How to show first commit by 'git log'?

I found that:

git log --reverse

shows commits from start.

Short answer

git rev-list --max-parents=0 HEAD

_{(from tiho's comment. As Chris Johnsen notices, --max-parents was introduced after this answer was posted.)}

Explanation

Technically, there may be more than one root commit. This happens when multiple previously independent histories are merged together. It is common when a project is integrated via a subtree merge.

The git.git repository has six root commits in its history graph (one each for Linus’s initial commit, gitk, some initially separate tools, git-gui, gitweb, and git-p4). In this case, we know that e83c516 is the one we are probably interested in. It is both the earliest commit and a root commit.

It is not so simple in the general case.

Imagine that libfoo has been in development for a while and keeps its history in a Git repository (libfoo.git). Independently, the “bar” project has also been under development (in bar.git), but not for as long libfoo (the commit with the earliest date in libfoo.git has a date that precedes the commit with the earliest date in bar.git). At some point the developers of “bar” decide to incorporate libfoo into their project by using a subtree merge. Prior to this merge it might have been trivial to determine the “first” commit in bar.git (there was probably only one root commit). After the merge, however, there are multiple root commits and the earliest root commit actually comes from the history of libfoo, not “bar”.

You can find all the root commits of the history DAG like this:

git rev-list --max-parents=0 HEAD

For the record, if --max-parents weren't available, this does also work:

git rev-list --parents HEAD | egrep "^[a-f0-9]{40}$"

If you have useful tags in place, then git name-rev might give you a quick overview of the history:

git rev-list --parents HEAD | egrep "^[a-f0-9]{40}$" | git name-rev --stdin

Bonus

Use this often? Hard to remember? Add a git alias for quick access

git config --global alias.first "rev-list --max-parents=0 HEAD"

Now you can simply do

git first

You can just reverse your log and just head it for the first result.

git log --pretty=oneline --reverse | head -1