git-filter-branch.html

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 735 <body class="manpage">
 736 <div id="header">
 737 <h1>
 738 git-filter-branch(1) Manual Page
 739 </h1>
 740 <h2>NAME</h2>
 741 <div class="sectionbody">
 742 <p>git-filter-branch -
 743    Rewrite branches
 744 </p>
 745 </div>
 746 </div>
 747 <div id="content">
 748 <div class="sect1">
 749 <h2 id="_synopsis">SYNOPSIS</h2>
 750 <div class="sectionbody">
 751 <div class="verseblock">
 752 <pre class="content"><em>git filter-branch</em> [--setup &lt;command&gt;] [--subdirectory-filter &lt;directory&gt;]
 753         [--env-filter &lt;command&gt;] [--tree-filter &lt;command&gt;]
 754         [--index-filter &lt;command&gt;] [--parent-filter &lt;command&gt;]
 755         [--msg-filter &lt;command&gt;] [--commit-filter &lt;command&gt;]
 756         [--tag-name-filter &lt;command&gt;] [--prune-empty]
 757         [--original &lt;namespace&gt;] [-d &lt;directory&gt;] [-f | --force]
 758         [--state-branch &lt;branch&gt;] [--] [&lt;rev-list options&gt;&#8230;]</pre>
 759 <div class="attribution">
 760 </div></div>
 761 </div>
 762 </div>
 763 <div class="sect1">
 764 <h2 id="_warning">WARNING</h2>
 765 <div class="sectionbody">
 766 <div class="paragraph"><p><em>git filter-branch</em> has a plethora of pitfalls that can produce non-obvious
 767 manglings of the intended history rewrite (and can leave you with little
 768 time to investigate such problems since it has such abysmal performance).
 769 These safety and performance issues cannot be backward compatibly fixed and
 770 as such, its use is not recommended.  Please use an alternative history
 771 filtering tool such as <a href="https://github.com/newren/git-filter-repo/">git
 772 filter-repo</a>.  If you still need to use <em>git filter-branch</em>, please
 773 carefully read <a href="#SAFETY">[SAFETY]</a> (and <a href="#PERFORMANCE">[PERFORMANCE]</a>) to learn about the land
 774 mines of filter-branch, and then vigilantly avoid as many of the hazards
 775 listed there as reasonably possible.</p></div>
 776 </div>
 777 </div>
 778 <div class="sect1">
 779 <h2 id="_description">DESCRIPTION</h2>
 780 <div class="sectionbody">
 781 <div class="paragraph"><p>Lets you rewrite Git revision history by rewriting the branches mentioned
 782 in the &lt;rev-list options&gt;, applying custom filters on each revision.
 783 Those filters can modify each tree (e.g. removing a file or running
 784 a perl rewrite on all files) or information about each commit.
 785 Otherwise, all information (including original commit times or merge
 786 information) will be preserved.</p></div>
 787 <div class="paragraph"><p>The command will only rewrite the <em>positive</em> refs mentioned in the
 788 command line (e.g. if you pass <em>a..b</em>, only <em>b</em> will be rewritten).
 789 If you specify no filters, the commits will be recommitted without any
 790 changes, which would normally have no effect.  Nevertheless, this may be
 791 useful in the future for compensating for some Git bugs or such,
 792 therefore such a usage is permitted.</p></div>
 793 <div class="paragraph"><p><strong>NOTE</strong>: This command honors <code>.git/info/grafts</code> file and refs in
 794 the <code>refs/replace/</code> namespace.
