Learn Bash by writing an interactive game

Learning a new programming language can be fun. Whenever I try to learn a new one, I focus on defining variables, writing a statement, and evaluating expressions. Once I have a general understanding of those concepts, I can usually figure out the rest on my own. Most programming languages have some similarities, so once you know one programming language, learning the next one is a matter of figuring out the unique details and recognizing the differences in it.

To help me practice a new programming language, I like to write a few test programs. One sample program I often write is a simple "guess the number" program, where the computer picks a number between one and 100 and asks me to guess the number. The program loops until I guess correctly.

The "guess the number" program exercises several concepts in programming languages: how to assign values to variables, how to write statements, and how to perform conditional evaluation and loops. It's a great practical experiment for learning a new programming language.

Guess the number in Bash

Bash is the standard shell for most Linux systems. Aside from providing a rich command-line user interface, Bash also supports a complete programming language in the form of scripts.

If you're not familiar with Bash, I recommend these introductions:

• What is Bash?
• Get started with Bash programming
• Get started with Bash scripting for sysadmins
• How to write functions in Bash
• Read more about Bash

You can explore Bash by writing a version of the "guess the number" game. Here is my implementation:

#!/bin/bash

number=$(( $RANDOM % 100 + 1 ))

echo "Guess a number between 1 and 100"

guess=0

while [ "0$guess" -ne $number ] ; do
	read guess
	[ "0$guess" -lt $number ] && echo "Too low"
	[ "0$guess" -gt $number ] && echo "Too high"
done

echo "That's right!"
exit 0

Breaking down the script

The first line in the script, #!/bin/bash tells Linux to run this script using the Bash shell. Every script starts with the #! character pair, which indicates this is a shell script. What immediately follows #! is the shell to run. In this case, /bin/bash is the Bash shell.

To assign a value to a variable, list the variable's name followed by the = sign. For example, the statement guess=0 assigns a zero value to the guess variable.

You can also prompt the user to enter a value using the read statement. If you write read guess, Bash waits for the user to enter some text then stores that value in the guess variable.

To reference the value of a variable, use $ before the variable name. So, having stored a value in the guess variable, you can retrieve it using $guess.

You can use whatever names you like for variables, but Bash reserves a few special variable names for itself. One special variable is RANDOM, which generates a very large random number every time you reference it.

If you want to perform an operation at the same time you store a value, you need to enclose the statement in special brackets. This tells Bash to execute that statement first, and the = stores the resulting value in the variable. To evaluate a mathematical expression, use $(( )) around your statement. The double parentheses indicate an arithmetic expression. In my example, number=$(( $RANDOM % 100 + 1 )) evaluates the expression $RANDOM % 100 + 1 and then stores the value in the number variable.

Standard arithmetic operators such as + (plus), - (minus), * (multiply), / (divide), and % (modulo) apply.

That means the statement number=$(( $RANDOM % 100 + 1 )) generates a random number between one and 100. The modulo operator (%) returns the remainder after dividing two numbers. In this case, Bash divides a random number by 100, leaving a remainder in the range zero to 99. By adding one to that value, you get a random number between one and 100.

Bash supports conditional expressions and flow control like loops. In the "guess the number" game, Bash continues looping as long as the value in guess is not equal to number. If the guess is less than the random number, Bash prints "Too low," and if the guess is greater than the number, Bash prints "Too high."

How it works

Now that you've written your Bash script, you can run it to play the "guess the number" game. Continue guessing until you find the correct number:

Guess a number between 1 and 100
50
Too high
30
Too high
20
Too high
10
Too low
15
Too high
13
Too low
14
That's right!

Every time you run the script, Bash will pick a different random number.

This "guess the number" game is a great introductory program when learning a new programming language because it exercises several common programming concepts in a pretty straightforward way. By implementing this simple game in different programming languages, you can demonstrate some core concepts and compare details in each language.

Do you have a favorite programming language? How would you write the "guess the number" game in it? Follow this article series to see examples of other programming languages that might interest you.