Why git blame does not follow renames?
$ pwd /data/mdi2/classes $ git blame -L22,+1 -- utils.js 99b7a802 mdi2/utils.js (user 2015-03-26 21:54:57 +0200 22) #comment $ git blame -L22,+1 99b7a802^ -- utils.js fatal: no such path mdi2/classes/utils.js in 99b7a802^
As you have noticed, the file were in different directory in that commit
$ git blame -L22,+1 99b7a802^ -- ../utils.js c5105267 (user 2007-04-10 08:00:20 +0000 22) #comment 2
Despite on doc
The origin of lines is automatically followed across whole-file renames (currently there is no option to turn the rename-following off)
blame does not follow renames. Why?
UPDATE: Short answer
git blame follow renames but not for
git blame COMMIT^ -- <filename>
But this is too hard to track file renames manually through bulk of renames and ton of history.
I think, this behaviour must be fixed to silently follow renames for
git blame COMMIT^ -- <filename>. Or, at least,
--follow must be implemented, so I can:
git blame --follow COMMIT^ -- <filename>
UPDATE2: That is impossible. Read below.
ANSWER FROM MAILLIST by Junio C Hamano
git blamefollow renames but not for
git blame COMMIT^ -- <filename>
Suppose you have file A and file B in your version v1.0.
Six month down the road, the code was much refactored, and you do
not need the contents of these two files separately. You have
removed A and B and much of what they had is now in file C. That is
the current state.
git blame -C HEAD -- C
may follow the contents from both just fine, but if you were
allowed to say
git blame v1.0 -- C
what does it even mean? C did not exist v1.0 at all. Are you
asking to follow the contents of A back then, or B? How did you
tell you meant A and not B when you told it C in this command?
“git blame” follows content movements, and never treats “renames” in
any special way, as it is a stupid thing to do to think a rename is
somehow special 😉
The way you tell what content to start digging from to the command
from its command line is to give starting point commit (defaults to
HEAD but you may give COMMIT^ as your example) and the path in that
starting point. As it does not make any sense to tell C to Git and
then magically make it guess you meant A in some cases and B in some
other. If v1.0 did not have C, the only sensible thing to do is to
exit instead of making a guess (and without telling the user how it
One Solution collect form web for “Why git blame does not follow renames?”
git blame does follow renames (as does
git log if you give it
--follow). The problem lies in the way it follows renames, which is a not-very-thorough hack: as it steps back one commit at a time (from each child to each parent), it makes a diff—the same kind of diff you can make manually with:
git diff -M SHA1^ SHA1
—and checks to see if this diff detected a rename.1
That’s all fine as far as it goes, but it means that for
git blame to detect a rename, (a)
git diff -M has to be able to detect it (fortunately that is the case here) and—here’s what’s causing you problems—it must step across the rename.
For instance, suppose the commit graph looks a bit like this:
A <-- B <-- ... Q <-- R <-- S <-- T
where each uppercase letter represents a commit. Suppose further that a file was renamed in commit
R, so that in commits
T it has name
newname while in commits
Q it has name
If you run
git blame -- newname, the sequence starts at
S, and compares
R. When it compares
git blame discovers the name-change, and starts looking for
oldname in commits
Q and earlier, so when it compares
Q it compares files
oldname in those two commits.
If, on the other hand, you run
git blame R^ -- newname (or
git blame Q -- newname) so that the sequence starts at commit
Q, there is no file
newname in that commit, and there is no rename when comparing
git blame simply gives up.
The trick is that if you’re starting from a commit in which the file had the previous name, you must give git the old name:
git blame R^ -- oldname
and then it all works again.
git diff documentation, you will see that there is a
-M option that controls how
git diff detects renames. The
blame code modifies this a bit (and in fact does two passes, one with
-M turned off and a second with
-M turned on) and uses its own (different)
-M option for somewhat different purposes, but ultimately it’s using this same code.
[Edit to add reply to comment (didn’t fit as a comment itself)]:
Is any tool that can show me file renames like: git renames <filename> SHA date oldname->newname
Not exactly, but
git diff -M comes close, and may be close enough.
I’m not sure what you mean by “SHA date” here, but
git diff -M allows you to supply two SHA-1s and compares left-vs-right. Add
--name-status to get just file names and dispositions. Hence
git diff -M --name-status HEAD oldsha1 may report that to convert from
oldsha1, git believes you should
Rename a file and will report the old name as the “new” name. For instance, in the git repository itself, there is a file currently named
Documentation/giteveryday.txt that used to have a slightly different name:
$ git diff -M --name-status HEAD 992cb206 M .gitignore M .mailmap [...snip...] M Documentation/diff-options.txt R097 Documentation/giteveryday.txt Documentation/everyday.txt D Documentation/everyday.txto [...]
If that’s the file you care about, you’re good. The two problems here are:
- finding an SHA1: where did
992cb206come from? If you already have an SHA-1, that’s easy; if not,
git rev-listis the SHA1-finding tool; read its documentation;
- and the fact that following a series of renames through each commit one commit at a time, as
git blamedoes, may produce quite different answers than comparing a much-later commit (
HEAD) against a much-earlier commit (
992cb206or whatever). In this case, it comes out the same, but the “similarity index” here is 97 out of 100. If it were to have been modified much more in some of the intermediate steps, that similarity index might fall below 50% … yet, if we were to compare a revision just a little after
git blamewould), perhaps the similarity index between those two files might be higher.
What’s needed (and missing) is for
git rev-list itself to implement
--follow, so that all commands that use
git rev-list internally—i.e., most commands that work on more than just one revision—can do the trick. Along the way, it would be nice if it worked in the other direction (currently
--follow is newer-to-older only, i.e., works fine with
git blame and works ok with
git log as long you don’t ask for oldest history first with