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</head>
<body class="manpage">
<div id="header">
<h1>git-filter-branch(1) Manual Page</h1>
<h2 id="_name">NAME</h2>
<div class="sectionbody">
<p>git-filter-branch - Rewrite branches</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="verseblock">
<pre class="content"><em>git filter-branch</em> [--setup &lt;command&gt;] [--subdirectory-filter &lt;directory&gt;]
        [--env-filter &lt;command&gt;] [--tree-filter &lt;command&gt;]
        [--index-filter &lt;command&gt;] [--parent-filter &lt;command&gt;]
        [--msg-filter &lt;command&gt;] [--commit-filter &lt;command&gt;]
        [--tag-name-filter &lt;command&gt;] [--prune-empty]
        [--original &lt;namespace&gt;] [-d &lt;directory&gt;] [-f | --force]
        [--state-branch &lt;branch&gt;] [--] [&lt;rev-list-options&gt;&#8230;&#8203;]</pre>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_warning">WARNING</h2>
<div class="sectionbody">
<div class="paragraph">
<p><em>git filter-branch</em> 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 <a href="https://github.com/newren/git-filter-repo/">git
filter-repo</a>.  If you still need to use <em>git filter-branch</em>, please
carefully read <a href="#SAFETY">SAFETY</a> (and <a href="#PERFORMANCE">PERFORMANCE</a>) to learn about the land
mines of filter-branch, and then vigilantly avoid as many of the hazards
listed there as reasonably possible.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Lets you rewrite Git revision history by rewriting the branches mentioned
in the &lt;rev-list-options&gt;, 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.</p>
</div>
<div class="paragraph">
<p>The command will only rewrite the <em>positive</em> refs mentioned in the
command line (e.g. if you pass <em>a..b</em>, only <em>b</em> 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.</p>
</div>
<div class="paragraph">
<p><strong>NOTE</strong>: This command honors .<code>git/info/grafts</code> file and refs in
the <code>refs/replace/</code> namespace.
If you have any grafts or replacement refs defined, running this command
will make them permanent.</p>
</div>
<div class="paragraph">
<p><strong>WARNING</strong>! 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 <a href="git-rebase.html">git-rebase(1)</a> for further information about
rewriting published history.)</p>
</div>
<div class="paragraph">
<p>Always verify that the rewritten version is correct: The original refs,
if different from the rewritten ones, will be stored in the namespace
<em>refs/original/</em>.</p>
</div>
<div class="paragraph">
<p>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
<code>-d</code> option, e.g. on tmpfs.  Reportedly the speedup is very noticeable.</p>
</div>
<div class="sect2">
<h3 id="_filters">Filters</h3>
<div class="paragraph">
<p>The filters are applied in the order as listed below.  The &lt;command&gt;
argument is always evaluated in the shell context using the <em>eval</em> command
(with the notable exception of the commit filter, for technical reasons).
Prior to that, the <code>$GIT_COMMIT</code> 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 <a href="git-commit-tree.html">git-commit-tree(1)</a> after the
filters have run.</p>
</div>
<div class="paragraph">
<p>If any evaluation of &lt;command&gt; returns a non-zero exit status, the whole
operation will be aborted.</p>
</div>
<div class="paragraph">
<p>A <em>map</em> 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 <em>map</em> function can
return several ids on separate lines if your commit filter emitted
multiple commits.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_options">OPTIONS</h2>
<div class="sectionbody">
<div class="dlist">
<dl>
<dt class="hdlist1">--setup &lt;command&gt;</dt>
<dd>
<p>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.</p>
</dd>
<dt class="hdlist1">--subdirectory-filter &lt;directory&gt;</dt>
<dd>
<p>Only look at the history which touches the given subdirectory.
