opam-search is a command to which you provide a list of packages and a command. For each given package, it fetches its source, runs your command on it, then cleans up.

This is a bit similar to opam-grep, but:

• opam-grep only gives you a boolean for each package, whereas this is more comparable to xargs or parallel: you can output whatever you want, store results in files, etc.
• opam-grep is very slow if you need to go through thousands of packages. From memory, opam-search may be 5x faster sequentially at fetching sources, and both fetching and the user command run in parallel, whereas opam-grep does everything sequentially. I've run the ocaml lexer on every ocaml in every package compatible with ocaml 5.2 (3200 packages) in a couple of minutes on a normal laptop with no caching, and 15s with caching of the source files. And 1.5s for a cheaper search like a recursive grep.

Concretely, a use might look like this:

opam-search -x 1 -- grep -r --include '*.ml' -F '(*)'

A second executable, syntax-grep, is provided. It can be used as argument for opam-search. It does nothing useful by default, but the way to use it is to modify vendor/ocamlformat/vendor/parser-standard/lexer.mll to detect the situations you want (see other branches in the repo for examples). An example of use is finding what would break if # started a line comment (without the actual lexer, you'd find "#" in string literals, or comments, or method calls, etc).