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Chapter 6.3 - Control Structures

Learning Objectives​

By the end of this chapter, you will be able to:

  • Use if/else conditions
  • Implement for and while loops
  • Use case for multiple selection
  • Understand test operators
  • Handle errors

if/else Conditions​

Basic Syntax​

#!/bin/bash
# simple if
if [ condition ]; then
    commands
fi

# if/else
if [ condition ]; then
    commands_if_true
else
    commands_if_false
fi

# if/elif/else
if [ condition1 ]; then
    commands1
elif [ condition2 ]; then
    commands2
else
    commands3
fi

Test Operators​

Files:

[ -f file ]      # File exists and is regular
[ -d directory ] # Directory exists
[ -e path ]      # Path exists
[ -r file ]      # Readable
[ -w file ]      # Writable
[ -x file ]      # Executable
[ -s file ]      # Not empty

Strings:

[ -z "$VAR" ]       # Empty
[ -n "$VAR" ]       # Not empty
[ "$VAR1" = "$VAR2" ]  # Equal
[ "$VAR1" != "$VAR2" ] # Not equal

Numbers:

[ $A -eq $B ]       # Equal
[ $A -ne $B ]       # Not equal
[ $A -lt $B ]       # Less than
[ $A -le $B ]       # Less than or equal
[ $A -gt $B ]       # Greater than
[ $A -ge $B ]       # Greater than or equal

Logic:

[ condition1 ] && [ condition2 ]  # AND
[ condition1 ] || [ condition2 ]  # OR
[ ! condition ]                    # NOT

Examples​

#!/bin/bash
# Check file
if [ -f "/etc/passwd" ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

# Check user
if [ "$USER" = "root" ]; then
    echo "You are root"
elif [ "$UID" -eq 0 ]; then
    echo "You are root (by UID)"
else
    echo "You are $USER"
fi

# Multiple conditions
if [ -f "$1" ] && [ -r "$1" ]; then
    echo "File exists and is readable"
fi

for Loop​

Syntax​

#!/bin/bash
# List of values
for item in value1 value2 value3; do
    echo "Item: $item"
done

# Iterate over arguments
for arg in "$@"; do
    echo "Argument: $arg"
done

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

# With increment
for i in {1..10..2}; do
    echo "Number: $i"
done

# C-style
for ((i=1; i<=10; i++)); do
    echo "Number: $i"
done

Examples​

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

# Iterate over directories
for dir in /home/*; do
    if [ -d "$dir" ]; then
        echo "Directory: $dir"
    fi
done

# Iterate over array
FRUITS=("apple" "banana" "orange")
for fruit in "${FRUITS[@]}"; do
    echo "Fruit: $fruit"
done

while Loop​

Syntax​

#!/bin/bash
# simple while
while [ condition ]; do
    commands
done

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

# infinite while
while true; do
    commands
    sleep 1
done

Examples​

#!/bin/bash
# Counter
i=1
while [ $i -le 10 ]; do
    echo "Number: $i"
    i=$((i + 1))
done

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

# Interactive menu
while true; do
    echo "1. Option 1"
    echo "2. Option 2"
    echo "3. Quit"
    read -p "Choice: " choice
    
    case $choice in
        1) echo "Option 1" ;;
        2) echo "Option 2" ;;
        3) break ;;
        *) echo "Invalid choice" ;;
    esac
done

until Loop​

#!/bin/bash
# until (until condition is true)
i=1
until [ $i -gt 10 ]; do
    echo "Number: $i"
    i=$((i + 1))
done

case - Multiple Selection​

Syntax​

#!/bin/bash
case $variable in
    pattern1)
        commands1
        ;;
    pattern2)
        commands2
        ;;
    pattern3|pattern4)
        commands3
        ;;
    *)
        default_commands
        ;;
esac

Examples​

#!/bin/bash
# Simple menu
read -p "Choice (1-3): " choice

case $choice in
    1)
        echo "Option 1 selected"
        ;;
    2)
        echo "Option 2 selected"
        ;;
    3)
        echo "Option 3 selected"
        ;;
    *)
        echo "Invalid choice"
        exit 1
        ;;
esac

# Patterns with wildcards
case "$1" in
    start)
        echo "Starting"
        ;;
    stop)
        echo "Stopping"
        ;;
    restart|reload)
        echo "Restarting"
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        exit 1
        ;;
esac

Flow Control​

break and continue​

#!/bin/bash
# break: Exit the loop
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break
    fi
    echo $i
done

# continue: Skip to the next iteration
for i in {1..10}; do
    if [ $i -eq 5 ]; then
        continue
    fi
    echo $i
done

exit​

#!/bin/bash
# exit: Exit the script
if [ ! -f "$1" ]; then
    echo "Error: File not found"
    exit 1
fi

# Return code
exit 0   # Success
exit 1   # General error
exit 2   # Specific error

Complete Example​

#!/bin/bash
# File processing script

# Check arguments
if [ $# -eq 0 ]; then
    echo "Usage: $0 FILE..."
    exit 1
fi

# Process each file
for file in "$@"; do
    # Check existence
    if [ ! -f "$file" ]; then
        echo "Warning: $file does not exist, ignored"
        continue
    fi
    
    # Check permissions
    if [ ! -r "$file" ]; then
        echo "Error: $file is not readable"
        continue
    fi
    
    # Process according to extension
    case "$file" in
        *.txt)
            echo "Text processing: $file"
            # Processing...
            ;;
        *.log)
            echo "Log processing: $file"
            # Processing...
            ;;
        *)
            echo "Unsupported type: $file"
            ;;
    esac
done

echo "Processing completed"

Summary​

In this chapter, you learned:

if/else: Conditions with test operators
for: Loops with lists, sequences, arrays
while: Conditional loops, file reading
case: Multiple selection with patterns
Control: break, continue, exit
Operators: Files, strings, numbers, logic

Next Steps​

Chapter 6.4: Functions and Cron
Module 7: Networking and Configuration

Chapter created on: December 2024