The result will contain that directory (and only that) as its
project root. Implies <a href="#Remap_to_ancestor">Remap to ancestor</a>.</p>
</dd>
<dt class="hdlist1">--env-filter &lt;command&gt;</dt>
<dd>
<p>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 <a href="git-commit-tree.html">git-commit-tree(1)</a> for details).</p>
</dd>
<dt class="hdlist1">--tree-filter &lt;command&gt;</dt>
<dd>
<p>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 <strong>HAVE ANY EFFECT</strong>!).</p>
</dd>
<dt class="hdlist1">--index-filter &lt;command&gt;</dt>
<dd>
<p>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 <code>git</code> <code>rm</code> <code>--cached</code>
<code>--ignore-unmatch</code> ..., see EXAMPLES below.  For hairy
cases, see <a href="git-update-index.html">git-update-index(1)</a>.</p>
</dd>
<dt class="hdlist1">--parent-filter &lt;command&gt;</dt>
<dd>
<p>This is the filter for rewriting the commit&#8217;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 <a href="git-commit-tree.html">git-commit-tree(1)</a>: empty for
the initial commit, "-p parent" for a normal commit and
"-p parent1 -p parent2 -p parent3 &#8230;&#8203;" for a merge commit.</p>
</dd>
<dt class="hdlist1">--msg-filter &lt;command&gt;</dt>
<dd>
<p>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.</p>
</dd>
<dt class="hdlist1">--commit-filter &lt;command&gt;</dt>
<dd>
<p>This is the filter for performing the commit.
If this filter is specified, it will be called instead of the
<em>git commit-tree</em> command, with arguments of the form
"&lt;TREE_ID&gt; [(-p &lt;PARENT_COMMIT_ID&gt;)&#8230;&#8203;]" and the log message on
stdin.  The commit id is expected on stdout.</p>
<div class="paragraph">
<p>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.</p>
</div>
<div class="paragraph">
<p>You can use the <em>map</em> convenience function in this filter, and other
convenience functions, too.  For example, calling <em>skip_commit "$@"</em>
will leave out the current commit (but not its changes! If you want
that, use <em>git rebase</em> instead).</p>
</div>
<div class="paragraph">
<p>You can also use the <code>git_commit_non_empty_tree</code> "$@" instead of
<code>git</code> <code>commit-tree</code> "$@" if you don&#8217;t wish to keep commits with a single parent
and that makes no change to the tree.</p>
</div>
</dd>
<dt class="hdlist1">--tag-name-filter &lt;command&gt;</dt>
<dd>
<p>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.</p>
<div class="paragraph">
<p>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.</p>
</div>
<div class="paragraph">
<p>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.</p>
</div>
</dd>
<dt class="hdlist1">--prune-empty</dt>
<dd>
<p>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
<code>--commit-filter</code>, though the same effect can be achieved by using the
provided <code>git_commit_non_empty_tree</code> function in a commit filter.</p>
</dd>
<dt class="hdlist1">--original &lt;namespace&gt;</dt>
<dd>
<p>Use this option to set the namespace where the original commits
will be stored. The default value is <em>refs/original</em>.</p>
</dd>
<dt class="hdlist1">-d &lt;directory&gt;</dt>
<dd>
<p>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 .<code>git-rewrite/</code> directory but you can override
that choice by this parameter.</p>
</dd>
<dt class="hdlist1">-f</dt>
<dt class="hdlist1">--force</dt>
<dd>
<p><em>git filter-branch</em> refuses to start with an existing temporary
directory or when there are already refs starting with
<em>refs/original/</em>, unless forced.</p>
</dd>
<dt class="hdlist1">--state-branch &lt;branch&gt;</dt>
<dd>
<p>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 <em>&lt;branch&gt;</em> does not exist it will be created.</p>
</dd>
<dt class="hdlist1">&lt;rev-list options&gt;&#8230;&#8203;</dt>
<dd>
<p>Arguments for <em>git rev-list</em>.  All positive refs included by
these options are rewritten.  You may also specify options
such as <code>--all</code>, but you must use <code>--</code> to separate them from
the <em>git filter-branch</em> options. Implies <a href="#Remap_to_ancestor">Remap to ancestor</a>.</p>
</dd>
</dl>
</div>
<div class="sect2">
<h3 id="Remap_to_ancestor">Remap to ancestor</h3>
<div class="paragraph">
<p>By using <a href="git-rev-list.html">git-rev-list(1)</a> 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&#8217;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.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_exit_status">EXIT STATUS</h2>
<div class="sectionbody">
<div class="paragraph">
<p>On success, the exit status is <code>0</code>.  If the filter can&#8217;t find any commits to
rewrite, the exit status is <code>2</code>.  On any other error, the exit status may be
any other non-zero value.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_examples">EXAMPLES</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Suppose you want to remove a file (containing confidential information
or copyright violation) from all commits:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --tree-filter 'rm filename' HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>However, if the file is absent from the tree of some commit,
a simple <code>rm</code> <code>filename</code> will fail for that tree and commit.
