FP concepts in shell pipelines

The other day I was faced with a little task at work: find in the current repository subdirectories that contain a file named render-javascript.js and zip those directories one by one. Here is what came out:

find . -name render-javascript.js | xargs -n1 dirname | xargs -n1 -i zip -r {} {}

Here is what’s going on:

In the first segment of the pipe I find the files; the result is similar to this:

~/src/xo ɀ  find . -name *Field.js
./app/utils/createField.js
./app/widgets/ActivityDateField.js
./app/widgets/DateField.js
./app/widgets/LabeledDateField.js
./app/widgets/LabeledLargeTextField.js
./app/widgets/LabeledSelectField.js
./app/widgets/LabeledTextField.js
~/src/xo ɀ  

Next, each line — this is what -n1 does for xargs — is passed to dirname which prints the directory portion of a path.

Having the required directory paths, I can pass each of them to zip. Here I use a little xargs trick: passing it -i gives you the ability to mention the input line multiple times within the command by using the {} placeholder. So given our command zip -r {} {} and a line like ./app/widgets the final command will be zip -r ./app/widgets ./app/widgets.

That’s it! Yes, I could do the same thing a for loop, but I like that the process is neatly split in discrete steps. 8-)

* * *

Marveling at this command, I realized that it’s a lot like a promise chain, or an FP composition. The latter may look like this:

var zipDirectoriesContaining = compose(zip, dirname, find);

zipDirectoriesContaining('render-javascript.js');

While code like this looks quite cool, as does a promise chain and a shell pipeline, it has 2 little drawbacks:

  1. The IO protocol is implicit: you have to assume that the output value each function is appropriate as input for the subsequent one. I guess this may be fine if all those functions have a filter-like API, but to me it feels a bit implicit.
  2. At any given step I don’t have access the value of previous ones. For example if in the zip function I wanted the name passed to find — for example to record it in a log file along with the final directory name — I can’t get that.

But “drawback” only means that that particular solution is not a perfect fit for the problem at hand.