The seventh batch

[thirdparty/git.git] / Documentation / git-filter-branch.txt

git-filter-branch(1)
====================

NAME
----
git-filter-branch - Rewrite branches

SYNOPSIS
--------
[verse]
'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>...]

WARNING
-------
'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 https://github.com/newren/git-filter-repo/[git
filter-repo].  If you still need to use 'git filter-branch', please
carefully read <<SAFETY>> (and <<PERFORMANCE>>) to learn about the land
mines of filter-branch, and then vigilantly avoid as many of the hazards
listed there as reasonably possible.

DESCRIPTION
-----------
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 linkgit: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 linkgit: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.


OPTIONS
-------

--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 <<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 linkgit: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 linkgit: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 linkgit: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 <<Remap_to_ancestor>>.


[[Remap_to_ancestor]]
Remap to ancestor
~~~~~~~~~~~~~~~~~

By using linkgit: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.


EXIT STATUS
-----------

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.


EXAMPLES
--------

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
---------------------------------------------------------------



CHECKLIST FOR SHRINKING A REPOSITORY
------------------------------------

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 linkgit: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).

[[PERFORMANCE]]
PERFORMANCE
-----------

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 https://github.com/newren/git-filter-repo/[git filter-repo] 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
filter-repo' also provides
https://github.com/newren/git-filter-repo/blob/master/contrib/filter-repo-demos/filter-lamely[filter-lamely],
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.

[[SAFETY]]
SAFETY
------

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.

GIT
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Part of the linkgit:git[1] suite