What does 'git blame' do?

Question

I saw a lot of questions about methods of using git blame, but I don't really understand them.

I see a Blame button on top of files on the GitHub interface. Upon clicking it, it shows some diff with usernames on the left bar. What does that indicate?

Why is git blame actually used, apart from GitHub?

Answer 1

From git-blame:

Annotates each line in the given file with information from the revision which last modified the line. Optionally, start annotating from the given revision.

When specified one or more times, -L restricts annotation to the requested lines.

Example:

[email protected]:~# git blame .htaccess ... ^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  4) allow from all ^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  5) ^e1fb2d7 (John Doe 2015-07-03 06:30:25 -0300  6) <IfModule mod_rewrite.c> ^72fgsdl (Arthur King 2015-07-03 06:34:12 -0300  7)     RewriteEngine On ... 

Please note that git blame does not show the per-line modifications history in the chronological sense. It only shows who was the last person to have changed a line in a document up to the last commit in HEAD.

That is to say that in order to see the full history/log of a document line, you would need to run a git blame path/to/file for each commit in your git log.