save not >: piping tar into a tuned xz compression in nushell

To compress a folder you first bundle it into a single file (tar), then compress that file (xz) — which naively means juggling three artifacts: the folder, the intermediate .tar, and the final .tar.xz. In a POSIX shell the byte-stream pipe gets you from three files to one:

tar -cf - folder_name/ | xz -9 -T0 > folder_name.tar.xz

In nushell the same pipeline shape works — but the way you write the output out is the gotcha.

The nushell version

tar -cf - folder_name/ | xz -9 -T0 | save folder_name.tar.xz

Two things to unpack.

External-to-external pipes stay raw bytes. Nushell’s docs are explicit here: data piped between two external commands flows the same way it would in Bash, so tar’s raw archive bytes reach xz untouched. The UTF-8 re-interpretation that nushell applies between internal commands never kicks in.

save, not >, is how you write pipeline output to a file. In a POSIX shell > opens (or truncates) the file and bytes stream straight in. Nushell’s > does not behave that way — the data-model-correct command is save:

tar -cf - folder_name/ | xz -9 -T0 | save folder_name.tar.xz
#                                    ^^^^ writes the stream to the file

For bytes, save --raw folder_name.tar.xz is the explicit form.

Why pipe to xz at all, instead of tar -J?

tar can compress in one shot with its built-in filters:

tar -cJf folder_name.tar.xz folder_name/

but that invokes xz with a fixed configuration you can’t tune. Piping to xz directly hands over the flags:

• -9 — maximum compression level (default is -6).
• -T0 — use all available CPU cores in parallel.

That control is the real payoff of the pipe: same three-line shape, but the compressor runs exactly how you want it to.

Side by side

Gotchas

• > silently does something different in nushell. Reach for save (and save --raw for binary). This is the single most common “my POSIX pipe broke” moment when moving to nushell.
• xz -9 -T0 can eat your RAM. One thread per core at max level spikes memory; on a constrained machine this can OOM and break the pipe. Use -T <n> below your core count or back off to -6.
• The - in tar -cf - is what enables the stream — it tells tar to write the archive to stdout instead of a file. Forget it and there’s nothing to pipe.
• tar -J is fine when tuning doesn’t matter; reach for the explicit pipe only when you actually want -T0/-9-style control.

Related

• nushell-splat-docker-stop — another nushell pipe trick
• nushell-par-each-parallel-tests — parallelizing across cores with par-each