find out which git commit a file was taken from

Question

A collaborator that does not use version control has sent me a file with some local modifications. Now he went on vacation. I would like to find out which version his edits were based on.

I suppose the most promising approach is to somehow iterate over the most recent commits and check for which the length of the diff is minimal.

Is there some ready-made functionality for this, or would I need to code this myself? Not being a git expert myself, what would be the most promising way to proceed?

Answer 1

I do not know about any standard git command to do this. But a simple script could aid this task. First, create a tmp-branch and commit the file to this branch. Then create a simple script like the one below to print how much different the file is from each of the 50 most recent versions of that file.

#!/bin/bash

BRANCH="tmp-branch"
FILE="path/to/file.txt"
RECENT_COMMITS=$(git rev-list -50 master -- $FILE)

for COMMIT in $RECENT_COMMITS
do
    echo -n "$COMMIT: "
    git diff $BRANCH $COMMIT --shortstat -- $FILE
done

Not fully automatic, but it would give you output like the following. In this output you identify the version with the least changes. In my example, the simplistic change i used as an example was based on edff0c0.

e2b2c157a81e0523e7d4a0a52df79cb4fce981ac:  1 file changed, 12 insertions(+), 16 deletions(-)
154d84736f4df3dd968450599dc254cda56f2057:  1 file changed, 12 insertions(+), 13 deletions(-)
ba11ecc3a4d8268f43589fb929f0877e65879f13:  1 file changed, 11 insertions(+), 13 deletions(-)
017a7a5abdffeb37671a03c0db2e32c37b0ee6bd:  1 file changed, 8 insertions(+), 9 deletions(-)
cc97d3453ebde37b02a42ca7263bf7a983222d4d:  1 file changed, 8 insertions(+), 5 deletions(-)
a84adb9e337d2cf1e851924cf27f5f0bfdca790f:  1 file changed, 7 insertions(+), 4 deletions(-)
9a3c10cefc133792377851b1b5cb8a69d3ffd788:  1 file changed, 7 insertions(+), 3 deletions(-)
edff0c0155b77e39599402574ba1c4aa02c1bbac:  1 file changed, 6 insertions(+), 2 deletions(-)
413800ab0de606548c0c69b4b35e50b527d33d7f:  1 file changed, 13 insertions(+), 2 deletions(-)
af689f1d6d76303d8e39311f48a977b87260586e:  1 file changed, 13 insertions(+), 2 deletions(-)
25123d4196533a0f3ce718a288bc3c5d975ad865:  1 file changed, 24 insertions(+), 3 deletions(-)
e7ca01b247f7e32010f256b55696c3ecb1d72144:  1 file changed, 26 insertions(+), 5 deletions(-)
6e9c2a561cc606f34ccb2cc918b297187c2e8c42:  1 file changed, 33 insertions(+), 23 deletions(-)

I'm not sure if this method is foolproof. You should probably have a look at a couple of the surrounding commits also.

Answer 2

I'd create a new branch with the colleague's change and then use git merge-base:

git merge-base finds best common ancestor(s) between two commits to use in a three-way merge. One common ancestor is better than another common ancestor if the latter is an ancestor of the former. A common ancestor that does not have any better common ancestor is a best common ancestor, i.e. a merge base. Note that there can be more than one merge base for a pair of commits.