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27/02 — 2022
56.74 cm   4.7 min

Scripting in markdown

I recently wrote yet another command-line utility in Rust called present that lets you interpolate the standard output of arbitrary commands that get interpreted by the shell into a markdown file, and I thought I’d share a bit about it here.


The main problem I kept running into that pushed me to write this tool was having to manually update the outputs of command line utility help messages in my project readme documents.

For instance, I wrote a command-line utility called vim-profiler that lets you profile the startup-time for your installed vim plugins and receive a nicely formatted output. In the readme document I have a usage section that includes the output of calling vp --help. Each time I make a version change, update the description, or update the API – the result of calling vp --help changes, which prompts me to update the readme.


Instead of invoking the binary with the --help flag, piping the result to pbcopy and then manually pasting the chunk into the appropriate section within the readme myself, I can now have present do all of that for me.

Here’s how that looks like:

  1. Include the command at the start of a fenced codeblock using the present prefix
## vim-profiler 🕒


### Usage

```present cargo run -- --help
  1. Run present on the markdown file
$ present --in-place --path readme.md
  1. View the modified document!
## vim-profiler 🕒


### Usage

```cargo run -- --help
vim-profiler 0.0.4
A vim profiling tool.

    vp [FLAGS] [OPTIONS]

    -h, --help       Prints help information
    -r, --reverse    Display the plugin times in reverse order (fastest first)
    -s, --sys        Show system plugins in the output
    -V, --version    Prints version information
    -v, --verbose    Add informative messages during program execution

    -c, --command   <command>      The command to run, e.g vim or neovim [default: vim]
    -n, --count     <count>        The number of plugins to list in the output
    -e, --export    <path>         Export the results to a CSV file
    -f, --file      <file>         A file to open
    -i, --iter      <iter>         The number of iterations
    -p, --plot      <path>         Plot the data and save it to a SVG file
    -x, --precision <precision>    Precision in the output

In practice, I’ll add the command in a justfile all recipe that I invoke before I commit any changes, such as shown in this justfile.

Moving forward

The project is still in its early stages of development, and there most certainly exists a few non-trivial bugs to be found and fixed.

Some things on the todo list, in no particular order, include:

Improve diff output in interactive mode

The project uses the similar crate to help with diff output in interactive mode, but it can be made nicer by taking advantage of additional features the crate has to offer.

Support same-line command interpolation

This isn’t that important, but it would be nice to support having backticks remain on the same line and have the command result get interpolated with the appropriate newlines. e.g

-> ```echo foo```

-> ```echo foo

Handle quotes as a single argument

This would let you actually write inline bash scripts, e.g /bin/bash -c 'for i in *; do echo "$i"; done'. As of right now, the program just splits the entire command string on whitespace, which the shell doesn’t like in certain situations.

This however, for now, can be hacked around by simply including the script in a justfile or makefile and invoking just <name> or make <name> within the markdown file.

Get rid of pulldown_cmark

The project uses the pulldown_cmark crate to get full codeblock ranges within the source, and then hackily turns them into two separate ranges (start, end) which represent the starting and ending range of a single codeblock. This could probably be done better, perhaps without depending on a library for that initial step.

Overall, I get what I need out of the program as it is. Whether or not the aformentioned todo’s provide useful to someone else will most likely have a non-trivial amount of impact on my motiviation for getting them done.

Feel free to check out the code over on github https://github.com/terror/present. It is licensed under the highly permissive CC0-1.0 license.


I'm Liam.

I am an incoming honours math/compsci undergrad @ mcgill, programmer and rationalist.

Send me a message by email: liam [at] scalzulli.com or matrix: worse:matrix.org

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