|
NAME | SYNOPSIS | WARNING | DESCRIPTION | OPTIONS | EXIT STATUS | EXAMPLES | CHECKLIST FOR SHRINKING A REPOSITORY | PERFORMANCE | SAFETY | GIT | NOTES | COLOPHON |
GIT-FILTER-BRANCH(1) Git Manual GIT-FILTER-BRANCH(1)
git-filter-branch - Rewrite branches
git filter-branch [--setup <command>] [--subdirectory-filter <directory>]
[--env-filter <command>] [--tree-filter <command>]
[--index-filter <command>] [--parent-filter <command>]
[--msg-filter <command>] [--commit-filter <command>]
[--tag-name-filter <command>] [--prune-empty]
[--original <namespace>] [-d <directory>] [-f | --force]
[--state-branch <branch>] [--] [<rev-list options>...]
git filter-branch has a plethora of pitfalls that can produce
non-obvious manglings of the intended history rewrite (and can leave
you with little time to investigate such problems since it has such
abysmal performance). These safety and performance issues cannot be
backward compatibly fixed and as such, its use is not recommended.
Please use an alternative history filtering tool such as git
filter-repo[1]. If you still need to use git filter-branch, please
carefully read the section called “SAFETY” (and the section called
“PERFORMANCE”) to learn about the land mines of filter-branch, and
then vigilantly avoid as many of the hazards listed there as
reasonably possible.
Lets you rewrite Git revision history by rewriting the branches
mentioned in the <rev-list options>, applying custom filters on each
revision. Those filters can modify each tree (e.g. removing a file or
running a perl rewrite on all files) or information about each
commit. Otherwise, all information (including original commit times
or merge information) will be preserved.
The command will only rewrite the positive refs mentioned in the
command line (e.g. if you pass a..b, only b will be rewritten). If
you specify no filters, the commits will be recommitted without any
changes, which would normally have no effect. Nevertheless, this may
be useful in the future for compensating for some Git bugs or such,
therefore such a usage is permitted.
NOTE: This command honors .git/info/grafts file and refs in the
refs/replace/ namespace. If you have any grafts or replacement refs
defined, running this command will make them permanent.
WARNING! The rewritten history will have different object names for
all the objects and will not converge with the original branch. You
will not be able to easily push and distribute the rewritten branch
on top of the original branch. Please do not use this command if you
do not know the full implications, and avoid using it anyway, if a
simple single commit would suffice to fix your problem. (See the
"RECOVERING FROM UPSTREAM REBASE" section in git-rebase(1) for
further information about rewriting published history.)
Always verify that the rewritten version is correct: The original
refs, if different from the rewritten ones, will be stored in the
namespace refs/original/.
Note that since this operation is very I/O expensive, it might be a
good idea to redirect the temporary directory off-disk with the -d
option, e.g. on tmpfs. Reportedly the speedup is very noticeable.
Filters
The filters are applied in the order as listed below. The <command>
argument is always evaluated in the shell context using the eval
command (with the notable exception of the commit filter, for
technical reasons). Prior to that, the $GIT_COMMIT environment
variable will be set to contain the id of the commit being rewritten.
Also, GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE,
GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL, and GIT_COMMITTER_DATE are
taken from the current commit and exported to the environment, in
order to affect the author and committer identities of the
replacement commit created by git-commit-tree(1) after the filters
have run.
If any evaluation of <command> returns a non-zero exit status, the
whole operation will be aborted.
A map function is available that takes an "original sha1 id" argument
and outputs a "rewritten sha1 id" if the commit has been already
rewritten, and "original sha1 id" otherwise; the map function can
return several ids on separate lines if your commit filter emitted
multiple commits.
--setup <command>
This is not a real filter executed for each commit but a one time
setup just before the loop. Therefore no commit-specific
variables are defined yet. Functions or variables defined here
can be used or modified in the following filter steps except the
commit filter, for technical reasons.
--subdirectory-filter <directory>
Only look at the history which touches the given subdirectory.
