Delete all local git branches

Question

I follow a development process where I create a new local branch for every new feature or story card. When finished I merge the branch into master and then push.

What tends to happen over time due to a combination of laziness or forgetfulness, is that I end up with a large list of local branches, some of which (such as spikes) may not have been merged.

I know how to list all my local branches and I know how to remove a single branch but I was wondering if there was a git command that allows me to delete all my local branches?

Below is the output of the git branch --merged command.

user@machine:~/projects/application[master]$ git branch --merged
  STORY-123-Short-Description
  STORY-456-Another-Description
  STORY-789-Blah-Blah
* master

All attempts to delete branches listed with grep -v \* (as per the answers below) result in errors:

error: branch 'STORY-123-Short-Description' not found.
error: branch 'STORY-456-Another-Description' not found.
error: branch 'STORY-789-Blah-Blah' not found.

I'm using:
git 1.7.4.1
ubuntu 10.04
GNU bash, version 4.1.5(1)-release
GNU grep 2.5.4

Answer 1

The 'git branch -d' subcommand can delete more than one branch. So, simplifying @sblom's answer but adding a critical xargs:

git branch -D `git branch --merged | grep -v \* | xargs`

or, further simplified to:

git branch --merged | grep -v \* | xargs git branch -D 

Importantly, as noted by @AndrewC, using git branch for scripting is discouraged. To avoid it use something like:

git for-each-ref --format '%(refname:short)' refs/heads | grep -v master | xargs git branch -D

Caution warranted on deletes!

$ mkdir br
$ cd br; git init
Initialized empty Git repository in /Users/ebg/test/br/.git/
$ touch README; git add README; git commit -m 'First commit'
[master (root-commit) 1d738b5] First commit
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 README
$ git branch Story-123-a
$ git branch Story-123-b
$ git branch Story-123-c
$ git branch --merged
  Story-123-a
  Story-123-b
  Story-123-c
* master
$ git branch --merged | grep -v \* | xargs
Story-123-a Story-123-b Story-123-c
$ git branch --merged | grep -v \* | xargs git branch -D
Deleted branch Story-123-a (was 1d738b5).
Deleted branch Story-123-b (was 1d738b5).
Deleted branch Story-123-c (was 1d738b5).