Document git-filter-branch

This moves the documentation in git-filter-branch.sh to its own
man page, with a few touch ups (incorporating comments by Frank
Lichtenheld).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 262 insertions, 0 deletions

diff --git a/Documentation/git-filter-branch.txt b/Documentation/git-filter-branch.txt
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+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>]
+	[-d <directory>] <new-branch-name> [<rev-list options>...]
+
+DESCRIPTION
+-----------
+Lets you rewrite git revision history by creating a new branch from
+your current branch, 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 takes the new branch name as a mandatory argument and
+the filters as optional arguments.  If you specify no filters, the
+commits will be recommitted without any changes, which would normally
+have no effect and result in the new branch pointing to the same
+branch as your current branch.  Nevertheless, this may be useful in
+the future for compensating for some git bugs or such, therefore
+such a usage is permitted.
+
+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.
+
+Always verify that the rewritten version is correct before disposing
+the original branch.
+
+Note that since this operation is extensively I/O expensive, it might
+be a good idea to redirect the temporary directory off-disk, 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 shell using the 'eval' command.
+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 is set according to the current commit.
+
+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, fails otherwise; the 'map' function can return several
+ids on separate lines if your commit filter emitted multiple commits.
+
+
+OPTIONS
+-------
+
+--env-filter <command>::
+	This is the filter for modifying the environment in which
+	the commit will be performed.  Specifically, you might want
+	to rewrite the author/committer name/email/time environment
+	variables (see gitlink:git-commit[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.  For hairy cases, see gitlink: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
+	a format accepted by gitlink: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
+	gitlink:git-commit-tree[1] 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, ancestors of the original commit will
+have all of them as parents.
+
+--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.
++
+Note that there is currently no support for proper rewriting of
+tag objects; in layman terms, if the tag has a message or signature
+attached, the rewritten tag won't have it.  Sorry.  (It is by
+definition impossible to preserve signatures at any rate.)
+
+--subdirectory-filter <directory>::
+	Only ever look at the history, which touches the given subdirectory.
+	The result will contain that directory as its project root.
+
+-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
+	temporary checkout 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.
+
+<rev-list-options>::
+	When options are given after the new branch name, they will
+	be passed to gitlink:git-rev-list[1].  Only commits in the resulting
+	output will be filtered, although the filtered commits can still
+	reference parents which are outside of that set.
+
+
+Examples
+--------
+
+Suppose you want to remove a file (containing confidential information
+or copyright violation) from all commits:
+
+-------------------------------------------------------
+git filter-branch --tree-filter 'rm filename' newbranch
+-------------------------------------------------------
+
+A significantly faster version:
+
+-------------------------------------------------------------------------------
+git filter-branch --index-filter 'git update-index --remove filename' newbranch
+-------------------------------------------------------------------------------
+
+Now, you will get the rewritten history saved in the branch 'newbranch'
+(your current branch is left untouched).
+
+To "etch-graft" a commit to the revision history (set a commit to be
+the parent of the current initial commit and propagate that):
+
+----------------------------------------------------------------------
+git filter-branch --parent-filter sed\ 's/^$/-p <graft-id>/' newbranch
+----------------------------------------------------------------------
+
+(if the parent string is empty - therefore 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 \
+	'cat; test $GIT_COMMIT = <commit-id> && echo "-p <graft-id>"' newbranch
+-------------------------------------------------------------------------------
+
+To remove commits authored by "Darl McBribe" from the history:
+
+------------------------------------------------------------------------------
+git filter-branch --commit-filter '
+	if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
+	then
+		shift;
+		while [ -n "$1" ];
+		do
+			shift;
+			echo "$1";
+			shift;
+		done;
+	else
+		git commit-tree "$@";
+	fi' newbranch
+------------------------------------------------------------------------------
+
+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.
+
+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.
+
+Note that the changes introduced by the commits, and 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 gitlink:git-rebase[1].
+
+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 ... new-H C..H
+--------------------------------
+
+To rewrite commits E,F,G,H, use one of these:
+
+----------------------------------------
+git filter-branch ... new-H C..H --not D
+git filter-branch ... new-H 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' directorymoved
+---------------------------------------------------------------
+
+
+Author
+------
+Written by Petr "Pasky" Baudis <pasky@suse.cz>,
+and the git list <git@vger.kernel.org>
+
+Documentation
+--------------
+Documentation by Petr Baudis and the git list.
+
+GIT
+---
+Part of the gitlink:git[7] suite