Conditional Logic: if Statements and case Switches

Conditional logic allows scripts to make decisions based on the state of the system or input values.

1. The if Statement Syntax

The basic structure of a Bash conditional block:

#!/bin/bash
DISK_USAGE=$(df / | awk 'NR==2 {print $5}' | tr -d '%')

if [ "$DISK_USAGE" -gt 90 ]; then
  echo "CRITICAL: Disk usage is above 90%!"
elif [ "$DISK_USAGE" -gt 75 ]; then
  echo "WARNING: Disk usage is above 75%."
else
  echo "OK: Disk usage is at ${DISK_USAGE}%."
fi

2. Common Test Operators

3. The case Switch Statement

For matching one variable against multiple patterns:

#!/bin/bash
ENVIRONMENT=$1

case "$ENVIRONMENT" in
  production)
    echo "Deploying to production server..."
    ;;
  staging)
    echo "Deploying to staging server..."
    ;;
  *)
    echo "Unknown environment: $ENVIRONMENT"
    exit 1
    ;;
esac