On , I learnt ...

How to use xargs with printf

This is obvious in retrospect but it hadn’t dawned on me that you can use xargs printf to use STDIN as printf arguments. Trivial example:

$ echo world | xargs printf "hello %s"
hello world

It’s useful for building strings from streams of data. Today’s usecase was building a pastable Python snippet from a JSON file containing object IDs:

$ cat loggly_events.json | jq '.events[].event.json.object_id' \
    | paste -sd, | xargs printf "object_ids = [%s]"
object_ids = [4,6,8,2,3,4]

Here jq is used to extract a newline-separated stream of IDs which are joined into a CSV-string using paste before being formatted into a Python snippet with printf.