am,shazam: retrieving and applying patches

Most commonly, b4 is used to retrieve, prepare, and apply patches sent via distribution lists. The base functionality is similar to that of b4 mbox:

b4 am

This will do the following:

  1. look up if that message-id is known on the specified public-inbox server (e.g.

  2. retrieve the full thread containing that message-id

  3. process all replies to collect code review trailers and apply them to the relevant patch commit messages

  4. perform attestation checks on patches and code review follow-ups

  5. put all patches in the correct order and prepare for “git am”

  6. write out the resulting mailbox so it is ready to be applied to a git tree

For example:

$ b4 am
Analyzing 5 messages in the thread
Checking attestation on all messages, may take a moment...
  ✓ [PATCH v2 1/2] selftests/harness: Move test child waiting logic
  ✓ [PATCH v2 2/2] selftests/harness: Handle timeouts cleanly
  ✓ Signed: DKIM/
Total patches: 2
Cover: ./v2_20200313_keescook_selftests_harness_handle_timeouts_cleanly.cover
 Base: not specified
       git am ./v2_20200313_keescook_selftests_harness_handle_timeouts_cleanly.mbx

b4 am vs. b4 shazam


b4 shazam was added in version v0.9.

The two commands are very similar – the main distinction is that b4 am will prepare the patch series for application to the git tree, but will not make any modifications to your current branch.

The b4 shazam command will do the same as b4 am and will apply the patch series to the current branch (if it is possible to do so cleanly).

Common flags

The following flags are common to both commands:

-m LOCALMBOX, --use-local-mbox LOCALMBOX

By default, b4 will retrieve threads from remote public-inbox servers, but it can also use a local mailbox/maildir. This is useful if you have a tool like mbsync or lei copying remote messages locally and you need to do some work while offline. You can pass - to read messages from stdin.

--stdin-pipe-sep STDIN_PIPE_SEP (0.11+)

When reading input from stdin, split messages using the string passed as parameter. Otherwise, b4 expects stdin to be a single message or a valid mbox.

This is most useful when piping threads directly from mutt. In your .muttrc add the following configuration parameter:

set pipe_sep = "\n---randomstr---\n"

Then invoke b4 with -m - --stdin-pipe-sep='\n---randomstr---\n'

-C, --no-cache

By default, b4 will cache the retrieved threads for about 10 minutes. This lets you force b4 to ignore cache and retrieve the latest results.

--single-message (0.13+)

By default, b4 will retrieve the entire thread, but sometimes you really just want a single message. This helps when someone posts a patch in the middle of a long thread and you just want that patch and ignore the rest of what is going on.

-v WANTVER, --use-version WANTVER

If a thread (or threads, when used with -c) contains multiple patch series revisions, b4 will automatically pick the highest numbered version. This switch lets you pick a different revision.

-S, --sloppy-trailers

B4 tries to be careful when collecting code review trailers and will refuse to consider the trailers where the email address in the From: header does not patch the address in the trailer itself.

For example, the following message will not be processed:

From: Alice Maintainer <>
Subject: Re: [PATCH v3 3/3] Some patch title

> [...]
Reviewed-by: Alice Maintainer <>

In such situations, b4 will print a warning and refuse to apply the trailer due to the email address mismatch. You can override this by passing the -S flag.

-T, --no-add-trailers

This tells b4 to ignore any follow-up trailers and just save the patches as sent by the contributor.

-s, --add-my-sob

Applies your own Signed-off-by: trailer to every commit.

-l, --add-link

Adds a Link: trailer with the URL of the retrieved message using the linkmask template. Note, that such trailers may be considered redundant by the upstream maintainer.


This allows you to select a subset of patches from a larger series. Here are a few examples.

This will pick patches 1, 3, 5, 6, 7, 9 and any others that follow:

b4 am -P 1,3,5-7,9- <msgid>

This will pick just the patch that matches the exact message-id provided:

b4 am -P _ <msgid>

This will pick all patches where the subject matches “iscsi”:

b4 am -P *iscsi*

Copies all addresses found in the message Cc’s into Cc: commit trailers.


