git blame
Show what revision and author last modified each line of a file
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.
The origin of lines is automatically followed across whole-file renames (currently there is no option to turn the rename-following off). To follow lines moved from one file to another, or to follow lines that were copied and pasted from another file, etc., see the -C and -M options.
The report does not tell you anything about lines which have been deleted or replaced; you need to use a tool such as git diff or the "pickaxe" interface briefly mentioned in the following paragraph.
Apart from supporting file annotation, Git also supports searching the development history for when a code snippet occurred in a change. This makes it possible to track when a code snippet was added to a file, moved or copied between files, and eventually deleted or replaced. It works by searching for a text string in the diff. A small example of the pickaxe interface that searches for blame_usage:
git log --pretty=oneline -S'blame_usage'
5040f17eba15504bad66b14a645bddd9b015ebb7 blame -S
ea4c7f9bf69e781dd0cd88d2bccb2bf5cc15c9a7 git-blame: Make the output