 795 If you have any grafts or replacement refs defined, running this command
 796 will make them permanent.</p></div>
 797 <div class="paragraph"><p><strong>WARNING</strong>! The rewritten history will have different object names for all
 798 the objects and will not converge with the original branch.  You will not
 799 be able to easily push and distribute the rewritten branch on top of the
 800 original branch.  Please do not use this command if you do not know the
 801 full implications, and avoid using it anyway, if a simple single commit
 802 would suffice to fix your problem.  (See the "RECOVERING FROM UPSTREAM
 803 REBASE" section in <a href="git-rebase.html">git-rebase(1)</a> for further information about
 804 rewriting published history.)</p></div>
 805 <div class="paragraph"><p>Always verify that the rewritten version is correct: The original refs,
 806 if different from the rewritten ones, will be stored in the namespace
 807 <em>refs/original/</em>.</p></div>
 808 <div class="paragraph"><p>Note that since this operation is very I/O expensive, it might
 809 be a good idea to redirect the temporary directory off-disk with the
 810 <code>-d</code> option, e.g. on tmpfs.  Reportedly the speedup is very noticeable.</p></div>
 811 <div class="sect2">
 812 <h3 id="_filters">Filters</h3>
 813 <div class="paragraph"><p>The filters are applied in the order as listed below.  The &lt;command&gt;
 814 argument is always evaluated in the shell context using the <em>eval</em> command
 815 (with the notable exception of the commit filter, for technical reasons).
 816 Prior to that, the <code>$GIT_COMMIT</code> environment variable will be set to contain
 817 the id of the commit being rewritten.  Also, GIT_AUTHOR_NAME,
 818 GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL,
 819 and GIT_COMMITTER_DATE are taken from the current commit and exported to
 820 the environment, in order to affect the author and committer identities of
 821 the replacement commit created by <a href="git-commit-tree.html">git-commit-tree(1)</a> after the
 822 filters have run.</p></div>
 823 <div class="paragraph"><p>If any evaluation of &lt;command&gt; returns a non-zero exit status, the whole
 824 operation will be aborted.</p></div>
 825 <div class="paragraph"><p>A <em>map</em> function is available that takes an "original sha1 id" argument
 826 and outputs a "rewritten sha1 id" if the commit has been already
 827 rewritten, and "original sha1 id" otherwise; the <em>map</em> function can
 828 return several ids on separate lines if your commit filter emitted
 829 multiple commits.</p></div>
 830 </div>
 831 </div>
 832 </div>
 833 <div class="sect1">
 834 <h2 id="_options">OPTIONS</h2>
 835 <div class="sectionbody">
 836 <div class="dlist"><dl>
 837 <dt class="hdlist1">
 838 --setup &lt;command&gt;
 839 </dt>
 840 <dd>
 841 <p>
 842         This is not a real filter executed for each commit but a one
 843         time setup just before the loop. Therefore no commit-specific
 844         variables are defined yet.  Functions or variables defined here
 845         can be used or modified in the following filter steps except
 846         the commit filter, for technical reasons.
 847 </p>
 848 </dd>
 849 <dt class="hdlist1">
 850 --subdirectory-filter &lt;directory&gt;
 851 </dt>
 852 <dd>
 853 <p>
 854         Only look at the history which touches the given subdirectory.
 855         The result will contain that directory (and only that) as its
 856         project root. Implies <a href="#Remap_to_ancestor">[Remap_to_ancestor]</a>.
 857 </p>
 858 </dd>
 859 <dt class="hdlist1">
 860 --env-filter &lt;command&gt;
 861 </dt>
 862 <dd>
 863 <p>
 864         This filter may be used if you only need to modify the environment
 865         in which the commit will be performed.  Specifically, you might
 866         want to rewrite the author/committer name/email/time environment
 867         variables (see <a href="git-commit-tree.html">git-commit-tree(1)</a> for details).
 868 </p>
 869 </dd>
 870 <dt class="hdlist1">
 871 --tree-filter &lt;command&gt;
 872 </dt>
 873 <dd>
 874 <p>
 875         This is the filter for rewriting the tree and its contents.
 876         The argument is evaluated in shell with the working
 877         directory set to the root of the checked out tree.  The new tree
 878         is then used as-is (new files are auto-added, disappeared files
 879         are auto-removed - neither .gitignore files nor any other ignore
 880         rules <strong>HAVE ANY EFFECT</strong>!).
 881 </p>
 882 </dd>
 883 <dt class="hdlist1">
 884 --index-filter &lt;command&gt;
 885 </dt>
 886 <dd>
 887 <p>
 888         This is the filter for rewriting the index.  It is similar to the
 889         tree filter but does not check out the tree, which makes it much
 890         faster.  Frequently used with <code>git rm --cached
 891         --ignore-unmatch ...</code>, see EXAMPLES below.  For hairy
 892         cases, see <a href="git-update-index.html">git-update-index(1)</a>.
