More accurately, it can be used for any command which expects a file and doesn't do anything too weird in reading it (e.g. doesn't seek to the beginning and read it again)

The '<( ... )' is just giving the path to that command's stdout as a file descriptor.

    $ ls <(echo hi)  
    /proc/self/fd/11
    $ vi <(echo hi)
    # opens vi with 'hi' as the contents

Thank you. I was just thinking of looking into how it was implemented.

It is called "process substitution", and can work the other way around:

http://www.tldp.org/LDP/abs/html/process-sub.html

With fish:

https://fishshell.com/docs/current/commands.html#psub

But there it only works one way:

https://github.com/fish-shell/fish-shell/issues/1786

Note that this is a bash feature; other shells may have different syntax.

It's the same in ksh at least, so one is covered in the BSDs. It could be also implemented as a separate tool, but not as convenient, because of additional quoting - I did once.

I use it in my frugal SSG project [0] to remove HTML files for every removed source file. It's great to use it this way as it doesn't use temp files.

[0] https://gist.github.com/hadrianw/060944011acfcadd889d937b960...