Advanced usage

How do I checkout the full CMSSW repository locally?

To checkout the full CMSSW repository locally you can do:

git clone cmssw-main src

in your SCRAM work area. Notice this will require a github account (see here for the relevant FAQ). Alternatively you can do:

git clone cmssw-main-ro src

for read only access. If you want to checkout a given tag, you need to specify it via the -b flag, e.g.:

git clone cmssw-main src -b CMSSW_6_1_0

See here for more detailed information on git clone command.

How do I browse / search the code?

The source code for the official CMSSW repository can be found in the main github page for CMSSW:

Branches and tags are available by clicking on the “branches” combo box in the top left part of the page:


The source-code of the official CMSSW can be searched by using .

If you prefer using the command line interface, you can do the same by using git grep command inside CMSSW/src area:

git grep <reg-expr>


git grep Plugin.*

which will give you:

Alignment/CSA06AlignmentAlgorithm/BuildFile:<use name=FWCore/PluginManager>
Alignment/CommonAlignmentAlgorithm/BuildFile.xml:<use   name="FWCore/PluginManager"/>
Alignment/CommonAlignmentAlgorithm/interface/AlignmentAlgorithmPluginFactory.h:#include "FWCore/PluginManager/interface/PluginFactory.h"
Alignment/CommonAlignmentAlgorithm/interface/IntegratedCalibrationPluginFactory.h:#include "FWCore/PluginManager/interface/PluginFactory.h"
Alignment/CommonAlignmentMonitor/BuildFile.xml:<use   name="FWCore/PluginManager"/>
Alignment/CommonAlignmentMonitor/interface/AlignmentMonitorPluginFactory.h:#include "FWCore/PluginManager/interface/PluginFactory.h"
Alignment/CommonAlignmentMonitor/plugins/ #include "PluginManager/ModuleDef.h"
Alignment/CommonAlignmentMonitor/plugins/ "FWCore/PluginManager/interface/ModuleDef.h"
Alignment/CommonAlignmentMonitor/plugins/BuildFile.xml:<use   name="FWCore/PluginManager"/>

for more information about git grep click here.

How do I tell who is responsible for a given change?

Let’s say you want to find out who is responsible for a given change in FWCore/Framework/BuildFile.xml. If you are happy with using the web based GUI you can simply browse to the given file (click here for this particular example).

Then click on the “Blame” button (again click here for the particular example).

blame button

You can get the full information about a given change (including which files changed in the same commit) by clicking on the specific commit-id (the hash in the left column, for example bd2fd326).

Alternatively if you prefer doing it from the command line, you can use the git blame command to find out for each line which commit-id is responsible for a given change.

> git blame FWCore/Framework/BuildFile.xml

which returns:

f4330d28 (wmtan 2010-02-18 23:10:32 +0000  1) <use   name="DataFormats/Common"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  2) <use   name="DataFormats/Provenance"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  3) <use   name="FWCore/Common"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  4) <use   name="FWCore/MessageLogger"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  5) <use   name="FWCore/ParameterSet"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  6) <use   name="FWCore/PluginManager"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  7) <use   name="FWCore/PythonParameterSet"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  8) <use   name="FWCore/ServiceRegistry"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000  9) <use   name="FWCore/Utilities"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 10) <use   name="FWCore/Version"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 11) <use   name="boost"/>
bd2fd326 (wmtan 2013-01-30 23:21:16 +0000 12) <use   name="rootcintex"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 13) <use   name="rootcore"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 14) <export>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 15)   <lib   name="1"/>
f4330d28 (wmtan 2010-02-18 23:10:32 +0000 16) </export>

Let’s say we are interested in line 12, i.e. <use name="rootcintex"/>. The first column gives us the commit-id (bd2fd326 in this case). To find out everything about that commit you can use the git show command:

git show bd2fd326

which returns:

commit bd2fd32657121cda0cc132a98b3b0d68773788b8
Author: wmtan <>
Date:   Wed Jan 30 23:21:16 2013 +0000

