Learning the bash Shell, Second Edition

Advanced Examples: pushd and popd

We will conclude this chapter with a couple of functions that are already built into bash but are useful in demonstrating some of the concepts we have covered in this chapter. ^[59]

Task 4-8

The functions pushd and popd implement a stack of directories that enable you to move to another directory temporarily and have the shell remember where you were. Implement them as shell functions.

We will start by implementing a significant subset of their capabilities and finish the implementation in Chapter 6.

Think of a stack as a spring-loaded dish receptacle in a cafeteria. When you place dishes on the receptacle, the spring compresses so that the top stays at roughly the same level. The dish most recently placed on the stack is the first to be taken when someone wants food; thus, the stack is known as a “last-in, first-out” or LIFO structure. Putting something onto a stack is known in computer science parlance as pushing, and taking something off the top is called popping.

A stack is very handy for remembering directories, as we will see; it can “hold your place” up to an arbitrary number of times. The cd - form of the cd command does this, but only to one level. For example: if you are in firstdir and then you change to seconddir, you can type cd - to go back. But if you start out in firstdir, then change to seconddir, and then go to thirddir, you can use cd - only to go back to seconddir. If you type cd - again, you will be back in thirddir, because ...