External commands

% and !

% resolves to the current filename. ! calls an external function. Together, %! executes a command over the file, and replaces the current buffer with the output.

Mapping over files with :w

You can generate derived JSON from an existing file with jq, which I sometimes use to generate and manipulate test vectors:
:w !jq '.key-to-filter-by' > filtered.json
You can easily pass Markdown (or any other text format) files straight through pandoc. This is useful for exporting documentation for sharing with non-engineers.
:w !pandoc -o FILENAME.pdf

Buffer processing with :%:

Processing can be achieved by calling %! !external_fn to replace buffer contents in-place.
In this example, we pipe a JSON file through jq to prettify:
:%: !jq '.'
In this example, we convert the current file from Markdown to HTML using pandoc:
:%: !pandoc -f markdown -t html
Last modified 11mo ago
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% and !
Mapping over files with :w
Buffer processing with :%: