06. If-else statements if, then, elif, else, fi

Just like any other language, the shell provides if-else statements to handle logic flow. However, the syntax is quite different - let's see how.

No curly braces!

Instead of curly braces or indentations to enclose logical constructs, the shell encloses if-statements within its keywords if, then, else, and fi. There is also the elif keyword that is the shorthand for else-if.

if command; then
  # If test condition returns exit status of 0 (success)
elif another command; then
  # Another command is true
else
  # Both commands have exit status not 0
fi

Notice anything different from other programming languages? Instead of using some logical expression bound by parentheses, the shell runs commands and checks their exit statuses! This concept may seem foreign and strange at first, but it'll grow on you with practice.

Be sure to get this concept down since it's used in loops as well! It can be confusing since many if statements use bracket notations [], which look like syntax. However, recall that the open bracket [ is just a shortcut for the test command.

Boolean Constructs

When stringing together a group of test commands, use the following syntax:

if [ $value -lt 2 ] && [ $value -gt 0 ]; then
  # value is between 0 and 2
fi

Sample script - flip a coin!

Now that we know enough control flow logic, let's create a simple coin tossing game! We'll have the user pick heads or tails, and the shell toss a coin. If the user is correct, then he/she wins; if not, he/she loses.

Try implementing it on your own with what we have learned thus far, then compare your answer below.

#!/bin/sh
 
printf "Choose (h)eads or (t)ails: "
read user_choice
 
# Make sure user chooses between heads or tails
if [ $user_choice != h ] && [ $user_choice != t ]; then
  echo "Invalid choice. Defaulting to (h)eads."
  user_choice=h
fi
 
# Value of 1 is heads, 2 is tails
computer_choice=$((RANDOM % 2 + 1))
 
if [ $computer_choice -eq 1 ]; then
  echo "Computer chose heads."
else
  echo "Computer chose tails."
fi
 
if [ $computer_choice -eq 1 ] && [ $user_choice = h ]; then
  # Correct
  echo "You win!"
elif [ $computer_choice -eq 1 ] && [ $user_choice = t     | ]; then
  # Incorrect
  echo "You lose!"
elif [ $computer_choice -eq 2 ] && [ $user_choice = t ]; then
  # Correct
  echo "You win!"
else 
  # Incorrect
  echo "You lose!"
fi

Great! You're well on your way to becoming a shell scripting master. Let's move onto case statements.

Aching back from coding all day?

Inversion Therapy Table

Aching back from coding all day? Try Back Problems

Stretch out your back and relieve your back muscles with inversion therapy. This device counteracts the forces of gravity on the body by decompressing and elongating the spine. By using this product just ten minutes a day, you can be well on your way to improved circulation and posture while relieving muscle aches, back pain and stress.

$$ Check price
119.98119.98Amazon 4.5 logo(1,700+ reviews)

More Back Problems resources

Take your Linux skills to the next level!

System Admin Handbook

Take your Linux skills to the next level! Try Linux & UNIX

This book approaches system administration in a practical way and is an invaluable reference for both new administrators and experienced professionals. It details best practices for every facet of system administration, including storage management, network design and administration, email, web hosting, scripting, and much more.

$ Check price
74.9974.99Amazon 4.5 logo(142+ reviews)

More Linux & UNIX resources

Ad