Comment by esoterae | Hacker News Reader

Comment by esoterae | Hacker News Reader

The way in which the author structured the language used to describe things may be the most confusing thing of all. For example:

"HEAD^ and HEAD~ are the same thing (1 commit ago)"

Followed by:

"But I guess they also wanted a way to refer to “3 commits ago”, so HEAD^3 is the third parent of the current commit, and HEAD~3 is the parent’s parent’s parent."

The author's language implies a contradiction they immediately prior said doesn't exist. If these two distinct constructs were indeed different ways to define the same relationship, the second paragraph would say "^3 and ~3 are both ways of saying the third parent of the current commit, or the parent's parent's parent." Instead, they've defined the constructs as different once again.

> If these two distinct constructs were indeed different ways to define the same relationship, the second paragraph would say "^3 and ~3 are both ways of saying the third parent of the current commit, or the parent's parent's parent."

They're not the same thing. Merge commits can have multiple parents, and HEAD^3 refers to the third parent. If HEAD is not a merge commit, then HEAD^3 doesn't refer to anything. HEAD~3, by contrast, refers to following the parent's parent's parent, independent of how many parents any of the commits in question had.

> HEAD~3, by contrast, refers to following the parent's parent's parent,

More specifically, following the first parent only. man git-rev-parse (surprisingly) has a nice explanation:

> A suffix ~<n> to a revision parameter means the commit object that is the <n>th generation ancestor of the named commit object, following only the first parents. I.e. <rev>~3 is equivalent to <rev>^^^ which is equivalent to <rev>^1^1^1.

To make it even clearer, "^3" is when a team of, say, 4 commits contributed to the current commit. "^3" is then the team member number 3.

"~3" however refres to grand-grand-parent to the current commit.

It's confusing because ^ looks like an up-arrow, so we think of it as a pointer to look n levels up. That's not the case.

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