Delete all local git branches

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 @adminndrewC, 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\|main" | 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).