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In git, what is the difference between the following?
HEADHEAD^HEAD~1HEAD~2And how do they relate to master? So is there a MASTER^, MASTER~1??
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HEAD~1 refers to the commit's first parent. HEAD~2 refers to the first parent of the commit's first parent. ^<n> refers to the the <n>th parent. HEAD^1 refers to the commit's first parent. HEAD^2 refers to the commit's second parent.
The difference between HEAD^ (Caret) and HEAD~ (Tilde) is how they traverse history backwards from a specified starting point, in this particular case HEAD .
git reference suffixes (^N, ~N, @{... }) ref~ is shorthand for ref~1 and means the commit's first parent. ref~2 means the commit's first parent's first parent. ref~3 means the commit's first parent's first parent's first parent.
When working with Git, only one branch can be checked out at a time - and this is what's called the "HEAD" branch. Often, this is also referred to as the "active" or "current" branch. Git makes note of this current branch in a file located inside the Git repository, in . git/HEAD .
HEAD is a synonym for the most recent commit on your current branch, whatever it is.
HEAD^ (or HEAD^1) means the first parent of HEAD. A merge commit has multiple parents, so HEAD^2 refers to the second immediate parent of HEAD that was involved in the merge that created HEAD.
HEAD~1 is the same as HEAD~. In this case, it is synonymous with HEAD^. To see the difference, consider that HEAD~2 is the grandparent of HEAD. Using ~ goes back generations.
If you happen to be on the master branch, then HEAD refers to master. If you are on branch topic/foo, then it refers to that branch while you are on it.
Case matters with git. MASTER^ or MASTER~1 is likely to produce errors of the form
fatal: ambiguous argument 'MASTER~1': unknown revision or path not in the working tree.
But master^ and master~1 are meaningful.
See the git rev-parse documentation for full details of the many ways you can address commits.
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