How often do you need to move to some other directory temporarily, look at some file, and then move back to the directory where you started? If you're like most users, you do this all the time. csh and bash have pushd and popd commands make this a lot easier. (If you use ksh, O'Reilly & Associates' Learning the Korn Shell shows you shell functions that do the same thing.)
These commands implement a "directory stack." The classical analogy for a stack is one of those spring-loaded plate stackers in a school cafeteria. The last plate put ("pushed") onto the stack is the first plate taken ("popped") from the stack. It's just the same with directories: each time you use pushd, the shell adds your current directory to the stack and moves you to the new directory. When you use popd, the shell takes the top directory off the stack, and moves you to the directory underneath.
You may as well learn about pushd the way I did: by watching. Let's say that I'm in the directory ~/power, working on this book. I want to change to my Mail directory briefly, to look at some old correspondence. Here's how: 
 If you've set ayou can use those short directory names with pushd.
pushd ~/Mail...current directory becomes ~/Mail ~/Mail ~/power
pushd prints the entire stack, giving me some confirmation about where I am, and where I can go. When I'm done reading the old mail, I want to move back:
popd...current directory becomes ~/power ~/power
We're back where we started; the Mail directory is no longer on the stack.
What if you want to move back and forth repeatedly? pushd, with no arguments, just switches the two top directories on the stack. Like this:
pwd...current directory is ~/power /home/los/mikel/power los%
pushd ~/Mail...current directory becomes ~/Mail ~/Mail ~/power los%
pushd...current directory becomes ~/power ~/power ~/Mail los%
pushd...current directory becomes ~/Mail ~/Mail ~/power
And so on.
If you like, you can let your directory stack get really long.
this case, two special commands are useful.
deletes the n entry in the stack.
Entries are counted "down"
from the top, starting with zero; that is, your current directory is 0.
popd +0 and
popd are the same.
If n is greater
than 0, your current directory does not change.
This may seem
surprising, but it isn't; after all, you haven't changed the top of
n "rotates" the stack, so that the
nth directory moves to the top, becoming the current directory.
Note that this is a "rotation": the whole stack moves.
I don't find
n commands too useful, but you should know about them.
The one drawback to pushd and popd is that you can easily build up a gigantic directory stack full of useless directories. I suppose this doesn't really hurt anything, but it's needless clutter. The only way to clear the stack is to popd repeatedly (except, in tcsh, the command dirs -c clears the stack). More to the point, the directories you're most likely to want are at the top of the stack. There's no really convenient way to save them. I mean, with 7 directories in the stack, you could conceivably do something like:
pushd +5 ; popd ; popd
to get rid of the bottom two elements. The pushd moves the bottom two elements of a 7-directory stack to the top. A bit inconvenient. [Clearing the whole stack is a good use for the C shellcommand. For example, if the stack has 7 directories, type:
repeat 6 popd
That's an easy way to start over when the stack gets too messy. –JP ]
NOTE: The Korn shell has some similar (but not quite as general) features. It keeps track of your previous working directory, and then defines the special command
cd -as "change to the previous working directory."