git-commit-vandalism/Documentation/git-filter-branch.txt

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

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

SYNOPSIS
--------
[verse]
'git filter-branch' [--env-filter <command>] [--tree-filter <command>]
	[--index-filter <command>] [--parent-filter <command>]
	[--msg-filter <command>] [--commit-filter <command>]
	[--tag-name-filter <command>] [--subdirectory-filter <directory>]
	[--prune-empty]
	[--original <namespace>] [-d <directory>] [-f | --force]
	[--] [<rev-list options>...]

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
-------

--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).  Do not forget
	to re-export the variables.

--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.

--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>>.

--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.

<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.


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:

-----------------------------------------------
echo "$commit-id $graft-id" >> .git/info/grafts
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 "$GIT_AUTHOR_EMAIL" = "root@localhost"
	then
		GIT_AUTHOR_EMAIL=john@example.com
		export GIT_AUTHOR_EMAIL
	fi
	if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
	then
		GIT_COMMITTER_EMAIL=john@example.com
		export GIT_COMMITTER_EMAIL
	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).

Notes
-----

git-filter-branch allows you to make complex shell-scripted rewrites
of your Git history, but you probably don't need this flexibility if
you're simply _removing unwanted data_ like large files or passwords.
For those operations you may want to consider
http://rtyley.github.io/bfg-repo-cleaner/[The BFG Repo-Cleaner],
a JVM-based alternative to git-filter-branch, typically at least
10-50x faster for those use-cases, and with quite different
characteristics:

* Any particular version of a file is cleaned exactly _once_. The BFG,
  unlike git-filter-branch, does not give you the opportunity to
  handle a file differently based on where or when it was committed
  within your history. This constraint gives the core performance
  benefit of The BFG, and is well-suited to the task of cleansing bad
  data - you don't care _where_ the bad data is, you just want it
  _gone_.

* By default The BFG takes full advantage of multi-core machines,
  cleansing commit file-trees in parallel. git-filter-branch cleans
  commits sequentially (i.e. in a single-threaded manner), though it
  _is_ possible to write filters that include their own parallelism,
  in the scripts executed against each commit.

* The http://rtyley.github.io/bfg-repo-cleaner/#examples[command options]
  are much more restrictive than git-filter branch, and dedicated just
  to the tasks of removing unwanted data- e.g:
  `--strip-blobs-bigger-than 1M`.

GIT
---
Part of the linkgit:git[1] suite