I knew about this neat shell expansion feature (very helpful for file selection):

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$ echo {foo,ba{r,z}}
foo bar baz

Today I stumbled upon this stackoverflow answer containing the command (shortened here):

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$ sudo rsync -avAX --delete --exclude={"/dev/*","/proc/*","/sys/*"} / /mnt/rootfs

I wondered how it would work since I thought it would expand to … --exclude="/dev/*" "/proc/*" "/sys/*" …, which wouldn’t exclude those other paths in rsync.

Turns out my understanding was incomplete:

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$ echo --exclude={foo,ba{r,z}}
--exclude=foo --exclude=bar --exclude=baz

It prepends --exclude= to every expanded, space-separated value! The same is applicable to suffixes too:

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$ echo foo{0,1}bar
foo0bar foo1bar

As usual, this works for prefixes and suffixes up to a space, unless the space is escaped:

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$ echo before\ foo{0,1}bar after
before foo0bar before foo1bar after
$ echo "before foo"{0,1}bar after
before foo0bar before foo1bar after

This feature is called “brace expansion”. And zsh’s manual does show this use case in the manual: https://zsh.sourceforge.io/Doc/Release/Expansion.html#Brace-Expansion. It also works in bash: https://mywiki.wooledge.org/BraceExpansion.

/me murmuring to myself: “RTFM”