Explain xargs code

What this xargs sample shows

-print0 and -0

xargs reads names from its input and adds them to the end of the command. -print0 ends each name with a NUL byte and -0 splits only there, so jobs 2.log stayed one name. The last line has neither, and xargs split on the space: ls got ./error and log.log, printed cannot access for each, and xargs exited with code 123. A file named it's.log stopped plain xargs with unmatched single quote.

-n and -P

-n 1 gives each command one name, so the 5 old logs ran gzip 5 times. Without -n, xargs ran one gzip with all 5 names. -P 4 runs up to 4 commands at once: 5 jobs that each sleep 1 second took 2.0 seconds with -P 4 and 5.0 with -P 1. Output can mix: the -v lines of 4 gzips came out tangled on one line.

-I {}

-I {} runs one command per name and puts the name where {} is, here in the middle, because mv needs the folder last. All 5 .log.gz files ended up in archive/, jobs 2.log.gz included, while the 2 newer logs stayed where they were.

-r on empty input

With no .tmp files, find printed nothing, and GNU xargs still ran rm -v once with no names: rm printed missing operand and xargs exited with code 123. With -r, nothing ran and the exit code was 0, which matters in a script that stops on the first error.

Good to know

• Run with GNU findutils 4.11.0 (find and xargs) and gzip 1.14 on Linux, in a test logs folder of 7 files: 5 last changed 10 days ago, jobs 2.log among them, and 2 new ones, error log.log and today.log.
• The xargs in macOS is a BSD version. Its manual in Apple's source (github.com/apple-oss-distributions/shell_cmds, shell_cmds-329) says it doesn't run the command on empty input, and that -r is accepted there but does nothing.

Watch out: Plain find | xargs rm can delete the wrong file. In a folder with a file named keep and a file named keep me.tmp, find . -name '*.tmp' | xargs rm -v printed removed './keep', then cannot remove 'me.tmp'. The .tmp file was still there and keep was gone. Use -print0 with -0 every time a command deletes or moves files.