Break thread at the msgid specified and ignore any parent messages. This is handy with very convoluted threads, for example when someone replies with a different patch series in the middle of a larger conversation and b4 gets confused about which patch series is being requested.


There are some clever tricks that can be accomplished with unicode control chars that make the code as printed on the screen (and reviewed by a human) to actually do something totally different when processed by a compiler. Such unicode control chars are almost never legitimately useful in the code, so b4 will print a warning and bail out when it finds them. However, just in case there are legitimate reasons for these characters to be in the code (e.g. as part of documentation translated into LTR languages), this behaviour can be overridden.

Flags only valid for b4 am

The following flags only make sense for b4 am:

-o OUTDIR, --outdir OUTDIR

Instead of writing the .mbox file to the current directory, write it to this location instead. You can also pass a path to an existing mbox or maildir location to have the results appended to that mailbox instead (see also the -f flag below).

When - is specified, the output is dumped to stdout.

-c, --check-newer-revisions

When retrieving patch series, check if a newer revision is available. For example, if you are trying to retrieve a series titled [PATCH v2 0/3], b4 will use a number of mechanisms to check if a v3 or later revision is also available and will add these results to the retrieved thread.

-n WANTNAME, --mbox-name WANTNAME

By default, the resulting mailbox file will use the message-id as the basis for its filename. This option lets you override this behaviour.

-M, --save-as-maildir

By default, the retrieved thread will be saved as an mbox file. However, due to subtle incompatibilities between various mbox formats (“mboxo” vs “mboxrd”, etc), you may want to instead save the results as a Maildir directory.

-Q, --quilt-ready

Saves the patches as a folder that can be fed directly to quilt. If you don’t know what quilt is, you don’t really need to worry about this option.

-b GUESSBRANCH [...], --guess-branch GUESSBRANCH [...]

When using --guess-base, you can restrict which branch(es) b4 will use to find the match. If not specified, b4 will use the entire tree history.

--guess-lookback GUESSDAYS

When using --guess-base, you can specify how far back b4 should look from the date of the patch to find the base commit. By default, b4 will only consider the last 14 days prior to the date of the patch, but you can expand or shrink it as necessary.

-3, --prep-3way

This will try to prepare your tree for a 3-way merge by doing some behind the scenes git magic and preparing some fake loose commits.


By default, b4 will save the cover letter as a separate file in the output directory specified. This flag turns it off (this is also the default when used with -o -).


For minor changes, it is common practice for contributors to send follow-ups to just the patches they have modified. For example:

[PATCH v1 1/3] foo: add foo to bar
[PATCH v1 2/3] bar: add bar to baz
 \- [PATCH v2 2/3] bar: add bar to baz
[PATCH v1 3/3] baz: add baz to quux

In this case, b4 will properly create a v2 of the entire series by reusing [PATCH v1 1/3] and [PATCH v1 3/3]. However, sometimes that is not the right thing to do, so you can turn off this feature using --no-partial-reroll.

Flags only valid for b4 shazam

By default, b4 shazam will apply the patch series directly to the git tree where the command is being executed. However, instead of just running git am and applying the patches directly on top of the current branch, it can also treat the series similar to a git pull request and either prepare a FETCH_HEAD that you can merge manually, or even automatically merge the series using the series cover letter as the basis for the merge commit.

-H, --make-fetch-head

This will prepare the series and place it into the FETCH_HEAD that can then be merged just as if it were a pull request:

  1. b4 will prepare a temporary sparse worktree

  2. b4 will apply the series to that worktree

  3. if git am completed successfully, b4 will fetch that tree into your current tree’s FETCH_HEAD (and get rid of the temporary tree)

  4. b4 will place the cover letter into .git/b4-cover

  5. b4 will offer the command you can run to merge the change into your current branch, e.g.:

    git merge --no-ff -F .git/b4-cover --edit FETCH_HEAD --signoff

Generally, this command is also a good test for “will this patch series apply cleanly to my tree.” You can perform any actions with the FETCH_HEAD as you normally would, e.g. run git diff, make a new branch out of it using git checkout, etc.

-M, --merge

Exactly the same as --make-fetch-head, but will actually execute the suggested git merge command.

Please also see the shazam settings section for some configuration file options that affect some of b4 shazam behaviour.