With git I can set a custom diff tool that is used for certain file extensions by the following in .git/config

[diff "csv_diff"]
    command = Tools/csv_diff

and this in .gitattributes (in the root of the repository)

*.csv diff=csv_diff

This works when using git diff, but it doesn't work with git show. My question is, how do I use a custom tool with git show?

Looks like you're looking for the --ext-diff option.

Here's what git show docs say about it:

--ext-diff

Allow an external diff helper to be executed. If you set an external diff driver with gitattributes, you need to use this option with git-log and friends.

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