The result will contain that directory (and only that) as its
project root. Implies the section called “Remap to ancestor”.
--env-filter <command>
This filter may be used if you only need to modify the
environment in which the commit will be performed. Specifically,
you might want to rewrite the author/committer name/email/time
environment variables (see git-commit-tree(1) for details).
--tree-filter <command>
This is the filter for rewriting the tree and its contents. The
argument is evaluated in shell with the working directory set to
the root of the checked out tree. The new tree is then used as-is
(new files are auto-added, disappeared files are auto-removed -
neither .gitignore files nor any other ignore rules HAVE ANY
EFFECT!).
--index-filter <command>
This is the filter for rewriting the index. It is similar to the
tree filter but does not check out the tree, which makes it much
faster. Frequently used with git rm --cached --ignore-unmatch
..., see EXAMPLES below. For hairy cases, see
git-update-index(1).
--parent-filter <command>
This is the filter for rewriting the commit’s parent list. It
will receive the parent string on stdin and shall output the new
parent string on stdout. The parent string is in the format
described in git-commit-tree(1): empty for the initial commit,
"-p parent" for a normal commit and "-p parent1 -p parent2 -p
parent3 ..." for a merge commit.
--msg-filter <command>
This is the filter for rewriting the commit messages. The
argument is evaluated in the shell with the original commit
message on standard input; its standard output is used as the new
commit message.
--commit-filter <command>
This is the filter for performing the commit. If this filter is
specified, it will be called instead of the git commit-tree
command, with arguments of the form "<TREE_ID> [(-p
<PARENT_COMMIT_ID>)...]" and the log message on stdin. The commit
id is expected on stdout.
As a special extension, the commit filter may emit multiple
commit ids; in that case, the rewritten children of the original
commit will have all of them as parents.
You can use the map convenience function in this filter, and
other convenience functions, too. For example, calling
skip_commit "$@" will leave out the current commit (but not its
changes! If you want that, use git rebase instead).
You can also use the git_commit_non_empty_tree "$@" instead of
git commit-tree "$@" if you don’t wish to keep commits with a
single parent and that makes no change to the tree.
--tag-name-filter <command>
This is the filter for rewriting tag names. When passed, it will
be called for every tag ref that points to a rewritten object (or
to a tag object which points to a rewritten object). The original
tag name is passed via standard input, and the new tag name is
expected on standard output.
The original tags are not deleted, but can be overwritten; use
"--tag-name-filter cat" to simply update the tags. In this case,
be very careful and make sure you have the old tags backed up in
case the conversion has run afoul.
Nearly proper rewriting of tag objects is supported. If the tag
has a message attached, a new tag object will be created with the
same message, author, and timestamp. If the tag has a signature
attached, the signature will be stripped. It is by definition
impossible to preserve signatures. The reason this is "nearly"
proper, is because ideally if the tag did not change (points to
the same object, has the same name, etc.) it should retain any
signature. That is not the case, signatures will always be
removed, buyer beware. There is also no support for changing the
author or timestamp (or the tag message for that matter). Tags
which point to other tags will be rewritten to point to the
underlying commit.
--prune-empty
Some filters will generate empty commits that leave the tree
untouched. This option instructs git-filter-branch to remove such
commits if they have exactly one or zero non-pruned parents;
merge commits will therefore remain intact. This option cannot be
used together with --commit-filter, though the same effect can be
achieved by using the provided git_commit_non_empty_tree function
in a commit filter.
--original <namespace>
Use this option to set the namespace where the original commits
will be stored. The default value is refs/original.
-d <directory>
Use this option to set the path to the temporary directory used
for rewriting. When applying a tree filter, the command needs to
temporarily check out the tree to some directory, which may
consume considerable space in case of large projects. By default
it does this in the .git-rewrite/ directory but you can override
that choice by this parameter.
-f, --force
git filter-branch refuses to start with an existing temporary
directory or when there are already refs starting with
refs/original/, unless forced.
