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

Summary

The bash shell offers four loops: for, while, until, and select, although while and until are almost identical in practice. These loops offer almost all of the features of more powerful languages, and when the right one is chosen for the task, they make even quite complicated flow control easy to write and maintain.

The addition of break and continue make the formal, structured aspects of loops more useful in the real world where shell scripts are often required to be rather more pragmatic than other languages that end up with more complicated control structures to deal with these situations.

Probably the hardest thing when using loops in a script is determining which type of loop to use. for loops are best at iterating over a predefined list of items; while and until loops are better at continuing to execute code until some test condition changes. The select loop makes for really quick and easy menuing systems.

The next chapter is an in-depth look at variables, particularly some of the useful features that do not exist in the original Bourne shell but are added by bash, such as ksh and zsh.