Sunday, October 7
filter-exec-stdin
I recently added a new script to my collection of personal utilities,
filter-exec-stdin.
It’s a little hard to explain this one without reference to its close
relatives, filter-exec and
filter-exec-raw. These are, as the name
strongly implies, filters: They take standard input, transform it, and
print the result to standard output.
This makes them easy to use in a text editor like vim, where you can
pass the contents of a buffer through an external program like so:
:%!some-script
I wrote filter-exec, the first of these, as a quick and dirty way to include
shell commands and their output when writing documentation (like
userland or DigitalOcean tutorials) in Markdown. Markdown parsers
tend to ignore HTML comments, so I used comments to mark the start and end of a
block, and $ to mimic a command prompt, followed by the command string:
<!-- exec -->
$ ack --thppppt
<!-- end -->
And the filter adds the output:
<!-- exec -->
$ ack --thppppt
_ /|
\'o.O'
=(___)=
U ack --thppppt!
<!-- end -->
The output is indented 4 spaces, which causes Markdown parsers to treat it as
preformatted text. It can be run repeatedly, and will retain the command but
update the output.
Later on I was cobbling together a system for generating simple invoices, and
wanted something similar without the command included in the rendered output or
the indentation, so I added filter-exec-raw:
<!-- exec-raw ack --thppppt -->
_ /|
\'o.O'
=(___)=
U ack --thppppt!
<!-- end -->
Here the command is inside the HTML comment, and the raw output is printed
inside the block, instead of an indented version.
exec-stdin is more meta. It goes something like this:
<!-- exec-stdin sort | uniq -->
1
2
3
4
<!-- end -->
…that is, it runs the command after exec-stdin, passes the contents of the
block off to it, and replaces the block with the command’s output. I’m not
sure how useful this is. It assumes that a command will have stable output for
given input - that is, something like sort, where once the input is sorted,
it won’t change again. The idea is that I might have a block of text that
should be sorted, checked for duplicates, formatted, etc., and I can just
invoke utilities like sort, uniq, fmt, and so forth to handle those
problems.
It’s possible that a more-generally-useful filter would retain the original
block while also outputting the results of the command.
markdown, notes, shell, technical, userland :: p1k3 /
2018 /
10 /
7