Git run server side hook when there are no updates

Question

I have a server side hook (post-update) that will run after every code push I do. Is there any way (or maybe another hook?) to run it even if there are no new changes to push?

Example:

$ git push origin master
remote: Hook running....
$
$ git push origin master
Everything up-to-date
$

I'd like it to be run again. Is that possible?

Answer 1

Many thanks to @pts for the answer (used all my votes today, can't immediately upvote it :)); for those (like me) having a slight problem understanding how it functions exactly, here is a simple command line log, demonstrating the use of non-existing-branch to trigger pre-receive:

# create original repo:

$ cd /tmp
$ mkdir repotest_git
$ cd repotest_git/
$ git init
Initialized empty Git repository in /tmp/repotest_git/.git/
$ git config user.name "Your Name"
$ git config user.email you@example.com

# populate repo with 1 commit:

$ echo "test" > test.txt
$ git add test.txt
$ git commit -m "initial commit"
[master (root-commit) 2b60608] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 test.txt
$ 

# create remote "bare" repo; and add pre-receive hook there:

$ cd /tmp
$ git clone --bare repotest_git repotest-remote.git
Cloning into bare repository 'repotest-remote.git'...
done.
$ cd repotest-remote.git
$ 
$ cat > hooks/pre-receive <<"EOF"
#!/bin/sh
echo from pre-receive: HELLO_VAR [$HELLO_VAR]
exit 1
EOF
$ chmod +x hooks/pre-receive

# go back to original repo, add a remote reference
# to the newly created remote "bare" repo; update with pull:

$ cd /tmp
$ cd repotest_git
$ git remote add origin file:///tmp/repotest-remote.git
$ git pull origin master
From file:///tmp/repotest-remote
 * branch            master     -> FETCH_HEAD
Already up-to-date.

# try testing the hook script;
# since we don't have any changes, Everything is 
# up-to-date, and the pre-receive won't trigger:

$ git push
Everything up-to-date

# try testing with HEAD:non-existing-branch
# pre-receive gets triggered - and 
# we can see variable is not there:

$ git push origin master HEAD:non-existing-branch
Total 0 (delta 0), reused 0 (delta 0)
remote: from pre-receive: HELLO_VAR []
To file:///tmp/repotest-remote.git
 ! [remote rejected] HEAD -> non-existing-branch (pre-receive hook declined)
error: failed to push some refs to 'file:///tmp/repotest-remote.git'

# try testing again, this time specify 
# env. variable on command line....
# pre-receive gets triggered - and 
# we can see variable is there:

$ HELLO_VAR=hello git push origin master HEAD:non-existing-branch
Total 0 (delta 0), reused 0 (delta 0)
remote: from pre-receive: HELLO_VAR [hello]
To file:///tmp/repotest-remote.git
 ! [remote rejected] HEAD -> non-existing-branch (pre-receive hook declined)
error: failed to push some refs to 'file:///tmp/repotest-remote.git'

Here, for local work, everything works as expected with variables; but, obviously, if your remote repo is behind servers etc, it can get problematic to see how/where variables ended up - so it would be mighty useful to debug that aspect only, without having to do changes to files + git add/commit/push every time, just to trigger the script.

Answer 2

Create a pre-receive hook, make it exit 1, and from the client git push origin master HEAD:non-existing-branch. This will trigger the pre-receive hook, even if there are no changes to master.

To avoid error messages, make the pre-receive hook exit successfully (exit 0), and manually remove the the non-existing-branch from the post-receive hook. However, this creates a small time window (when the file non-existing-branch exists) with the git push ... initiated from another client won't run the hooks.