Hi Wei,

I wanted seqkit split / split2 subcommands to have an option to feed output to a shell command, just as implemented in GNU coreutils' split as --filter option (https://man7.org/linux/man-pages/man1/split.1.html).

What I did are:

• added WopenPipe function in xopen (I haven't created PR yet: https://github.com/ocxtal/xopen/tree/wopenpipe)
• added openWriter in helper.go, which opens pipe with WopenPipe if postprocessing command is nonempty.
• modified writeSeqs in helper.go to use openWriter
• modified calls to writeSeqs in split{2}.go to pass postprocessing command string
• replaced calls to Wopen in split{2}.go to openWriter
• added --post (-P) option to split{2}.go

However the patch is incomplete in the following points:

• lacks tests for this new option (I tested it just worked. I didn't care about performance.)
• might have compatibility problems around forking child process in WopenPipe

Since I'm not familiar with golang or unix processes, I'd like ask you to give any kind of comment on the patch. I'd also happy if you could take over and complete this work.

Thanks in advance,

Hajime Suzuki