    With Reflex usage partially removed, EventProcessor needs CINT dictionaries

diff --git a/FWCore/Framework/BuildFile.xml b/FWCore/Framework/BuildFile.xml
index a5cae15..52e584d 100644
--- a/FWCore/Framework/BuildFile.xml
+++ b/FWCore/Framework/BuildFile.xml
@@ -9,6 +9,7 @@
<use   name="FWCore/Utilities"/>
<use   name="FWCore/Version"/>
<use   name="boost"/>
+<use   name="rootcintex"/>
<use   name="rootcore"/>
<lib   name="1"/>
diff --git a/FWCore/Framework/src/ b/FWCore/Framework/src/
index a8fe6c3..fbe4bc3 100644
--- a/FWCore/Framework/src/
+++ b/FWCore/Framework/src/
@@ -83,6 +83,9 @@
#include <sched.h>

+//Needed for introspection
+#include "Cintex/Cintex.h"
namespace edm {

namespace event_processor {
@@ -585,6 +588,8 @@ namespace edm {
                        serviceregistry::ServiceLegacy iLegacy) {

    //std::cerr << processDesc->dump() << std::endl;
+    ROOT::Cintex::Cintex::Enable();

    boost::shared_ptr<ParameterSet> parameterSet = processDesc->getProcessPSet();
    //std::cerr << parameterSet->dump() << std::endl;

How can I find out who is developing in a given package?

If you are interested in the general developers for a given package, the best option is probably to use git log -- <subsytem>/<package> in the toplevel directory and have a look at recent developments. A slightly fancier way of doing it is via:

git log --pretty="%an" --since 1y -- <subsytem>/<package> | sort -u

which will print only the authors ( --pretty="%an" ) who have committed something in the given <subsytem>/<package> in the last year ( --since 1y ).

Downloading from github is painfully slow, how can I improve the situation?

If you have a local mirror of CMSSW repository at your site / university you can use it by setting the CMSSW_MIRROR environment variable. E.g.:


for CERN.

git cms-addpkg will then use it to fetch most of the data, and then switch to the official repository for fetching the missing parts.

If you happen to have a local copy of the reposiry on the machine you are working on you can speed up things even further by having:

export CMSSW_GIT_REFERENCE=<repository-path>

in particular people working on lxplus can use:


or in case you have CVMFS available you can set it to:

export CMSSW_GIT_REFERENCE=/cvmfs/

How do I setup a local mirror?

Git allows you to mirror a repository by doing:

git clone --mirror --bare $CMSSW_GIT_REFERENCE

Done that you can update the mirror via:

git remote update

How do I collapse multiple commits into one?

A very common pattern when developing a feature with git is to commit very often, to keep track of even small set of changes and once one is happy with the feature branch to go back, collapse all the minor changes and write a nice commit message.

Let’s assume for example you have a very long commit history for a topic branch my-topic.

* 00b32e7 - Sat, 6 Apr 2013 11:07:19 +0200 (10 minutes ago) (HEAD, my-topic)
|           A typo.  Giulio Eulisse
* 6ed6e45 - Sat, 6 Apr 2013 11:06:49 +0200 (11 minutes ago)
|           Some documentation added.  Giulio Eulisse
* 74f8fd0 - Sat, 6 Apr 2013 11:06:34 +0200 (11 minutes ago)
|           Some more bug fixes.  Giulio Eulisse
* 39165d9 - Sat, 6 Apr 2013 11:06:00 +0200 (12 minutes ago)
|           Some bugfix.  Giulio Eulisse
* 3a775f1 - Sat, 6 Apr 2013 11:05:21 +0200 (12 minutes ago)
            Initial commit for topic.  Giulio Eulisse

once you are completely happy with your topic branch, you might not particularly care about all the intermediate steps, and you might want to squash all of them into one. This is done by using the git reset command: it will rever you back to a given commit, but it will keep your changes in the working tree, ready to be committed. E.g.:

git reset --soft 3a775f1

will reset the commit index to the “Initial commit for topic.” but it will leave all code changes ready to be committed. You can then amend the initial commit with all the changes you previously did by simply doing:

git commit --amend

which will prompt for an updated message and will results in one single commit:

* f7400d3 - Sat, 6 Apr 2013 11:05:21 +0200 (19 minutes ago) (HEAD, master)
            Some Feature added and tested.  Giulio Eulisse

For more information about how to rewrite history, you can have a look at the git guide. In particular once you are familiar with the concept you might want to look at git rebase --interactive which provides more flexibility when you want to do more that simply merge a set of commits into one.

