The double parentheses indicate an arithmetic expression. To evaluate a mathematical expression, use $(( )) around your statement. This tells Bash to execute that statement first, and the = stores the resulting value in the variable. If you want to perform an operation at the same time you store a value, you need to enclose the statement in special brackets. One special variable is RANDOM, which generates a very large random number every time you reference it. You can use whatever names you like for variables, but Bash reserves a few special variable names for itself. So, having stored a value in the guess variable, you can retrieve it using $guess. To reference the value of a variable, use $ before the variable name. If you write read guess, Bash waits for the user to enter some text then stores that value in the guess variable. You can also prompt the user to enter a value using the read statement. For example, the statement guess=0 assigns a zero value to the guess variable.

To assign a value to a variable, list the variable's name followed by the = sign. In this case, /bin/bash is the Bash shell.

What immediately follows #! is the shell to run. Every script starts with the #! character pair, which indicates this is a shell script. The first line in the script, #!/bin/bash tells Linux to run this script using the Bash shell. You can explore Bash by writing a version of the 'guess the number' game.

Get started with Bash scripting for sysadmins.

If you're not familiar with Bash, I recommend these introductions: Aside from providing a rich command-line user interface, Bash also supports a complete programming language in the form of scripts. Guess the number in Bashīash is the standard shell for most Linux systems. It's a great practical experiment for learning a new programming language. 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.

eBook: An introduction to programming with Bash.

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