| From: Junio C Hamano <gitster@pobox.com> |
| To: git@vger.kernel.org |
| Subject: [HOWTO] Reverting an existing commit |
| Abstract: In this article, JC gives a small real-life example of using |
| 'git revert' command, and using a temporary branch and tag for safety |
| and easier sanity checking. |
| Date: Mon, 29 Aug 2005 21:39:02 -0700 |
| Content-type: text/asciidoc |
| Message-ID: <7voe7g3uop.fsf@assigned-by-dhcp.cox.net> |
| |
| How to revert an existing commit |
| ================================ |
| |
| One of the changes I pulled into the 'master' branch turns out to |
| break building Git with GCC 2.95. While they were well-intentioned |
| portability fixes, keeping things working with gcc-2.95 was also |
| important. Here is what I did to revert the change in the 'master' |
| branch and to adjust the 'seen' branch, using core Git tools and |
| barebone Porcelain. |
| |
| First, prepare a throw-away branch in case I screw things up. |
| |
| ------------------------------------------------ |
| $ git checkout -b revert-c99 master |
| ------------------------------------------------ |
| |
| Now I am on the 'revert-c99' branch. Let's figure out which commit to |
| revert. I happen to know that the top of the 'master' branch is a |
| merge, and its second parent (i.e. foreign commit I merged from) has |
| the change I would want to undo. Further I happen to know that that |
| merge introduced 5 commits or so: |
| |
| ------------------------------------------------ |
| $ git show-branch --more=4 master master^2 | head |
| * [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
| ! [master^2] Replace C99 array initializers with code. |
| -- |
| - [master] Merge refs/heads/portable from http://www.cs.berkeley.... |
| *+ [master^2] Replace C99 array initializers with code. |
| *+ [master^2~1] Replace unsetenv() and setenv() with older putenv(). |
| *+ [master^2~2] Include sys/time.h in daemon.c. |
| *+ [master^2~3] Fix ?: statements. |
| *+ [master^2~4] Replace zero-length array decls with []. |
| * [master~1] tutorial note about git branch |
| ------------------------------------------------ |
| |
| The '--more=4' above means "after we reach the merge base of refs, |
| show until we display four more common commits". That last commit |
| would have been where the "portable" branch was forked from the main |
| git.git repository, so this would show everything on both branches |
| since then. I just limited the output to the first handful using |
| 'head'. |
| |
| Now I know 'master^2~4' (pronounce it as "find the second parent of |
| the 'master', and then go four generations back following the first |
| parent") is the one I would want to revert. Since I also want to say |
| why I am reverting it, the '-n' flag is given to 'git revert'. This |
| prevents it from actually making a commit, and instead 'git revert' |
| leaves the commit log message it wanted to use in '.msg' file: |
| |
| ------------------------------------------------ |
| $ git revert -n master^2~4 |
| $ cat .msg |
| Revert "Replace zero-length array decls with []." |
| |
| This reverts 6c5f9baa3bc0d63e141e0afc23110205379905a4 commit. |
| $ git diff HEAD ;# to make sure what we are reverting makes sense. |
| $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. |
| $ make clean test ;# make sure it did not cause other breakage. |
| ------------------------------------------------ |
| |
| The reverted change makes sense (from reading the 'diff' output), does |
| fix the problem (from 'make CC=gcc-2.95' test), and does not cause new |
| breakage (from the last 'make test'). I'm ready to commit: |
| |
| ------------------------------------------------ |
| $ git commit -a -s ;# read .msg into the log, |
| # and explain why I am reverting. |
| ------------------------------------------------ |
| |
| I could have screwed up in any of the above steps, but in the worst |
| case I could just have done 'git checkout master' to start over. |
| Fortunately I did not have to; what I have in the current branch |
| 'revert-c99' is what I want. So merge that back into 'master': |
| |
| ------------------------------------------------ |
