am,shazam: retrieving and applying patches

The most common use of b4 is to retrieve, prepare, and apply patches sent via distribution lists. For example, you can use it to retrieve a set of patches using the Message-ID:

b4 am

This does the following:

  1. Looks up that Message-ID on the specified public-inbox server, using by default.

  2. Retrieves the full thread containing that message-id.

  3. Processes all replies to collect code review trailers and apply them to the corresponding commit messages.

  4. Performs attestation checks on patches and follow-ups containing code-review trailers.

  5. Puts all patches in the correct order and prepares them for “git am”

  6. Writes 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

The two commands are similar – the main distinction is that b4 am prepares the patch series so you can git am the resulting mbox file, but it doesn’t make any actual modifications to your current branch.

The b4 shazam command dos the same as b4 am and actually applies the patch series to the current branch (if it’s possible to do so cleanly).

Common flags

The following flags are common to both commands:

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

By default, b4 retrieves 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

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 caches 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 retrieves 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 the messages.

-v WANTVER, --use-version WANTVER

If a thread (or threads, when used with -c) contains multiple patch series revisions, b4 automatically picks 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 refuses to consider the trailers where the email address in the From: header doesn’t match the address in the trailer itself.

For example, this follow-up trailer doesn’t match and b4 ignores it by default:

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

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

When b4 encounters such situations, it prints a warning and refuses to apply the trailer due to the email address mismatch. You can override this behavior 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.

-i, --add-message-id

Adds a Message-ID: trailer with the Message-ID of the retrieved message. Cannot be used together with the -l switch.


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

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

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

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

b4 am -P _ <msgid>

This picks just the last patch from a series:

b4 am -P -1 <msgid>

This picks all patches where the subject matches “iscsi”:

b4 am -P *iscsi*

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


Break thread at the message-id specified and ignore any parent messages. This is handy with long 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 you are requesting.


There are malicious tricks that someone can do with unicode control chars that make the code as printed on the screen and reviewed by a human do something totally different when processed by a compiler. Such unicode control chars are almost never legitimately useful in the code, so b4 prints a warning and bails out when it finds them. However, just in case there are legitimate reasons for these characters to be in the code, for example, as part of documentation translated into left-to-right languages), you can override the default behavior with this switch.

--check (0.14+)

Tells b4 to run a series of local checks on each patch of the series and display any problems. When b4 finds a valid patchwork project definition in the configuration settings, it also looks up the CI status of each patch.

For the Linux kernel tree, b4 runs the following checkpatch command:

./scripts/ -q --terse --no-summary --mailback

You can specify a different command to run by setting the configuration setting, e.g.:

am-perpatch-check-cmd = ./scripts/ -q --terse --no-summary --mailback --strict

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 goes to stdout.

-c, --check-newer-revisions

When retrieving a patch series, perform a lookup to see if a newer revision is available. For example, if you are trying to retrieve a series titled [PATCH v2 0/3], b4 tries a number of mechanisms to look up if a v3 or later revision exists and adds these results to the retrieved thread.

-n WANTNAME, --mbox-name WANTNAME

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

-M, --save-as-maildir

By default, b4 saves the retrieved thread as an mbox file. However, due to subtle incompatibilities between various mbox formats (“mboxo” vs “mboxrd”, etc), you may instead want to save the results as a Maildir directory.

-Q, --quilt-ready

Saves the patches as a folder that you can pass 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 uses to find the match. If not specified, b4 uses 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 only considers the last 14 days prior to the date of the patch, but you can expand or shrink this range as necessary.

-3, --prep-3way

This tries 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 saves 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’s 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

When b4 encounters this situation, it properly creates a v2 of the entire series by reusing [PATCH v1 1/3] and [PATCH v1 3/3]. However, sometimes that isn’t 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 applies the patch series directly to the current git tree and the current branch in the directory where you run it. However, instead of just running git am and applying the patches directly, it can also treat the series as if it were a git pull request and either prepare a FETCH_HEAD that you can merge manually, or even automatically merge the series using the cover letter as the basis for the merge commit.

-H, --make-fetch-head

This prepares the series and places it into the FETCH_HEAD that you can merge just as if it were a pull request:

  1. b4 prepares a temporary sparse worktree

  2. b4 applies the series to that worktree

  3. if git am completes successfully, b4 fetches that tree into your current tree’s FETCH_HEAD, and then gets rid of the temporary tree

  4. b4 places the cover letter into .git/b4-cover

  5. b4 suggests 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 to see if a patch series is going to apply cleanly to a tree. You can perform any actions with the FETCH_HEAD as you normally would, such as 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 am and shazam settings section for some configuration file options that affect some of b4 shazam behaviour.