HOWTO: git - change branch without touching working copy (at all)

Did you ever had the need in a git repository to change to another branch without altering AT ALL the working copy and ever wondered how that's done?

Usual use cases might be when you mde some changes to the working copy thinking you were on another branch, or you double-track in git a directory which is also tracked by another VCS (e.g. ClearCase).

What you need, in fact, is to update the index and not touch the working copy. The command that does that is

git read-tree otherbranch

If you also need to commit the state of your working tree to the otherbranch, you also need to tell git to actually associate the curent HEAD with the branch you just switched to:

git symbolic-ref HEAD refs/heads/otherbranch

I use this approach at my work place* to develop/experiment with possible code improvements on my machine before considering the merge into the official code.

* The preferred VCS is (Base) ClearCase, and I keep a git repository over the relevant part of the project in the ClerCase Dynamic View, so for synchronisations, the files in the working copy are updated by ClearCase and I have to resync my git branch (clearcaseint) following the latest official code from time to time, so I can pull in my local disk git repository the clearcaseint branch and merge it with my experimental changes in my git feature branches. 

If people are curious about how I work with ClearCase and git, I can expand on this in another post.

(Not a) GNU Make quirk, or why logs should be provided

About two months ago I was writing about a quirk I found in GNU Make related to the $(patsubst ) function.

I have just tried this on my Debian Wheezy laptop which has make 3.81, but I wasn't able to reproduce the issue with the version from Debian (3.81-8.2).

The makefile looks like this:

PATH := ../some/prefixCPU12suf/include
CPUINC := $(patsubst ../some/prefix%,%,$(PATH))
CPU := $(patsubst %/include,%,$(CPUINC))

default:
    @echo "PATH   = $(PATH)"
    @echo "CPUINC = $(CPUINC)"
    @echo "CPU    = $(CPU)"

And the result was correct:

0 eddy@heidi /tmp $ make
PATH   = ../some/prefixCPU12/include
CPUINC = CPU12/include
CPU    = CPU12
0 eddy@heidi /tmp $ make --version
GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for x86_64-pc-linux-gnu

The worst part is that I know I tested this issue on 3.82 on Cygwin and on Linux with the 3.82 version and it failed, but I wasn't able to remember how I did it. I started searching through the directory where I knew there could be the test makefile, I wasn't able to find it, until I remembered what I was trying to achieve.

From a path like ../some/prefixCPU12suf/include I wanted to use % to remove the parts 'some/prefix' and 'suf/include' because in the directory ../CPU12 there were some files that needed to be processed.

The actual issue is that GNU Make's '%' is not analogous to shell's '*', so that means code like this does not work as I assumed anf the 'pref' part is not an anchor:

PATH := ../some/prefCPU12suf/include
CPUINC := $(patsubst pref%,%,$(PATH))
CPU := $(patsubst %suf/include,%,$(CPUINC))

default:
    @echo "PATH   = $(PATH)"
    @echo "CPUINC = $(CPUINC)"
    @echo "CPU    = $(CPU)"

Which leads to these results, no matter the version:

0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ ./make -f /tmp/makefile
PATH   = ../some/prefCPU12suf/include
CPUINC = ../some/prefCPU12suf/include
CPU    = ../some/prefCPU12
0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ ./make --version
GNU Make 3.82
Built for x86_64-unknown-linux-gnu
Copyright (C) 2010  Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ make -f /tmp/makefile
PATH   = ../some/prefCPU12suf/include
CPUINC = ../some/prefCPU12suf/include
CPU    = ../some/prefCPU12
0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ make --version
GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for x86_64-pc-linux-gnu

Not sure if this could be qualified as a true bug, or a if the way I expected is a nice to have feature, but, in any case, the behaviour is consistent, unlike my brain which failed to initially identify the inconsistency in my code:

0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ grep patsubst /tmp/makefile
CPUINC := $(patsubst pref%,%,$(PATH))
CPU := $(patsubst %suf,%,$(CPUINC))
0 eddy@heidi ~/usr/src/make/make-profiler/make-3.82 $ make -f /tmp/makefile
PATH   = ../some/prefCPU12suf/include
CPUINC = ../some/prefCPU12suf/include
CPU    = ../some/prefCPU12suf/include

Note that this behaviour of patsubst is asymtric to how subst works, as explained in the updated old post.

This took some extra effort to remember what was the actual issue, and shows why logs are important when reporting an issue, and why reporting issues as soon as they were encountered: because human brains are faulty. (Yes, yours, too!)

HOWTO: git - remove those pesky remotes/origin/

Often people create branches to fix various bugs or implement various features, and publish those to a public repository to be pulled in the official repository (e.g. pull requests in the github model).

The problem is that after the fix is merged the local repo still has the branch that tracks the origin (your public repository).

To remove the local branch it is simple:

git branch -d feature_x

To remove the actual branch in the remote repository:

git push origin --delete feature_x

But, still the local tracking branches remain, those pesky 'remotes/origin/feature_x'. Here is an example from one of my repos:

$ git branch -a
  activestate
  master
  next
* py.test
  sysplatformtest
  remotes/activestate/master
  remotes/origin/HEAD -> origin/master
  remotes/origin/appauthor-fallback
  remotes/origin/linux-fixes
  remotes/origin/master
  remotes/origin/next
  remotes/origin/py.test
  remotes/origin/root-directories
  remotes/origin/sysplatformtest
  remotes/origin/wrapper-defaults

All of those can be removed with this command:

git remote prune origin

 And the output for my appdirs repository is:

Pruning origin
URL: git@github.com:eddyp/appdirs.git
 * [pruned] origin/appauthor-fallback
 * [pruned] origin/linux-fixes
 * [pruned] origin/next
 * [pruned] origin/root-directories
 * [pruned] origin/sysplatformtest
 * [pruned] origin/wrapper-defaults

Which, finally removes those damned refs:

$ git branch -a
  activestate
* master
  py.test
  remotes/activestate/master
  remotes/origin/HEAD -> origin/master
  remotes/origin/master
  remotes/origin/py.test

Great!

Update 2013-07-24:  There have been also other solutions proposed in the comments section. I am quoting them here for better visibility.

Besides the solution I proposed, people commented about other alternatives for pruning remote branches, by using git fetch --prune or by using git-push's --prune option.

Brice provided a method to remove specific branches one by one with git branch -dr command:

git branch -dr origin/foo

What is ironic is that in the past I have used this format, and before writing the original article I was looking for this, but haven't found it that easily. I hope this article will help some other poor soul or myself in the future.

Interesting talk on the scalability of git

http://vimeo.com/m/64716825