Having a long-lived CLI means that there is some functionality that’s going to change over time. One of these is usually flag changes.

Take xargs for instance. You could have a system where you have a list of commands in a file that you want to run in no particular order, and you want to be able to run many at once (assuming they can be run atomically)

You could use:

xargs --arg-file=myargs -P 4 --replace /bin/sh -c "{}"

This reads as: “Run xargs with myargs file, use 4 parallel processes, and replace any line with /bin/sh -c "{}"”, where I’m replacing {} with the line from the file, and -c relates to the /bin/sh flag of allowing to be read from string.

However, --replace is deprecated. There’s another flag, -i, but that’s also deprecated.

You can use -I however, but it doesn’t give the same output as a drop-in replacement for --replace

What does work however, is:

xargs --arg-file=myargs -P 4 -I {} /bin/sh -c "{}"

The {} says this is the replacement string for the command. The "{}" is a string that will replace the line from the file with a string that’s read from the file.

Because substitution with multiple characters can be confusing, the following is also functionally equivalent:

xargs --arg-file=myargs -P 4 -I % /bin/sh -c "%"

Also, the reason the first {} wasn’t required in the --replace version is because {} is the assumed replacement value.

To solve this particular issue I had, the man page for xargs didn’t help me, apart from telling me I was using deprecated flags. What did help was a well thought out answer from Stack Overflow. Thankfully it’s now the accepted answer, even though it was a ‘late submission’.

Sometimes it’s the well thought out answers that are the most helpful.