2022-12-31
When I specify an ancestor commit object in Git, I'm confused between HEAD^ and HEAD~.
Both have a "numbered" version like HEAD^3 and HEAD~2.
They seem very similar or the same to me, but are there any differences between the tilde and the caret?
~ most of the time — to go back a number of generations, usually what you want^ on merge commits — because they have two or more (immediate) parentsMnemonics:
~ is almost linear in appearance and wants to go backward in a straight line^ suggests an interesting segment of a tree or a fork in the roadThe “Specifying Revisions” section of the git rev-parse documentation defines ~ as
<rev>~<n>, e.g.master~3
A suffix~<n>to a revision parameter means the commit object that is the nth generation ancestor of the named commit object, following only the first parents. For example,<rev>~3is equivalent to<rev>^^^which is equivalent to<rev>^1^1^1…
You can get to parents of any commit, not just HEAD. You can also move back through generations: for example, master~2 means the grandparent of the tip of the master branch, favoring the first parent on merge commits.
Git history is nonlinear: a directed acyclic graph (DAG) or tree. For a commit with only one parent, rev~ and rev^ mean the same thing. The caret selector becomes useful with merge commits because each one is the child of two or more parents — and strains language borrowed from biology.
HEAD^ means the first immediate parent of the tip of the current branch. HEAD^ is short for HEAD^1, and you can also address HEAD^2 and so on as appropriate. The same section of the git rev-parse documentation defines it as
<rev>^, e.g.HEAD^,v1.5.1^0
A suffix^to a revision parameter means the first parent of that commit object.^<n>means the nth parent ([e.g.]<rev>^is equivalent to<rev>^1). As a special rule,<rev>^0means the commit itself and is used when<rev>is the object name of a tag object that refers to a commit object.
These specifiers or selectors can be chained arbitrarily, e.g., topic~3^2 in English is the second parent of the merge commit that is the great-grandparent (three generations back) of the current tip of the branch topic.
The aforementioned section of the git rev-parse documentation traces many paths through a notional git history. Time flows generally downward. Commits D, F, B, and A are merge commits.
Here is an illustration, by Jon Loeliger. Both commit nodes B and C are parents of commit node A. Parent commits are ordered left-to-right. (N.B. The
git log --graphcommand displays history in the opposite order.)G H I J \ / \ / D E F \ | / \ \ | / | \|/ | B C \ / \ / AA = = A^0 B = A^ = A^1 = A~1 C = A^2 D = A^^ = A^1^1 = A~2 E = B^2 = A^^2 F = B^3 = A^^3 G = A^^^ = A^1^1^1 = A~3 H = D^2 = B^^2 = A^^^2 = A~2^2 I = F^ = B^3^ = A^^3^ J = F^2 = B^3^2 = A^^3^2
Run the code below to create a git repository whose history matches the quoted illustration.
#! /usr/bin/env perl
use strict;
use warnings;
use subs qw/ postorder /;
use File::Temp qw/ mkdtemp /;
my %sha1;
my %parents = (
A => [ qw/ B C / ],
B => [ qw/ D E F / ],
C => [ qw/ F / ],
D => [ qw/ G H / ],
F => [ qw/ I J / ],
);
sub postorder {
my($root,$hash) = @;
my @parents = @{ $parents{$root} || [] };
postorder($, $hash) for @parents;
return if $sha1{$root};
@parents = map “-p $sha1{$_}”, @parents;
chomp($sha1{$root} = git commit-tree @parents -m "$root" $hash);
die “$0: git commit-tree failed” if $?;
system(“git tag -a -m ‘$sha1{$root}’ ‘$root’ ‘$sha1{$root}’”) == 0 or die “$0: git tag failed”;
}
$0 =~ s!^.*/!!; # / fix Stack Overflow highlighting
my $repo = mkdtemp “repoXXXXXXXX”;
chdir $repo or die “$0: chdir: $!”;
system(“git init”) == 0 or die “$0: git init failed”;
chomp(my $tree = git write-tree); die “$0: git write-tree failed” if $?;
postorder ‘A’, $tree;
system “git update-ref HEAD $sha1{A}”; die “$0: git update-ref failed” if $?;
system “git update-ref master $sha1{A}”; die “$0: git update-ref failed” if $?;
for browsing history - http://blog.kfish.org/2010/04/git-lola.html
system “git config alias.lol ’log –graph –decorate –pretty=oneline –abbrev-commit’”;
system “git config alias.lola ’log –graph –decorate –pretty=oneline –abbrev-commit –all’”;
It adds aliases in the new throwaway repo only for git lol and git lola so you can view history as in
$ git lol
* 29392c8 (HEAD -> master, tag: A) A
|\
| * a1ef6fd (tag: C) C
| |
| \
*-. \ 8ae20e9 (tag: B) B
|\ \ \
| | |/
| | * 03160db (tag: F) F
| | |\
| | | * 9df28cb (tag: J) J
| | * 2afd329 (tag: I) I
| * a77cb1f (tag: E) E
* cd75703 (tag: D) D
|\
| * 3043d25 (tag: H) H
* 4ab0473 (tag: G) G
Note that on your machine the SHA-1 object names will differ from those above, but the tags allow you to address commits by name and check your understanding.
$ git log -1 --format=%f $(git rev-parse A^)
B
$ git log -1 --format=%f $(git rev-parse A~^3~)
I
$ git log -1 --format=%f $(git rev-parse A^2~)
F
The “Specifying Revisions” in the git rev-parse documentation is full of great information and is worth an in-depth read. See also Git Tools - Revision Selection from the book Pro Git.
The commit 89e4fcb0dd from git’s own history is a merge commit, as git show 89e4fcb0dd indicates with the Merge header line that displays the immediate ancestors’ object names.
commit 89e4fcb0dd01b42e82b8f27f9a575111a26844df Merge: c670b1f876 649bf3a42f b67d40adbb Author: Junio C Hamano <[email protected]> Date: Mon Oct 29 10:15:31 2018 +0900Merge branches 'bp/reset-quiet' and 'js/mingw-http-ssl' into nd/config-split […]
We can confirm the ordering by asking git rev-parse to show 89e4fcb0dd’s immediate parents in sequence.
$ git rev-parse 89e4fcb0dd^1 89e4fcb0dd^2 89e4fcb0dd^3
c670b1f876521c9f7cd40184bf7ed05aad843433
649bf3a42f344e71b1b5a7f562576f911a1f7423
b67d40adbbaf4f5c4898001bf062a9fd67e43368
Querying the non-existent fourth parent results in an error.
$ git rev-parse 89e4fcb0dd^4
89e4fcb0dd^4
fatal: ambiguous argument '89e4fcb0dd^4': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
If you want to extract the parents only, use pretty format %P for the full hashes
$ git log -1 --pretty=%P 89e4fcb0dd
c670b1f876521c9f7cd40184bf7ed05aad843433 649bf3a42f344e71b1b5a7f562576f911a1f7423 b67d40adbbaf4f5c4898001bf062a9fd67e43368
or %p for abbreviated parents.
$ git log -1 --pretty=%p 89e4fcb0dd
c670b1f876 649bf3a42f b67d40adbb