--state-branch <branch>
This option will cause the mapping from old to new objects to be
loaded from named branch upon startup and saved as a new commit
to that branch upon exit, enabling incremental of large trees. If
<branch> does not exist it will be created.
<rev-list options>...
Arguments for git rev-list. All positive refs included by these
options are rewritten. You may also specify options such as
--all, but you must use -- to separate them from the git
filter-branch options. Implies the section called “Remap to
ancestor”.
Remap to ancestor
By using git-rev-list(1) arguments, e.g., path limiters, you can
limit the set of revisions which get rewritten. However, positive
refs on the command line are distinguished: we don’t let them be
excluded by such limiters. For this purpose, they are instead
rewritten to point at the nearest ancestor that was not excluded.
On success, the exit status is 0. If the filter can’t find any
commits to rewrite, the exit status is 2. On any other error, the
exit status may be any other non-zero value.
Suppose you want to remove a file (containing confidential
information or copyright violation) from all commits:
git filter-branch --tree-filter 'rm filename' HEAD
However, if the file is absent from the tree of some commit, a simple
rm filename will fail for that tree and commit. Thus you may instead
want to use rm -f filename as the script.
Using --index-filter with git rm yields a significantly faster
version. Like with using rm filename, git rm --cached filename will
fail if the file is absent from the tree of a commit. If you want to
"completely forget" a file, it does not matter when it entered
history, so we also add --ignore-unmatch:
git filter-branch --index-filter 'git rm --cached --ignore-unmatch filename' HEAD
Now, you will get the rewritten history saved in HEAD.
To rewrite the repository to look as if foodir/ had been its project
root, and discard all other history:
git filter-branch --subdirectory-filter foodir -- --all
Thus you can, e.g., turn a library subdirectory into a repository of
its own. Note the -- that separates filter-branch options from
revision options, and the --all to rewrite all branches and tags.
To set a commit (which typically is at the tip of another history) to
be the parent of the current initial commit, in order to paste the
other history behind the current history:
git filter-branch --parent-filter 'sed "s/^\$/-p <graft-id>/"' HEAD
(if the parent string is empty - which happens when we are dealing
with the initial commit - add graftcommit as a parent). Note that
this assumes history with a single root (that is, no merge without
common ancestors happened). If this is not the case, use:
git filter-branch --parent-filter \
'test $GIT_COMMIT = <commit-id> && echo "-p <graft-id>" || cat' HEAD
or even simpler:
git replace --graft $commit-id $graft-id
git filter-branch $graft-id..HEAD
To remove commits authored by "Darl McBribe" from the history:
git filter-branch --commit-filter '
if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
then
skip_commit "$@";
else
git commit-tree "$@";
fi' HEAD
The function skip_commit is defined as follows:
skip_commit()
{
shift;
while [ -n "$1" ];
do
shift;
map "$1";
shift;
done;
}
The shift magic first throws away the tree id and then the -p
parameters. Note that this handles merges properly! In case Darl
committed a merge between P1 and P2, it will be propagated properly
and all children of the merge will become merge commits with P1,P2 as
their parents instead of the merge commit.
NOTE the changes introduced by the commits, and which are not
reverted by subsequent commits, will still be in the rewritten
branch. If you want to throw out changes together with the commits,
you should use the interactive mode of git rebase.
You can rewrite the commit log messages using --msg-filter. For
example, git svn-id strings in a repository created by git svn can be
removed this way:
git filter-branch --msg-filter '
sed -e "/^git-svn-id:/d"
'
If you need to add Acked-by lines to, say, the last 10 commits (none
of which is a merge), use this command:
git filter-branch --msg-filter '
cat &&
echo "Acked-by: Bugs Bunny <bunny@bugzilla.org>"
' HEAD~10..HEAD
The --env-filter option can be used to modify committer and/or author
identity. For example, if you found out that your commits have the
wrong identity due to a misconfigured user.email, you can make a
correction, before publishing the project, like this:
git filter-branch --env-filter '
if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
then
GIT_AUTHOR_EMAIL=john@example.com
fi
if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
then
GIT_COMMITTER_EMAIL=john@example.com
fi
' -- --all
To restrict rewriting to only part of the history, specify a revision
range in addition to the new branch name. The new branch name will
point to the top-most revision that a git rev-list of this range will
print.
