What is the intended purpose of .orig files created by merge tool?

Question

The Documentation does not state why .orig files are created by default. Also, their content after the next merge is undefined. Still, there must be some reason for having them created by default.

My best guess is that correcting a failed merge is common and also easier when existing .orig files are present (as opposed to repeating the merge) but apparently most users do not know what to do with them.

Answer 1

It's not that they are created by default, but rather that they are left behind by default. The git mergetool command always copies the conflict-marker-ized file to a .orig before running the command you selected as the merge tool.

The real reason the git mergetool command creates them is so that it can compare the date stamp on the supposedly-merged file to the date stamp on the .orig file it recently created.

Suppose the command that git mergetool ran, that supposedly merged the three base, local (HEAD / --ours), and remote (--theirs) files, actually did nothing at all. For instance, suppose you chose the command true, which is a no-op that simply succeeds. In this case, the "merged" file, which the command did not write-to, has the same time stamp it had when git mergetool made it. Meanwhile the .orig file also has the same time stamp it had when git mergetool made it. This indicates to git mergetool that the merge-tool you chose actually failed, and the file is not actually merged after all. If the supposedly-merged file is newer than the .orig file, though, then the command you selected must have written to the file, so it's probably merged the way you want, now.

(The git mergetool command has a separate configuration knob, "trust exit code", that is also intended for the same purpose: to try to decide whether the tool actually did merge the files. It uses both of these tricks to decide whether to run git add. For merge tools that git mergetool knows about, it has a built-in "trust exit code" setting. Obviously, the true command is a terrible merge tool and is not built in.)

If we modify the question to read: Why does git mergetool leave the .orig file behind by default? then we have to ask the authors of git mergetool why they chose that as the default. Only they actually know. We can speculate: perhaps it is because git checkout -m, which can re-create the conflicts if the merge is still going on, did not exist as a command at the time. Perhaps it is because they thought this was the more convenient default: it's easier to rm *.orig than it is to run git checkout -m. But the truth is, I don't really know for sure.