local variable
    ((count++))
    echo $count      # 6 (local)
}
increment
echo $count          # 10 (global, unchanged)

8. Exit Codes and Error Handling

bash
#!/bin/bash
# Exit on error
set -e               # Exit on first error
set -u               # Treat unset variables as error
set -x               # Print commands before executing (debug)
set -o pipefail      # Fail if any command in pipeline fails
# Check exit code
if ! command_that_might_fail; then
    echo "Command failed!"
    exit 1
fi
# Or use $?
cp file.txt backup/
if [ $? -ne 0 ]; then
    echo "Backup failed!"
    exit 1
fi

9. Practice Questions

Q1: Write a script that prints "Hello, [name]!" where name is the first argument.

bash
#!/bin/bash
name=${1:-"World"}
echo "Hello, $name!"

Q2: How do you check if a file exists before processing it?

bash
if [ -f "$filename" ]; then
    echo "Processing $filename"
    wc -l "$filename"
else
    echo "File not found: $filename"
    exit 1
fi

Q3: What does $@ represent in a script?

Answer: All command-line arguments as separate quoted words. "$@" preserves spaces in arguments. Contrast with $* which treats all args as a single string. Q4: How do you loop through all .txt files in a directory?

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

Q5: What does set -e do in a shell script?

Answer: Causes the script to exit immediately if any command returns a non-zero exit status. Prevents the script from continuing after errors. Use with caution — some commands may exit non-zero for non-error conditions. Q6: Write a function that returns the sum of two numbers.

bash
sum() {
    echo $(( $1 + $2 ))
}
result=$(sum 10 20)
echo $result  # 30

Q7: What's the difference between local and global variables in functions?

Answer: local variables are scoped to the function — they don't affect variables outside. Global variables persist after the function returns. By default, all variables in bash are global; use local to limit scope. Q8: Write a script that backs up all .txt files to a backup directory.

bash
#!/bin/bash
backup_dir="backup_$(date +%Y%m%d)"
mkdir -p "$backup_dir"
for file in *.txt; do
    cp "$file" "$backup_dir/"
    echo "Backed up $file"
done
echo "Backup complete: $backup_dir"

📐 Key Concepts

🔗 Cross-References

• Next: Advanced Shell Scripting Join Discord Previous6.1 Process ManagementNext8.1 Advanced Shell Scripting