Consider this history:
D--E--F--G--H
/ /
A--B-----C
To rewrite only commits D,E,F,G,H, but leave A, B and C alone, use:
git filter-branch ... C..H
To rewrite commits E,F,G,H, use one of these:
git filter-branch ... C..H --not D
git filter-branch ... D..H --not C
To move the whole tree into a subdirectory, or remove it from there:
git filter-branch --index-filter \
'git ls-files -s | sed "s-\t\"*-&newsubdir/-" |
GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
git update-index --index-info &&
mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD
git-filter-branch can be used to get rid of a subset of files,
usually with some combination of --index-filter and
--subdirectory-filter. People expect the resulting repository to be
smaller than the original, but you need a few more steps to actually
make it smaller, because Git tries hard not to lose your objects
until you tell it to. First make sure that:
· You really removed all variants of a filename, if a blob was
moved over its lifetime. git log --name-only --follow --all --
filename can help you find renames.
· You really filtered all refs: use --tag-name-filter cat -- --all
when calling git-filter-branch.
Then there are two ways to get a smaller repository. A safer way is
to clone, that keeps your original intact.
· Clone it with git clone file:///path/to/repo. The clone will not
have the removed objects. See git-clone(1). (Note that cloning
with a plain path just hardlinks everything!)
If you really don’t want to clone it, for whatever reasons, check the
following points instead (in this order). This is a very destructive
approach, so make a backup or go back to cloning it. You have been
warned.
· Remove the original refs backed up by git-filter-branch: say git
for-each-ref --format="%(refname)" refs/original/ | xargs -n 1
git update-ref -d.
· Expire all reflogs with git reflog expire --expire=now --all.
· Garbage collect all unreferenced objects with git gc --prune=now
(or if your git-gc is not new enough to support arguments to
--prune, use git repack -ad; git prune instead).
The performance of git-filter-branch is glacially slow; its design
makes it impossible for a backward-compatible implementation to ever
be fast:
· In editing files, git-filter-branch by design checks out each and
every commit as it existed in the original repo. If your repo has
10^5 files and 10^5 commits, but each commit only modifies five
files, then git-filter-branch will make you do 10^10
modifications, despite only having (at most) 5*10^5 unique blobs.
· If you try and cheat and try to make git-filter-branch only work
on files modified in a commit, then two things happen
· you run into problems with deletions whenever the user is
simply trying to rename files (because attempting to delete
files that don’t exist looks like a no-op; it takes some
chicanery to remap deletes across file renames when the
renames happen via arbitrary user-provided shell)
· even if you succeed at the map-deletes-for-renames chicanery,
you still technically violate backward compatibility because
users are allowed to filter files in ways that depend upon
topology of commits instead of filtering solely based on file
contents or names (though this has not been observed in the
wild).
· Even if you don’t need to edit files but only want to e.g. rename
or remove some and thus can avoid checking out each file (i.e.
you can use --index-filter), you still are passing shell snippets
for your filters. This means that for every commit, you have to
have a prepared git repo where those filters can be run. That’s a
significant setup.
· Further, several additional files are created or updated per
commit by git-filter-branch. Some of these are for supporting the
convenience functions provided by git-filter-branch (such as
map()), while others are for keeping track of internal state (but
could have also been accessed by user filters; one of
git-filter-branch’s regression tests does so). This essentially
amounts to using the filesystem as an IPC mechanism between
git-filter-branch and the user-provided filters. Disks tend to be
a slow IPC mechanism, and writing these files also effectively
represents a forced synchronization point between separate
processes that we hit with every commit.