Thus you may instead want to use <code>rm</code> <code>-f</code> <code>filename</code> as the script.</p>
</div>
<div class="paragraph">
<p>Using <code>--index-filter</code> with <em>git rm</em> yields a significantly faster
version.  Like with using <code>rm</code> <code>filename</code>, <code>git</code> <code>rm</code> <code>--cached</code> <code>filename</code>
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 <code>--ignore-unmatch</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --index-filter 'git rm --cached --ignore-unmatch filename' HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>Now, you will get the rewritten history saved in HEAD.</p>
</div>
<div class="paragraph">
<p>To rewrite the repository to look as if <code>foodir/</code> had been its project
root, and discard all other history:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --subdirectory-filter foodir -- --all</pre>
</div>
</div>
<div class="paragraph">
<p>Thus you can, e.g., turn a library subdirectory into a repository of
its own.  Note the <code>--</code> that separates <em>filter-branch</em> options from
revision options, and the <code>--all</code> to rewrite all branches and tags.</p>
</div>
<div class="paragraph">
<p>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:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --parent-filter 'sed "s/^\$/-p &lt;graft-id&gt;/"' HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>(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:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --parent-filter \
        'test $GIT_COMMIT = &lt;commit-id&gt; &amp;&amp; echo "-p &lt;graft-id&gt;" || cat' HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>or even simpler:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git replace --graft $commit-id $graft-id
git filter-branch $graft-id..HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>To remove commits authored by "Darl McBribe" from the history:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --commit-filter '
        if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
        then
                skip_commit "$@";
        else
                git commit-tree "$@";
        fi' HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>The function <em>skip_commit</em> is defined as follows:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>skip_commit()
{
        shift;
        while [ -n "$1" ];
        do
                shift;
                map "$1";
                shift;
        done;
}</pre>
</div>
</div>
<div class="paragraph">
<p>The shift magic first throws away the tree id and then the -p
parameters.  Note that this handles merges properly! In case Darl
committed a merge between P1 and P2, it will be propagated properly
and all children of the merge will become merge commits with P1,P2
as their parents instead of the merge commit.</p>
</div>
<div class="paragraph">
<p><strong>NOTE</strong> the changes introduced by the commits, and which are not reverted
by subsequent commits, will still be in the rewritten branch. If you want
to throw out <em>changes</em> together with the commits, you should use the
interactive mode of <em>git rebase</em>.</p>
</div>
<div class="paragraph">
<p>You can rewrite the commit log messages using <code>--msg-filter</code>.  For
example, <em>git svn-id</em> strings in a repository created by <em>git svn</em> can
be removed this way:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --msg-filter '
        sed -e "/^git-svn-id:/d"
'</pre>
</div>
</div>
<div class="paragraph">
<p>If you need to add <em>Acked-by</em> lines to, say, the last 10 commits (none
of which is a merge), use this command:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --msg-filter '
        cat &amp;&amp;
        echo "Acked-by: Bugs Bunny &lt;bunny@bugzilla.org&gt;"
' HEAD~10..HEAD</pre>
</div>
</div>
<div class="paragraph">
<p>The <code>--env-filter</code> option can be used to modify committer and/or author
identity.  For example, if you found out that your commits have the wrong
identity due to a misconfigured user.email, you can make a correction,
before publishing the project, like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --env-filter '
        if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
        then
                GIT_AUTHOR_EMAIL=john@example.com
        fi
        if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
        then
                GIT_COMMITTER_EMAIL=john@example.com
        fi
' -- --all</pre>
</div>
</div>
<div class="paragraph">
<p>To restrict rewriting to only part of the history, specify a revision
range in addition to the new branch name.  The new branch name will
point to the top-most revision that a <em>git rev-list</em> of this range
will print.</p>
</div>
<div class="paragraph">
<p>Consider this history:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>     D--E--F--G--H
