The short question is: how to make
git diff
invoke
git difftool -t opendiff -y
? The second line is to invoke opendiff to diff some files, even when there is no configuration whatsoever done beforehand (I am using git version 1.9.2 and the current Xcode 12.1 on macOS Catalina and Big Sur).
The line
git config --global diff.external 'git difftool -t opendiff -y'
does not work.
Longer details:
We can readily invoke opendiff using git difftool -t opendiff -y, so git knows how to use opendiff as a visual diff tool without any additional configuration. And there is a way on github about how to use diff.external to invoke a separate shell script to invoke opendiff.
So since git readily knows how to invoke opendiff using difftool, we really should not need to use an external shell script. For the fact that diff.external can invoke any external command, then it would appear logical to even invoke itself, which is to invoke git difftool -t opendiff -y, but the line
git config --global diff.external 'git difftool -t opendiff -y'
won't work and can cause an error when we do a git diff afterwards. (it may appear it is due to git passing additional arguments to the "external" command?) The error it gives is:
fatal: 234c9c6e7d7f2798068d2f6ee434af9a9dd88123: no such path in the working tree.
Use 'git <command> -- <path>...' to specify paths that do not exist locally.
external diff died, stopping at foo.rb.
How to make it work so that git diff can invoke the git difftool with opendiff? It seems git diff and git difftool is a little bit adhoc, because we even configure the default difftool to use using git config --global diff.tool opendiff so we are touching diff to configure difftool. So in a way git diff and git difftool look like separate things but in a way they look like they are one unit.
One way to make it work but using an alias is
git config --global alias.diffy 'difftool -t opendiff -y'
but then we will need to use git diffy to run it instead of git diff. We can also create a Bash or Zsh alias gitdiff to run git difftool -t opendiff -y but it seems we should be able to use git diff without setting up an additional shell script.