· The user-provided shell commands will likely involve a pipeline
of commands, resulting in the creation of many processes per
commit. Creating and running another process takes a widely
varying amount of time between operating systems, but on any
platform it is very slow relative to invoking a function.
· git-filter-branch itself is written in shell, which is kind of
slow. This is the one performance issue that could be
backward-compatibly fixed, but compared to the above problems
that are intrinsic to the design of git-filter-branch, the
language of the tool itself is a relatively minor issue.
· Side note: Unfortunately, people tend to fixate on the
written-in-shell aspect and periodically ask if
git-filter-branch could be rewritten in another language to
fix the performance issues. Not only does that ignore the
bigger intrinsic problems with the design, it’d help less
than you’d expect: if git-filter-branch itself were not
shell, then the convenience functions (map(), skip_commit(),
etc) and the --setup argument could no longer be executed
once at the beginning of the program but would instead need
to be prepended to every user filter (and thus re-executed
with every commit).
The git filter-repo[1] tool is an alternative to git-filter-branch
which does not suffer from these performance problems or the safety
problems (mentioned below). For those with existing tooling which
relies upon git-filter-branch, git repo-filter also provides
filter-lamely[2], a drop-in git-filter-branch replacement (with a few
caveats). While filter-lamely suffers from all the same safety issues
as git-filter-branch, it at least ameliorates the performance issues
a little.
git-filter-branch is riddled with gotchas resulting in various ways
to easily corrupt repos or end up with a mess worse than what you
started with:
· Someone can have a set of "working and tested filters" which they
document or provide to a coworker, who then runs them on a
different OS where the same commands are not working/tested (some
examples in the git-filter-branch manpage are also affected by
this). BSD vs. GNU userland differences can really bite. If
lucky, error messages are spewed. But just as likely, the
commands either don’t do the filtering requested, or silently
corrupt by making some unwanted change. The unwanted change may
only affect a few commits, so it’s not necessarily obvious
either. (The fact that problems won’t necessarily be obvious
means they are likely to go unnoticed until the rewritten history
is in use for quite a while, at which point it’s really hard to
justify another flag-day for another rewrite.)
· Filenames with spaces are often mishandled by shell snippets
since they cause problems for shell pipelines. Not everyone is
familiar with find -print0, xargs -0, git-ls-files -z, etc. Even
people who are familiar with these may assume such flags are not
relevant because someone else renamed any such files in their
repo back before the person doing the filtering joined the
project. And often, even those familiar with handling arguments
with spaces may not do so just because they aren’t in the mindset
of thinking about everything that could possibly go wrong.
· Non-ascii filenames can be silently removed despite being in a
desired directory. Keeping only wanted paths is often done using
pipelines like git ls-files | grep -v ^WANTED_DIR/ | xargs git
rm. ls-files will only quote filenames if needed, so folks may
not notice that one of the files didn’t match the regex (at least
not until it’s much too late). Yes, someone who knows about
core.quotePath can avoid this (unless they have other special
characters like \t, \n, or "), and people who use ls-files -z
with something other than grep can avoid this, but that doesn’t
mean they will.
· Similarly, when moving files around, one can find that filenames
with non-ascii or special characters end up in a different
directory, one that includes a double quote character. (This is
technically the same issue as above with quoting, but perhaps an
interesting different way that it can and has manifested as a
problem.)
· It’s far too easy to accidentally mix up old and new history.
It’s still possible with any tool, but git-filter-branch almost
invites it. If lucky, the only downside is users getting
frustrated that they don’t know how to shrink their repo and
remove the old stuff. If unlucky, they merge old and new history
and end up with multiple "copies" of each commit, some of which
have unwanted or sensitive files and others which don’t. This
comes about in multiple different ways:
· the default to only doing a partial history rewrite (--all is
not the default and few examples show it)
· the fact that there’s no automatic post-run cleanup
· the fact that --tag-name-filter (when used to rename tags)
doesn’t remove the old tags but just adds new ones with the
new name
· the fact that little educational information is provided to
inform users of the ramifications of a rewrite and how to
avoid mixing old and new history. For example, this man page
discusses how users need to understand that they need to
rebase their changes for all their branches on top of new
history (or delete and reclone), but that’s only one of
multiple concerns to consider. See the "DISCUSSION" section
of the git filter-repo manual page for more details.
