How to use if statements and case statements for conditional judgment in Shell scripts?

Common conditional statements in Shell scripts include if statements, case statements, and test commands.

if Statement

Basic Syntax

bash
if [ condition ]; then
    commands
elif [ condition ]; then
    commands
else
    commands
fi

File Test Operators

bash
[ -f file ]    # File exists and is a regular file
[ -d dir ]     # Directory exists
[ -e file ]    # File or directory exists
[ -r file ]    # File is readable
[ -w file ]    # File is writable
[ -x file ]    # File is executable
[ -s file ]    # File size is greater than zero
[ -L file ]    # File is a symbolic link

String Test Operators

bash
[ "$str1" = "$str2" ]   # Strings are equal
[ "$str1" != "$str2" ]  # Strings are not equal
[ -z "$str" ]           # String is empty
[ -n "$str" ]           # String is not empty

Numeric Test Operators

bash
[ $num1 -eq $num2 ]     # Equal
[ $num1 -ne $num2 ]     # Not equal
[ $num1 -gt $num2 ]     # Greater than
[ $num1 -lt $num2 ]     # Less than
[ $num1 -ge $num2 ]     # Greater than or equal
[ $num1 -le $num2 ]     # Less than or equal

Logical Operators

bash
[ condition1 ] && [ condition2 ]  # Logical AND
[ condition1 ] || [ condition2 ]  # Logical OR
! [ condition ]                   # Logical NOT
[ condition1 -a condition2 ]      # Logical AND (inside [])
[ condition1 -o condition2 ]      # Logical OR (inside [])

if Statement Examples

bash
#!/bin/bash

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

# Check numeric value
age=25
if [ $age -ge 18 ]; then
    echo "Adult"
elif [ $age -ge 13 ]; then
    echo "Teenager"
else
    echo "Child"
fi

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

# Use [[ ]] for more complex tests
if [[ $name == "John" || $name == "Jane" ]]; then
    echo "Welcome $name"
fi

case Statement

Basic Syntax

bash
case $variable in
    pattern1)
        commands
        ;;
    pattern2)
        commands
        ;;
    *)
        commands
        ;;
esac

case Statement Examples

bash
#!/bin/bash

# Simple menu selection
echo "Select an option:"
echo "1. Start"
echo "2. Stop"
echo "3. Restart"
read -p "Enter choice: " choice

case $choice in
    1)
        echo "Starting service..."
        ;;
    2)
        echo "Stopping service..."
        ;;
    3)
        echo "Restarting service..."
        ;;
    *)
        echo "Invalid choice"
        ;;
esac

# Use pattern matching
file="document.txt"
case $file in
    *.txt)
        echo "Text file"
        ;;
    *.jpg|*.png)
        echo "Image file"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

Test Commands

test Command

bash
test condition
# Equivalent to
[ condition ]

Double Brackets [[ ]]

bash
# [[ ]] is a Bash extension with more powerful features
[[ $name == "John" && $age -gt 18 ]]

Double Parentheses (( ))

bash
# (( )) is used for arithmetic operations
(( age++ ))
if (( age >= 18 )); then
    echo "Adult"
fi

Practical Application Example

bash
#!/bin/bash

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

file=$1

# Check file type
if [ -f "$file" ]; then
    echo "File exists"
    
    # Check file permissions
    if [ -r "$file" ]; then
        echo "File is readable"
    else
        echo "File is not readable"
    fi
    
    if [ -w "$file" ]; then
        echo "File is writable"
    else
        echo "File is not writable"
    fi
    
elif [ -d "$file" ]; then
    echo "Directory exists"
    
    # Check directory contents
    file_count=$(ls -1 "$file" | wc -l)
    if [ $file_count -eq 0 ]; then
        echo "Directory is empty"
    else
        echo "Directory contains $file_count items"
    fi
    
else
    echo "File or directory does not exist"
    exit 1
fi

# Use case to handle file extensions
extension="${file##*.}"
case $extension in
    txt|md)
        echo "Text document"
        ;;
    sh)
        echo "Shell script"
        ;;
    py)
        echo "Python script"
        ;;
    *)
        echo "Unknown file type"
        ;;
esac

Best Practices

1. Always quote variables: "$variable" prevents issues with spaces and special characters
2. Prefer [[ ]] over []: More powerful and safer
3. Use case for multi-condition matching: Cleaner code
4. Check command execution status: Use $? or if command; then
5. Avoid using -a and -o: Use && and || for clarity