EDIT:
See @Simba Answer for valid solution
submodule.<name>.updateis what you want to change, see the docs – defaultcheckout
submodule.<name>.branchspecify remote branch to be tracked – defaultmaster
OLD ANSWER:
Personally I hate answers here which direct to external links which may stop working over time and check my answer here (Unless question is duplicate) – directing to question which does cover subject between the lines of other subject, but overall equals: “I’m not answering, read the documentation.”
So back to the question: Why does it happen?
Situation you described
After pulling changes from server, many times my submodule head gets detached from master branch.
This is a common case when one does not use submodules too often or has just started with submodules. I believe that I am correct in stating, that we all have been there at some point where our submodule‘s HEAD gets detached.
- Cause: Your submodule is not tracking correct branch (default master).
Solution: Make sure your submodule is tracking the correct branch
$ cd <submodule-path>
# if the master branch already exists locally:
# (From git docs - branch)
# -u <upstream>
# --set-upstream-to=<upstream>
# Set up <branchname>'s tracking information so <upstream>
# is considered <branchname>'s upstream branch.
# If no <branchname> is specified, then it defaults to the current branch.
$ git branch -u <origin>/<branch> <branch>
# else:
$ git checkout -b <branch> --track <origin>/<branch>
- Cause: Your parent repo is not configured to track submodules branch.
Solution: Make your submodule track its remote branch by adding new submodules with the following two commands.- First you tell git to track your remote
<branch>. - you tell git to perform rebase or merge instead of checkout
- you tell git to update your submodule from remote.
- First you tell git to track your remote
$ git submodule add -b <branch> <repository> [<submodule-path>]
$ git config -f .gitmodules submodule.<submodule-path>.update rebase
$ git submodule update --remote
- If you haven’t added your existing submodule like this you can easily fix that:
- First you want to make sure that your submodule has the branch checked out which you want to be tracked.
$ cd <submodule-path>
$ git checkout <branch>
$ cd <parent-repo-path>
# <submodule-path> is here path releative to parent repo root
# without starting path separator
$ git config -f .gitmodules submodule.<submodule-path>.branch <branch>
$ git config -f .gitmodules submodule.<submodule-path>.update <rebase|merge>
In the common cases, you already have fixed by now your DETACHED HEAD since it was related to one of the configuration issues above.
fixing DETACHED HEAD when .update = checkout
$ cd <submodule-path> # and make modification to your submodule
$ git add .
$ git commit -m"Your modification" # Let's say you forgot to push it to remote.
$ cd <parent-repo-path>
$ git status # you will get
Your branch is up-to-date with '<origin>/<branch>'.
Changes not staged for commit:
modified: path/to/submodule (new commits)
# As normally you would commit new commit hash to your parent repo
$ git add -A
$ git commit -m"Updated submodule"
$ git push <origin> <branch>.
$ git status
Your branch is up-to-date with '<origin>/<branch>'.
nothing to commit, working directory clean
# If you now update your submodule
$ git submodule update --remote
Submodule path 'path/to/submodule': checked out 'commit-hash'
$ git status # will show again that (submodule has new commits)
$ cd <submodule-path>
$ git status
HEAD detached at <hash>
# as you see you are DETACHED and you are lucky if you found out now
# since at this point you just asked git to update your submodule
# from remote master which is 1 commit behind your local branch
# since you did not push you submodule chage commit to remote.
# Here you can fix it simply by. (in submodules path)
$ git checkout <branch>
$ git push <origin>/<branch>
# which will fix the states for both submodule and parent since
# you told already parent repo which is the submodules commit hash
# to track so you don't see it anymore as untracked.
But if you managed to make some changes locally already for submodule and commited, pushed these to remote then when you executed ‘git checkout ‘, Git notifies you:
$ git checkout <branch>
Warning: you are leaving 1 commit behind, not connected to any of your branches:
If you want to keep it by creating a new branch, this may be a good time to do so with:
The recommended option to create a temporary branch can be good, and then you can just merge these branches etc. However I personally would use just git cherry-pick <hash> in this case.
$ git cherry-pick <hash> # hash which git showed you related to DETACHED HEAD
# if you get 'error: could not apply...' run mergetool and fix conflicts
$ git mergetool
$ git status # since your modifications are staged just remove untracked junk files
$ rm -rf <untracked junk file(s)>
$ git commit # without arguments
# which should open for you commit message from DETACHED HEAD
# just save it or modify the message.
$ git push <origin> <branch>
$ cd <parent-repo-path>
$ git add -A # or just the unstaged submodule
$ git commit -m"Updated <submodule>"
$ git push <origin> <branch>
Although there are some more cases you can get your submodules into DETACHED HEAD state, I hope that you understand now a bit more how to debug your particular case.