· Annotated tags can be accidentally converted to lightweight tags,
due to either of two issues:
· Someone can do a history rewrite, realize they messed up,
restore from the backups in refs/original/, and then redo
their git-filter-branch command. (The backup in
refs/original/ is not a real backup; it dereferences tags
first.)
· Running git-filter-branch with either --tags or --all in your
<rev-list options>. In order to retain annotated tags as
annotated, you must use --tag-name-filter (and must not have
restored from refs/original/ in a previously botched
rewrite).
· Any commit messages that specify an encoding will become
corrupted by the rewrite; git-filter-branch ignores the encoding,
takes the original bytes, and feeds it to commit-tree without
telling it the proper encoding. (This happens whether or not
--msg-filter is used.)
· Commit messages (even if they are all UTF-8) by default become
corrupted due to not being updated — any references to other
commit hashes in commit messages will now refer to
no-longer-extant commits.
· There are no facilities for helping users find what unwanted crud
they should delete, which means they are much more likely to have
incomplete or partial cleanups that sometimes result in confusion
and people wasting time trying to understand. (For example, folks
tend to just look for big files to delete instead of big
directories or extensions, and once they do so, then sometime
later folks using the new repository who are going through
history will notice a build artifact directory that has some
files but not others, or a cache of dependencies (node_modules or
similar) which couldn’t have ever been functional since it’s
missing some files.)
· If --prune-empty isn’t specified, then the filtering process can
create hoards of confusing empty commits
· If --prune-empty is specified, then intentionally placed empty
commits from before the filtering operation are also pruned
instead of just pruning commits that became empty due to
filtering rules.
· If --prune-empty is specified, sometimes empty commits are missed
and left around anyway (a somewhat rare bug, but it happens...)
· A minor issue, but users who have a goal to update all names and
emails in a repository may be led to --env-filter which will only
update authors and committers, missing taggers.
· If the user provides a --tag-name-filter that maps multiple tags
to the same name, no warning or error is provided;
git-filter-branch simply overwrites each tag in some undocumented
pre-defined order resulting in only one tag at the end. (A
git-filter-branch regression test requires this surprising
behavior.)
Also, the poor performance of git-filter-branch often leads to safety
issues:
· Coming up with the correct shell snippet to do the filtering you
want is sometimes difficult unless you’re just doing a trivial
modification such as deleting a couple files. Unfortunately,
people often learn if the snippet is right or wrong by trying it
out, but the rightness or wrongness can vary depending on special
circumstances (spaces in filenames, non-ascii filenames, funny
author names or emails, invalid timezones, presence of grafts or
replace objects, etc.), meaning they may have to wait a long
time, hit an error, then restart. The performance of
git-filter-branch is so bad that this cycle is painful, reducing
the time available to carefully re-check (to say nothing about
what it does to the patience of the person doing the rewrite even
if they do technically have more time available). This problem is
extra compounded because errors from broken filters may not be
shown for a long time and/or get lost in a sea of output. Even
worse, broken filters often just result in silent incorrect
rewrites.
· To top it all off, even when users finally find working commands,
they naturally want to share them. But they may be unaware that
their repo didn’t have some special cases that someone else’s
does. So, when someone else with a different repository runs the
same commands, they get hit by the problems above. Or, the user
just runs commands that really were vetted for special cases, but
they run it on a different OS where it doesn’t work, as noted
above.
Part of the git(1) suite
1. git filter-repo
https://github.com/newren/git-filter-repo/
2. filter-lamely
https://github.com/newren/git-filter-repo/blob/master/contrib/filter-repo-demos/filter-lamely
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