Task 4-7 In the C shell, the commands 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. The dirs command prints the stack. The Korn shell does not provide these commands. Implement them as shell functions.

• pushd dir does a cd to dir and then pushes dir onto the stack.

• popd does a cd to the top directory, then pops it off the stack.

Table 4-12. pushd/popd example

Your directory stack should be initialized to your home directory when you log in. To do so, put this in your .profile:

DIRSTACK="$PWD"
export DIRSTACK

Do not put this in your environment file if you have one. The export statement guarantees that DIRSTACK is known to all subprocesses; you want to initialize it only once. If you put this code in an environment file, it will get reinitialized in every interactive shell subprocess, which you probably don't want.

Next, we need to implement dirs, pushd, and popd as functions. Here are our initial versions:

function dirs {        # print directory stack (easy)
    print $DIRSTACK
}

function pushd {       # push current directory onto stack
    dirname=$1
    cd ${dirname:?"missing directory name."}
    DIRSTACK="$PWD $DIRSTACK"
    print "$DIRSTACK"
}

function popd {        # cd to top, pop it off stack
    top=${DIRSTACK%% *}
    DIRSTACK=${DIRSTACK#* }
    cd $top
    print "$PWD"
}

Notice that there isn't much code! Let's go through the functions and see how they work. dirs is easy; it just prints the stack. The fun starts with pushd. The first line merely saves the first argument in the variable dirname for readability reasons.

The second line's main purpose is to change to the new directory. We use the :? operator to handle the error when the argument is missing: if the argument is given, the expression ${dirname:?"missing directory name."} evaluates to $dirname, but if it is not given, the shell prints the message ksh: pushd: line 2: dirname: missing directory name. and exits from the function.

The third line of the function pushes the new directory onto the stack. The expression within double quotes consists of the full pathname for the current directory, followed by a single space, followed by the contents of the directory stack ($DIRSTACK). The double quotes ensure that all of this is packaged into a single string for assignment back to DIRSTACK.

The last line merely prints the contents of the stack, with the implication that the leftmost directory is both the current directory and at the top of the stack. (This is why we chose spaces to separate directories, rather than the more customary colons as in PATH and MAILPATH.)

The third line actually changes directory to the previous top of the stack. (Note that popd doesn't care where you are when you run it; if your current directory is the one on the top of the stack, you won't go anywhere.) The final line just prints a confirmation message.

This code is deficient in the following ways: first, it has no provision for errors. For example:

• What if the user tries to push a directory that doesn't exist or is invalid?

• What if the user tries popd and the stack is empty?

Test your understanding of the code by figuring out how it would respond to these error conditions. The second deficiency is that the code implements only some of the functionality of the C shell's pushd and popd commands -- albeit the most useful parts. In the next chapter, we will see how to overcome both of these deficiencies.