[QUESTION] mergetool environment variables

I am trying to integrate a custom mergetool with a shell wrapper. What I get from the online help is the following description referring to the command and environment variables.

mergetool.<tool>.cmd Specify the command to invoke the specified merge tool. The specified command is evaluated in shell with the following variables available: BASE is the name of a temporary file containing the common base of the files to be merged, if available; LOCAL is the name of a temporary file containing the contents of the file on the current branch; REMOTE is the name of a temporary file containing the contents of the file from the branch being merged; MERGED contains the name of the file to which the merge tool should write the results of a successful merge.

When I try to use this from a shell, simply with: #!/bin/sh env exit 1

the described environment variables: BASE, LOCAL, REMOTE, and MERGED, are not present.

The history is trivial, but has a 1 line conflict after a git merge. The file is called a, with two lines at the parent commit and the middle line changed in both branches being merged. There are correct temp files in the directory while the shell script is being run. The merged file looks as follows:

Hello <<<<<<< HEAD Ours ======= Theirs

World

This applies to git 2.45.1 in cygwin and 2.51.0 on NonStop. There are no upstreams. Is this the problem?

Any thoughts on this?

Thanks, Randall

-- Brief whoami: NonStop&UNIX developer since approximately UNIX(421664400) NonStop(211288444200000000) -- In real life, I talk too much.