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git grep foo

The main advantage of git grep is that it can find the patterns in the git repository, i. e. also in others than the current version of the source. This cannot be done using the standard grep of course. Also there are a lot more features in the git grep like pattern arithmetic (things like git grep -e pattern1 --and --not \( -e pattern2 -e pattern3 \)), tree search using glob (things like git grep pattern -- '*.[ch]' to search only in .c and .h files) and some more.

Here's an example session for searching in an older revision:

$ mkdir git-test                 # create fresh repository
$ cd git-test/
$ git init .
Initialized empty Git repository in /home/alfe/git-test/.git/
$ echo eins zwei drei > bla      # create example file
$ git add bla                    # add and commit it
$ git commit bla
[master (root-commit) 7494515] .
 1 file changed, 1 insertion(+)
 create mode 100644 bla
$ echo vier fuenf sechs > bla    # perform a change on that file
$ git commit -m 'increase' bla   # commit it
[master 062488e] increase
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git grep eins | cat            # grep for outdated pattern in current version
                                  # (finds nothing)
$ git grep eins master^ | cat    # grep for outdated pattern on former version
                                  # finds it:
master^:bla:eins zwei drei