| $ git checkout master |
| $ git merge revert-c99 ;# this should be a fast-forward |
| Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c... |
| cache.h | 8 ++++---- |
| commit.c | 2 +- |
| ls-files.c | 2 +- |
| receive-pack.c | 2 +- |
| server-info.c | 2 +- |
| 5 files changed, 8 insertions(+), 8 deletions(-) |
| ------------------------------------------------ |
| |
| There is no need to redo the test at this point. We fast-forwarded |
| and we know 'master' matches 'revert-c99' exactly. In fact: |
| |
| ------------------------------------------------ |
| $ git diff master..revert-c99 |
| ------------------------------------------------ |
| |
| says nothing. |
| |
| Then we rebase the 'seen' branch as usual. |
| |
| ------------------------------------------------ |
| $ git checkout seen |
| $ git tag seen-anchor seen |
| $ git rebase master |
| * Applying: Redo "revert" using three-way merge machinery. |
| First trying simple merge strategy to cherry-pick. |
| * Applying: Remove git-apply-patch-script. |
| First trying simple merge strategy to cherry-pick. |
| Simple cherry-pick fails; trying Automatic cherry-pick. |
| Removing Documentation/git-apply-patch-script.txt |
| Removing git-apply-patch-script |
| * Applying: Document "git cherry-pick" and "git revert" |
| First trying simple merge strategy to cherry-pick. |
| * Applying: mailinfo and applymbox updates |
| First trying simple merge strategy to cherry-pick. |
| * Applying: Show commits in topo order and name all commits. |
| First trying simple merge strategy to cherry-pick. |
| * Applying: More documentation updates. |
| First trying simple merge strategy to cherry-pick. |
| ------------------------------------------------ |
| |
| The temporary tag 'seen-anchor' is me just being careful, in case 'git |
| rebase' screws up. After this, I can do these for sanity check: |
| |
| ------------------------------------------------ |
| $ git diff seen-anchor..seen ;# make sure we got the master fix. |
| $ make CC=gcc-2.95 clean test ;# make sure it fixed the breakage. |
| $ make clean test ;# make sure it did not cause other breakage. |
| ------------------------------------------------ |
| |
| Everything is in the good order. I do not need the temporary branch |
| or tag anymore, so remove them: |
| |
| ------------------------------------------------ |
| $ rm -f .git/refs/tags/seen-anchor |
| $ git branch -d revert-c99 |
| ------------------------------------------------ |
| |
| It was an emergency fix, so we might as well merge it into the |
| 'release candidate' branch, although I expect the next release would |
| be some days off: |
| |
| ------------------------------------------------ |
| $ git checkout rc |
| $ git pull . master |
| Packing 0 objects |
| Unpacking 0 objects |
| |
| * commit-ish: e3a693c... refs/heads/master from . |
| Trying to merge e3a693c... into 8c1f5f0... using 10d781b... |
| Committed merge 7fb9b7262a1d1e0a47bbfdcbbcf50ce0635d3f8f |
| cache.h | 8 ++++---- |
| commit.c | 2 +- |
| ls-files.c | 2 +- |
| receive-pack.c | 2 +- |
| server-info.c | 2 +- |
| 5 files changed, 8 insertions(+), 8 deletions(-) |
| ------------------------------------------------ |
| |
| And the final repository status looks like this: |
| |
| ------------------------------------------------ |
| $ git show-branch --more=1 master seen rc |
| ! [master] Revert "Replace zero-length array decls with []." |
| ! [seen] git-repack: Add option to repack all objects. |
| * [rc] Merge refs/heads/master from . |
| --- |
| + [seen] git-repack: Add option to repack all objects. |
| + [seen~1] More documentation updates. |
| + [seen~2] Show commits in topo order and name all commits. |
| + [seen~3] mailinfo and applymbox updates |
| + [seen~4] Document "git cherry-pick" and "git revert" |
| + [seen~5] Remove git-apply-patch-script. |
| + [seen~6] Redo "revert" using three-way merge machinery. |
| - [rc] Merge refs/heads/master from . |
| ++* [master] Revert "Replace zero-length array decls with []." |
| - [rc~1] Merge refs/heads/master from . |
| ... [master~1] Merge refs/heads/portable from http://www.cs.berkeley.... |
| ------------------------------------------------ |
