Shell Scripting: Expert Recipes for Linux, Bash, and More

Chapter 6

Flow Control Using Loops

Loops are a vital tool for writing useful code. Much of the benefit of programming, and of computers in general, is that the machine can do the mundane work faster and more efficiently than a human, so it is often the case that you spend a long time carefully writing a few lines of code, which the machine then iterates over tens, hundreds, maybe even thousands of times or more. The basic structure of a loop is that it has a block of code to execute, and something telling it when to stop going around the loop and continue execution of the program. As you will see, the shell has four different loop structures: for, while, until, and select. Each of these has its own purpose and its own strengths and weaknesses. It is often (though not always) quite obvious up front which type of loop you will want to use for a particular task.