编辑：
请参阅@Simba 答案以获取有效的解决方案
submodule.<name>.update是您要更改的内容，请参见文档 - 默认checkout
submodule.<name>.branch指定要跟踪的远程分支- 默认master
老答案：
我个人不喜欢这里的答案，这些答案直接指向可能随着时间的推移而停止工作的外部链接，并在此处 检查我的答案（除非有重复的问题） -指向确实涵盖其他主题之间的主题的问题，但总体上等于：不回答，请阅读文档。”
回到问题：为什么会发生？
您描述的情况
从服务器提取更改后，很多时候我的子模块头都与master分支分离。
这是当一个不使用常见的情况的子模块过于频繁或刚开始与子模块。我相信我的陈述是正确的，我们 都曾在某个时候分离过子模块的HEAD。
- 原因：您的子模块未跟踪正确的分支（默认主节点）。
解决方案：确保您的子模块正在跟踪正确的分支
$ 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>
- 原因：您的父仓库未配置为跟踪子模块分支。
解决方案：通过使用以下两个命令添加新的子模块，使子模块跟踪其远程分支。
- 首先，您告诉git跟踪您的remote
<branch>。
- 你告诉git执行rebase或merge而不是checkout
- 您告诉git从远程更新您的子模块。
$ git submodule add -b <branch> <repository> [<submodule-path>]
$ git config -f .gitmodules submodule.<submodule-path>.update rebase
$ git submodule update --remote
- 如果您还没有添加这样的现有子模块，则可以轻松地解决该问题：
$ 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>
在通常情况下，由于与上面的配置问题之一相关，因此您现在已经修复了DETACHED HEAD。
修复DETACHED HEAD时 .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.
但是，如果您已经设法在本地对子模块进行了一些更改并提交了这些更改，请将这些更改推送到远程，然后在执行“ git checkout”时，Git会通知您：
$ 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:
推荐的创建临时分支的选项可能很好，然后您可以合并这些分支等。但是git cherry-pick <hash>在这种情况下，我个人会使用。
$ 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>
尽管在某些情况下可以使子模块进入DETACHED HEAD状态，但我希望您现在对如何调试特定情况有更多的了解。