As a (at present) Mac user, I spend nearly all my time in the terminal, and around 2002 a friend taught me about the xargs command, which I probably use way too much, but it's pretty handy. However, I keep forgetting specific tricks, so I'm writing them up here, in my online searchable diary.

A brief introduction to xargs

xargs is a command line tool that allows you to use STDIN as the argument to another program.

For instance, I want to see the first entry the web.archive.org has for a particular domain, first I must curl their API, then taking the 2nd field, I would pass this as an argument to date (for legible formatting). I could do this by hand:

getting first entry in archive.org

…or I could use xargs to complete it in a single command:

$ curl http://web.archive.org/cdx/search/cdx\?limit\=1\&url\=remysharp.com | \
  cut -d' ' -f2 | \
  xargs date -j -f "%Y%m%d%H%M%S"

The limitation somtimes I run into though, is that xargs by default puts the argument at the end (it also treats everything in STDIN as a single argument, but I'll come on to that). In particular, if you want to do something like pipe the output of the above command into a file, you can't just add > output.txt to the end of the line. xargs gets all messed up, and the whole command just hangs and waits.

The solution, and the whole point of this post, is using the filename placeholder with xargs.

Filename placeholder

xargs allows you to specify a placeholder using -I <marker> and then you can re-use the marker later on in the command. Most reading material on the web uses a marker of {}, but you can use anything, like FILE:

$ curl http://web.archive.org/cdx/search/cdx\?limit\=1\&url\=remysharp.com | \
  cut -d' ' -f2 | \
  xargs -I FILE date -j -f "%Y%m%d%H%M%S" FILE > output.txt

That's how I solved a specific problem: search every node project for a specific dependency.

Finding node projects with a specific dependency

I had a specific problem where I was trying to remember a dependency that I had included for date mocking, except I had forgotten the name.

I wanted to search just the devDependencies so I intended to run each package.json through the json command line too. Except to complicate things, the json tool requires the JSON as STDIN (or…so I thought!).

So, I had to combine a number of features:

  1. Filename placeholder: -I file
  2. Run command for each line: -L 1
  3. Shell out: sh -c '<command>'

The result was this (multiline for readability):

$ ls */package.json | \
  xargs -I {} -L 1 sh -c 'json devDependencies < "{}"'

Of course, when I found out (afterwards!) that json can take a filename, it simplifies considerably:

$ ls */package.json | xargs -I {} -L 1 json -f {} devDependencies

Aside, this could also be done using find, but it's quite a bit slower (for me) since there's a lot more files it work its way through:

$ find . -name package.json -depth 2 -exec sh -c 'json -f "{}" devDependencies'  \;

I hope that's helpful, and next time you might use xargs to do a bit of your CLI magic! Also, if that was all new to you, perhaps you might want my new book: Working the Command Line!