Building a better cd

Unix shell is very flexible and easy to customise. Here is how I made "cd" better for me than the builtin one.

One of the key strengths of unix systems is the philosophy of small tools which can be combined in flexible ways. Writing your own scripts, functions and aliases to customise your user experience not only makes your life better and more productive, its also a great way to learn about unix.

This is the antithesis of the mindset that brings about Integrated Development Environments. The great thing about the unix shell as an integrated environment is that it is infinite in potential given that you can always extend the facilities you have by making other tools using scripting or by getting them from the web. The shell itself is extensible in that you can add little scripts or write functions to add features or do things your way.

For example, the cd builtin in ksh has this tremendous feature for dealing with long paths. Say I'm in /foo/bar/nerf/2003-04-26/conf/frazzle and I realise I need to be in /foo/baz/nerf/2003-04-26/conf/frazzle, all I do is type cd bar baz . That's pretty useful.

I also often find myself typing vi /the/path/to/a/file, then wishing I could do cd !$ to get to the directory to check the file in or whatever. So I built a cd function that changes to the parent directory if the target is a file. When a colleague of mine explained the ksh cd behaviour, I added his ksh-like cd function to mine to get this:

cd()
{
    if [ -z "$1" ] ; then
        builtin cd
    else
        if [ -n "$2" ]; then
            TRY="${PWD/$1/$2}"
        else
            TRY="$1"
        fi

        if [ -f "${TRY}" ]; then
            builtin cd "${TRY:h}"
        else
            builtin cd "${TRY}"
        fi
    fi
}

Notice that this includes a few zsh-isms which you'll need to change if you want to use this in bash. ${TRY:h} is the same as $( dirname "${TRY}" ) but I get to skip the backticks and save a fork. I think you can do this in bash using a string substitution something like "${TRY##*/}" but I haven't bothered to try. If you are using a csh-like shell I pity you and this cd mechanism will not be enough to save you from your folly anyway.


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