How to find the Git commit that introduced a string in any branch?

You can do:

git log -S <search string> --source --all

To find all commits that added or removed the fixed string search string. The --all parameter means to start from every branch and --source means to show which of those branches led to finding that commit. It's often useful to add -p to show the patches that each of those commits would introduce as well.

Versions of git since 1.7.4 also have a similar -G option, which takes a regular expression. This actually has different (and rather more obvious) semantics, explained in this blog post from Junio Hamano.

As thameera points out in the comments, you need to put quotes around the search term if it contains spaces or other special characters, for example:

git log -S 'hello world' --source --all
git log -S "dude, where's my car?" --source --all

Here's an example using -G to find occurrences of function foo() {:

git log -G "^(\s)*function foo[(][)](\s)*{$" --source --all

Mark Longair’s answer is excellent, but I have found this simpler version to work for me.

git log -S whatever

--reverse is also helpful since you want the first commit that made the change:

git log --all -p --reverse --source -S 'needle'

This way older commits will appear first.

Messing around with the same answers:

$ git config --global alias.find '!git log --color -p -S '

• ! is needed because other way, git do not pass argument correctly to -S. See this response
• --color and -p helps to show exactly "whatchanged"

Now you can do

$ git find <whatever>

or

$ git find <whatever> --all
$ git find <whatever> master develop