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Comment by saljam | Hacker News Reader

There's a caveat with process substitution; the command can't seek() on the input file.

If if a command, for example, needs to do 2 passes, it'll will either have to be 'smart' enough to stick the output in a buffer or fail.

Yes, this is a problem in bash. In zsh you can use temporary files for process substitution, like this:

vimdiff =(ls /bin) =(ls /usr/bin)

You can use it even from bash:

zsh -c 'vimdiff =(ls /bin) =(ls /usr/bin)'

Actually vimdiff seems to have no problem with:

    vimdiff <(ls /bin) <(ls /usr/bin)