This is to be reused by the coming git-switch-branch.txt man page which also deals with detached HEAD. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx> --- Documentation/detach-head.txt | 132 ++++++++++++++++++++++++++++++++ Documentation/git-checkout.txt | 133 +-------------------------------- 2 files changed, 133 insertions(+), 132 deletions(-) create mode 100644 Documentation/detach-head.txt diff --git a/Documentation/detach-head.txt b/Documentation/detach-head.txt new file mode 100644 index 0000000000..bb6f5d7843 --- /dev/null +++ b/Documentation/detach-head.txt @@ -0,0 +1,132 @@ +HEAD normally refers to a named branch (e.g. 'master'). Meanwhile, each +branch refers to a specific commit. Let's look at a repo with three +commits, one of them tagged, and with branch 'master' checked out: + +------------ + HEAD (refers to branch 'master') + | + v +a---b---c branch 'master' (refers to commit 'c') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +When a commit is created in this state, the branch is updated to refer to +the new commit. Specifically, 'git commit' creates a new commit 'd', whose +parent is commit 'c', and then updates branch 'master' to refer to new +commit 'd'. HEAD still refers to branch 'master' and so indirectly now refers +to commit 'd': + +------------ +$ edit; git add; git commit + + HEAD (refers to branch 'master') + | + v +a---b---c---d branch 'master' (refers to commit 'd') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +It is sometimes useful to be able to checkout a commit that is not at +the tip of any named branch, or even to create a new commit that is not +referenced by a named branch. Let's look at what happens when we +checkout commit 'b' (here we show two ways this may be done): + +------------ +$ git checkout v2.0 # or +$ git checkout master^^ + + HEAD (refers to commit 'b') + | + v +a---b---c---d branch 'master' (refers to commit 'd') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +Notice that regardless of which checkout command we use, HEAD now refers +directly to commit 'b'. This is known as being in detached HEAD state. +It means simply that HEAD refers to a specific commit, as opposed to +referring to a named branch. Let's see what happens when we create a commit: + +------------ +$ edit; git add; git commit + + HEAD (refers to commit 'e') + | + v + e + / +a---b---c---d branch 'master' (refers to commit 'd') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +There is now a new commit 'e', but it is referenced only by HEAD. We can +of course add yet another commit in this state: + +------------ +$ edit; git add; git commit + + HEAD (refers to commit 'f') + | + v + e---f + / +a---b---c---d branch 'master' (refers to commit 'd') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +In fact, we can perform all the normal Git operations. But, let's look +at what happens when we then checkout master: + +------------ +$ git checkout master + + HEAD (refers to branch 'master') + e---f | + / v +a---b---c---d branch 'master' (refers to commit 'd') + ^ + | + tag 'v2.0' (refers to commit 'b') +------------ + +It is important to realize that at this point nothing refers to commit +'f'. Eventually commit 'f' (and by extension commit 'e') will be deleted +by the routine Git garbage collection process, unless we create a reference +before that happens. If we have not yet moved away from commit 'f', +any of these will create a reference to it: + +------------ +$ git checkout -b foo <1> +$ git branch foo <2> +$ git tag foo <3> +------------ + +<1> creates a new branch 'foo', which refers to commit 'f', and then +updates HEAD to refer to branch 'foo'. In other words, we'll no longer +be in detached HEAD state after this command. + +<2> similarly creates a new branch 'foo', which refers to commit 'f', +but leaves HEAD detached. + +<3> creates a new tag 'foo', which refers to commit 'f', +leaving HEAD detached. + +If we have moved away from commit 'f', then we must first recover its object +name (typically by using git reflog), and then we can create a reference to +it. For example, to see the last two commits to which HEAD referred, we +can use either of these commands: + +------------ +$ git reflog -2 HEAD # or +$ git log -g -2 HEAD +------------ diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt index 65bd1bc50d..25887a6087 100644 --- a/Documentation/git-checkout.txt +++ b/Documentation/git-checkout.txt @@ -306,138 +306,7 @@ leave out at most one of `A` and `B`, in which case it defaults to `HEAD`. DETACHED HEAD ------------- -HEAD normally refers to a named branch (e.g. 'master'). Meanwhile, each -branch refers to a specific commit. Let's look at a repo with three -commits, one of them tagged, and with branch 'master' checked out: - ------------- - HEAD (refers to branch 'master') - | - v -a---b---c branch 'master' (refers to commit 'c') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -When a commit is created in this state, the branch is updated to refer to -the new commit. Specifically, 'git commit' creates a new commit 'd', whose -parent is commit 'c', and then updates branch 'master' to refer to new -commit 'd'. HEAD still refers to branch 'master' and so indirectly now refers -to commit 'd': - ------------- -$ edit; git add; git commit - - HEAD (refers to branch 'master') - | - v -a---b---c---d branch 'master' (refers to commit 'd') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -It is sometimes useful to be able to checkout a commit that is not at -the tip of any named branch, or even to create a new commit that is not -referenced by a named branch. Let's look at what happens when we -checkout commit 'b' (here we show two ways this may be done): - ------------- -$ git checkout v2.0 # or -$ git checkout master^^ - - HEAD (refers to commit 'b') - | - v -a---b---c---d branch 'master' (refers to commit 'd') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -Notice that regardless of which checkout command we use, HEAD now refers -directly to commit 'b'. This is known as being in detached HEAD state. -It means simply that HEAD refers to a specific commit, as opposed to -referring to a named branch. Let's see what happens when we create a commit: - ------------- -$ edit; git add; git commit - - HEAD (refers to commit 'e') - | - v - e - / -a---b---c---d branch 'master' (refers to commit 'd') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -There is now a new commit 'e', but it is referenced only by HEAD. We can -of course add yet another commit in this state: - ------------- -$ edit; git add; git commit - - HEAD (refers to commit 'f') - | - v - e---f - / -a---b---c---d branch 'master' (refers to commit 'd') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -In fact, we can perform all the normal Git operations. But, let's look -at what happens when we then checkout master: - ------------- -$ git checkout master - - HEAD (refers to branch 'master') - e---f | - / v -a---b---c---d branch 'master' (refers to commit 'd') - ^ - | - tag 'v2.0' (refers to commit 'b') ------------- - -It is important to realize that at this point nothing refers to commit -'f'. Eventually commit 'f' (and by extension commit 'e') will be deleted -by the routine Git garbage collection process, unless we create a reference -before that happens. If we have not yet moved away from commit 'f', -any of these will create a reference to it: - ------------- -$ git checkout -b foo <1> -$ git branch foo <2> -$ git tag foo <3> ------------- - -<1> creates a new branch 'foo', which refers to commit 'f', and then -updates HEAD to refer to branch 'foo'. In other words, we'll no longer -be in detached HEAD state after this command. - -<2> similarly creates a new branch 'foo', which refers to commit 'f', -but leaves HEAD detached. - -<3> creates a new tag 'foo', which refers to commit 'f', -leaving HEAD detached. - -If we have moved away from commit 'f', then we must first recover its object -name (typically by using git reflog), and then we can create a reference to -it. For example, to see the last two commits to which HEAD referred, we -can use either of these commands: - ------------- -$ git reflog -2 HEAD # or -$ git log -g -2 HEAD ------------- +include::detach-head.txt[] ARGUMENT DISAMBIGUATION ----------------------- -- 2.20.0.rc1.380.g3eb999425c.dirty