How do I pick up a commit from a branch and apply it to another branch?

Have a look at the git-cherry-pick command. E.g.:

git cherry-pick <hash-of-commit-on-another-branch>

will take <hash-of-commit-on-another-branch> and apply it on the current branch. Notice that the new commit will have a different hash, because the history is different.

I need to checkout a file from the old CVS HEAD / a release tag, how can I do it?

The CVS HEAD at the time of the migration is available in the git branch imported-CVS-HEAD. You can checkout files from the by simply fetching that branch in your local workarea and then using git checkout for it.

For example lets assume you want to checkout the HEAD of PhysicsTools/Configuration/test/ in CMSSW_5_3_11, where it was never released. First you need to set up the area and fetch the branch:

scram project CMSSW_5_3_11
cd CMSSW_5_3_11/src

this needs to be done only once per workarea. Then you do:

git cms-addpkg PhysicsTools/Configuration
git fetch official-cmssw imported-CVS-HEAD:imported-CVS-HEAD
git checkout imported-CVS-HEAD -- PhysicsTools/Configuration/test/

Notice the git cms-addpkg is needed to bring in the package (i.e. make the sparse checkout). Finally notice you can use the same recipe to checkout files in different release tags (e.g. CMSSW_5_3_11).

I made a mess and I do not know what I’ve currently checked out!

It can happen that due to wrong manipulations of the history, merging or rebasing, one finds itself with the current branch ref pointing to something unexpected. In order to view the history of the actions done, one can use the

git reflog

command which will print out the various steps of the HEAD. E.g:

7225357 HEAD@{0}: commit: Align header.
a59e3bd HEAD@{1}: rebase -i (finish): returning to refs/heads/gh-pages
a59e3bd HEAD@{2}: rebase -i (pick): Typos.
fbb9019 HEAD@{3}: rebase -i (pick): Nicer tables.
83a21a1 HEAD@{4}: rebase -i (pick): Migration postponed.
5acef5f HEAD@{5}: rebase -i (squash): Move Rosetta stone in a glorified page.
9911088 HEAD@{6}: rebase -i (squash): updating HEAD
d3f80cb HEAD@{7}: checkout: moving from gh-pages to d3f80cb
f48873e HEAD@{8}: commit: Move rosetta.
ff1e4b0 HEAD@{9}: rebase -i (finish): returning to refs/heads/gh-pages

One can then use git reset --hard <some-hash> to revert back to some point in history.

To learn more about git reflog you can look at its man page or the Pro Git section about it.

How can I prevent automatic forward porting of a pull request?

CMS uses a cronjob to automate the forward porting of pull requests. For examples changes which get applied to CMSSW_7_0_X get automatically forward ported to CMSSW_7_1_X and from there they then get to CMSSW_7_2_X, etc. The forward porting is smart enough to make sure that if there is any conflicting changes in a newer version, the forward port of the conflicting part will not happen. However sometimes its desiderable to avoid the forward port completely, simply because a given bug-fix / new feature might apply on an old release and not a new one. In order to avoid this one needs to prepare an additional pull request which will stop the propagation in a given release.

Let’s say we have locally a branch my-non-forwardable-feature which I only want in CMSSW_7_0_X and not in CMSSW_7_1_X. First of all I need to move from my development branch to CMSSW_7_1_X:

git fetch official-cmssw
git reset --hard official-cmssw/CMSSW_7_1_X

then I need to merge the unwanted feature there, using the -s ours option, which will tell git to ignore any change and consider our current branch (hence the name of the option) as the one from which all the changes will be taken, regardless of the merge being successful or not.

git merge -s ours my-non-forwardable-feature

Finally I need to push my branch, and open a pull request in CMSSW_7_1_X. Such a pull request will say that thre are 0 changes compared to the current CMSSW_7_1_X. Once such a Pull Request is merged, we can happily merge the one which was done for CMSSW_7_0_X which will then be ignored by the subsequent automatic forward port.

How do I ask a question?

If you have more questions about git and CMSSW on git, please use this form.