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Conditionals in Shell Scripting

Conditionals allow your scripts to make decisions and execute different code blocks based on specific conditions.

The if-else Statement​

The if statement evaluates a condition. If it is true, it executes a block of code.

Basic if​

if [ condition ]; then
    # commands to run if true
fi

if-else​

AGE=18

if [ "$AGE" -ge 18 ]; then
    echo "You are an adult."
else
    echo "You are a minor."
fi

elif (Else If)​

Used for evaluating multiple conditions.

SCORE=85

if [ "$SCORE" -ge 90 ]; then
    echo "Grade: A"
elif [ "$SCORE" -ge 80 ]; then
    echo "Grade: B"
else
    echo "Grade: C or lower"
fi

File Test Operators​

Conditionals are frequently used to check file states.

  • -e file : Exists
  • -f file : Is a regular file
  • -d dir : Is a directory
  • -r file : Is readable
  • -w file : Is writable
  • -x file : Is executable
FILE="/etc/passwd"

if [ -f "$FILE" ]; then
    echo "$FILE exists and is a regular file."
fi

The case Statement​

The case statement is an elegant alternative to deeply nested if-elif blocks, especially when checking a variable against a set of fixed values.

read -p "Enter start, stop, or restart: " ACTION

case "$ACTION" in
    start)
        echo "Starting service..."
        ;;
    stop)
        echo "Stopping service..."
        ;;
    restart)
        echo "Restarting service..."
        ;;
    *)
        echo "Invalid action. Use start, stop, or restart."
        ;;
esac

Best Practices​

  • Terminate if statements with fi.
  • Terminate each case block with ;; and the entire case statement with esac.
  • Use the * wildcard in a case statement as a default/catch-all handler.
  • Remember to quote variables inside [ ] brackets.