    /     /
A--B-----C</pre>
</div>
</div>
<div class="paragraph">
<p>To rewrite only commits D,E,F,G,H, but leave A, B and C alone, use:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch ... C..H</pre>
</div>
</div>
<div class="paragraph">
<p>To rewrite commits E,F,G,H, use one of these:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch ... C..H --not D
git filter-branch ... D..H --not C</pre>
</div>
</div>
<div class="paragraph">
<p>To move the whole tree into a subdirectory, or remove it from there:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>git filter-branch --index-filter \
        'git ls-files -s | sed "s-\t\"*-&amp;newsubdir/-" |
                GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
                        git update-index --index-info &amp;&amp;
         mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_checklist_for_shrinking_a_repository">CHECKLIST FOR SHRINKING A REPOSITORY</h2>
<div class="sectionbody">
<div class="paragraph">
<p>git-filter-branch can be used to get rid of a subset of files,
usually with some combination of <code>--index-filter</code> and
<code>--subdirectory-filter</code>.  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:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>You really removed all variants of a filename, if a blob was moved
over its lifetime.  <code>git</code> <code>log</code> <code>--name-only</code> <code>--follow</code> <code>--all</code> <code>--</code> <code>filename</code>
can help you find renames.</p>
</li>
<li>
<p>You really filtered all refs: use <code>--tag-name-filter</code> <code>cat</code> <code>--</code> <code>--all</code>
when calling git-filter-branch.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Then there are two ways to get a smaller repository.  A safer way is
to clone, that keeps your original intact.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Clone it with <code>git</code> <code>clone</code> <code>file:///path/to/repo</code>.  The clone
will not have the removed objects.  See <a href="git-clone.html">git-clone(1)</a>.  (Note
that cloning with a plain path just hardlinks everything!)</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>If you really don&#8217;t want to clone it, for whatever reasons, check the
following points instead (in this order).  This is a very destructive
approach, so <strong>make a backup</strong> or go back to cloning it.  You have been
warned.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Remove the original refs backed up by git-filter-branch: say <code>git</code>
<code>for-each-ref</code> <code>--format=</code>"%(<code>refname</code>)" <code>refs/original/</code> | <code>xargs</code> <code>-n</code> <code>1</code> <code>git</code>
<code>update-ref</code> <code>-d</code>.</p>
</li>
<li>
<p>Expire all reflogs with <code>git</code> <code>reflog</code> <code>expire</code> <code>--expire=now</code> <code>--all</code>.</p>
</li>
<li>
<p>Garbage collect all unreferenced objects with <code>git</code> <code>gc</code> <code>--prune=now</code>
(or if your git-gc is not new enough to support arguments to
<code>--prune</code>, use <code>git</code> <code>repack</code> <code>-ad</code>; <code>git</code> <code>prune</code> instead).</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="PERFORMANCE">PERFORMANCE</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The performance of git-filter-branch is glacially slow; its design makes it
impossible for a backward-compatible implementation to ever be fast:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>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
<code>10^5</code> files and <code>10^5</code> commits, but each commit only modifies five
files, then git-filter-branch will make you do <code>10^10</code> modifications,
despite only having (at most) <code>5</code>*10^5 unique blobs.</p>
</li>
<li>
<p>If you try and cheat and try to make git-filter-branch only work on
files modified in a commit, then two things happen</p>
<div class="ulist">
<ul>
<li>
<p>you run into problems with deletions whenever the user is simply
trying to rename files (because attempting to delete files that
don&#8217;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)</p>
</li>
<li>
<p>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).</p>
</li>
</ul>
</div>
</li>
<li>
<p>Even if you don&#8217;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&#8217;s a
significant setup.</p>
</li>
<li>
<p>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&#8217;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.</p>
</li>
<li>
<p>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.</p>
</li>
<li>
<p>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.</p>
<div class="ulist">
<ul>
<li>
<p>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&#8217;d help less than you&#8217;d expect: if
git-filter-branch itself were not shell, then the convenience
