Using Git, how could I search for a string across all branches?

In many cases git rev-list --all can return a huge number of commits, taking forever to scan. If you, instead of searching through every commit on every branch in your repository history, just want to search all branch tips, you can replace it with git show-ref -s --heads. So in total:

git grep "string" `git show-ref -s --heads`

or:

git show-ref -s --heads | xargs git grep "string"

Tip: You can write output in file to view in an editor:

nano ~/history.txt
git show-ref -s --heads | xargs git grep "search string here" >> ~/history.txt

If you use @manojlds Git grep command and get an error:

-bash: /usr/bin/git: Argument list too long" 

then you should use xargs:

git rev-list --all | xargs git grep "string/regexp"

Also see How to grep (search) committed code in the Git history

You can do this on a Git repository:

git grep "string/regexp" $(git rev-list --all)

GitHub advanced search has code search capability:

The code search will look through all of the code publicly hosted on GitHub. You can also filter by:

• the language: language:
• the repository name (including the username): repo:
• the file path: path: