Instead of git rebase -i and editing manually is there a possibility to supply git rebase with the pick/drop/squash list as a text file, or do I need to redirect GIT_EDITOR to a script that supplies it ?
You'll need to use GIT_EDITOR or GIT_SEQUENCE_EDITOR to edit the file in place. That program will receive the file name as its argument, and can be a shell command, so you can do something like this:
GIT_SEQUENCE_EDITOR='sed -i -e '\''2,$s/^pick/fixup/'\' git rebase
The difference between the two editors is that GIT_SEQUENCE_EDITOR only applies to the rebase todo list, but not to other things like commit messages, whereas GIT_EDITOR applies to both. So, for example, if you want to automatically squash in all squash and fixup commits created by git commit --squash and git commit --fixup without prompting, you can do this:
GIT_SEQUENCE_EDITOR=true git rebase -i --autosquash main
That automatically accepts the squash and fixup commands without prompting, but still leaves your normal editor for editing commit messages due to squash commits.