functions (map(), skip_commit(), etc) and the <code>--setup</code> 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).</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p>The <a href="https://github.com/newren/git-filter-repo/">git filter-repo</a> 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, <em>git
filter-repo</em> also provides
<a href="https://github.com/newren/git-filter-repo/blob/master/contrib/filter-repo-demos/filter-lamely">filter-lamely</a>,
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.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="SAFETY">SAFETY</h2>
<div class="sectionbody">
<div class="paragraph">
<p>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:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>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&#8217;t
do the filtering requested, or silently corrupt by making some
unwanted change.  The unwanted change may only affect a few commits,
so it&#8217;s not necessarily obvious either.  (The fact that problems
won&#8217;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&#8217;s really hard to justify another flag-day for another
rewrite.)</p>
</li>
<li>
<p>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&#8217;t in the mindset of thinking about
everything that could possibly go wrong.</p>
</li>
<li>
<p>Non-ascii filenames can be silently removed despite being in a
desired directory.  Keeping only wanted paths is often done using
pipelines like <code>git</code> <code>ls-files</code> | <code>grep</code> <code>-v</code> <code>^WANTED_DIR/</code> | <code>xargs</code> <code>git</code> <code>rm</code>.
ls-files will only quote filenames if needed, so folks may not
notice that one of the files didn&#8217;t match the regex (at least not
until it&#8217;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&#8217;t mean they
will.</p>
</li>
<li>
<p>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.)</p>
</li>
<li>
<p>It&#8217;s far too easy to accidentally mix up old and new history.  It&#8217;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&#8217;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&#8217;t.  This comes about in
multiple different ways:</p>
<div class="ulist">
<ul>
<li>
<p>the default to only doing a partial history rewrite (<em>--all</em> is not
the default and few examples show it)</p>
</li>
<li>
<p>the fact that there&#8217;s no automatic post-run cleanup</p>
</li>
<li>
<p>the fact that --tag-name-filter (when used to rename tags) doesn&#8217;t
remove the old tags but just adds new ones with the new name</p>
</li>
<li>
<p>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&#8217;s only one of multiple concerns to consider.  See the
"DISCUSSION" section of the git filter-repo manual page for more
details.</p>
</li>
</ul>
</div>
</li>
<li>
<p>Annotated tags can be accidentally converted to lightweight tags,
due to either of two issues:</p>
<div class="ulist">
<ul>
<li>
<p>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.)</p>
</li>
<li>
<p>Running git-filter-branch with either --tags or --all in your
&lt;rev-list-options&gt;.  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).</p>
</li>
</ul>
</div>
</li>
<li>
<p>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.)</p>
</li>
<li>
<p>Commit messages (even if they are all UTF-8) by default become
corrupted due to not being updated&#8201;&#8212;&#8201;any references to other commit
hashes in commit messages will now refer to no-longer-extant
commits.</p>
</li>
<li>
<p>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&#8217;t have ever been
functional since it&#8217;s missing some files.)</p>
</li>
<li>
<p>If --prune-empty isn&#8217;t specified, then the filtering process can
create hoards of confusing empty commits</p>
</li>
<li>
<p>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.</p>
</li>
<li>
<p>If --prune-empty is specified, sometimes empty commits are missed
and left around anyway (a somewhat rare bug, but it happens&#8230;&#8203;)</p>
</li>
<li>
<p>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.</p>
</li>
<li>
<p>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.)</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Also, the poor performance of git-filter-branch often leads to safety
issues:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Coming up with the correct shell snippet to do the filtering you
want is sometimes difficult unless you&#8217;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.</p>
</li>
<li>
<p>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&#8217;t have some special cases that someone else&#8217;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&#8217;t work, as noted above.</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_git">GIT</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Part of the <a href="git.html">git(1)</a> suite</p>
</div>
</div>
</div>
</div>
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