 893 </p>
 894 </dd>
 895 <dt class="hdlist1">
 896 --parent-filter &lt;command&gt;
 897 </dt>
 898 <dd>
 899 <p>
 900         This is the filter for rewriting the commit&#8217;s parent list.
 901         It will receive the parent string on stdin and shall output
 902         the new parent string on stdout.  The parent string is in
 903         the format described in <a href="git-commit-tree.html">git-commit-tree(1)</a>: empty for
 904         the initial commit, "-p parent" for a normal commit and
 905         "-p parent1 -p parent2 -p parent3 &#8230;" for a merge commit.
 906 </p>
 907 </dd>
 908 <dt class="hdlist1">
 909 --msg-filter &lt;command&gt;
 910 </dt>
 911 <dd>
 912 <p>
 913         This is the filter for rewriting the commit messages.
 914         The argument is evaluated in the shell with the original
 915         commit message on standard input; its standard output is
 916         used as the new commit message.
 917 </p>
 918 </dd>
 919 <dt class="hdlist1">
 920 --commit-filter &lt;command&gt;
 921 </dt>
 922 <dd>
 923 <p>
 924         This is the filter for performing the commit.
 925         If this filter is specified, it will be called instead of the
 926         <em>git commit-tree</em> command, with arguments of the form
 927         "&lt;TREE_ID&gt; [(-p &lt;PARENT_COMMIT_ID&gt;)&#8230;]" and the log message on
 928         stdin.  The commit id is expected on stdout.
 929 </p>
 930 <div class="paragraph"><p>As a special extension, the commit filter may emit multiple
 931 commit ids; in that case, the rewritten children of the original commit will
 932 have all of them as parents.</p></div>
 933 <div class="paragraph"><p>You can use the <em>map</em> convenience function in this filter, and other
 934 convenience functions, too.  For example, calling <em>skip_commit "$@"</em>
 935 will leave out the current commit (but not its changes! If you want
 936 that, use <em>git rebase</em> instead).</p></div>
 937 <div class="paragraph"><p>You can also use the <code>git_commit_non_empty_tree "$@"</code> instead of
 938 <code>git commit-tree "$@"</code> if you don&#8217;t wish to keep commits with a single parent
 939 and that makes no change to the tree.</p></div>
 940 </dd>
 941 <dt class="hdlist1">
 942 --tag-name-filter &lt;command&gt;
 943 </dt>
 944 <dd>
 945 <p>
 946         This is the filter for rewriting tag names. When passed,
 947         it will be called for every tag ref that points to a rewritten
 948         object (or to a tag object which points to a rewritten object).
 949         The original tag name is passed via standard input, and the new
 950         tag name is expected on standard output.
 951 </p>
 952 <div class="paragraph"><p>The original tags are not deleted, but can be overwritten;
 953 use "--tag-name-filter cat" to simply update the tags.  In this
 954 case, be very careful and make sure you have the old tags
 955 backed up in case the conversion has run afoul.</p></div>
 956 <div class="paragraph"><p>Nearly proper rewriting of tag objects is supported. If the tag has
 957 a message attached, a new tag object will be created with the same message,
 958 author, and timestamp. If the tag has a signature attached, the
 959 signature will be stripped. It is by definition impossible to preserve
 960 signatures. The reason this is "nearly" proper, is because ideally if
 961 the tag did not change (points to the same object, has the same name, etc.)
 962 it should retain any signature. That is not the case, signatures will always
 963 be removed, buyer beware. There is also no support for changing the
 964 author or timestamp (or the tag message for that matter). Tags which point
 965 to other tags will be rewritten to point to the underlying commit.</p></div>
 966 </dd>
 967 <dt class="hdlist1">
 968 --prune-empty
 969 </dt>
 970 <dd>
 971 <p>
 972         Some filters will generate empty commits that leave the tree untouched.
 973         This option instructs git-filter-branch to remove such commits if they
 974         have exactly one or zero non-pruned parents; merge commits will
 975         therefore remain intact.  This option cannot be used together with
 976         <code>--commit-filter</code>, though the same effect can be achieved by using the
 977         provided <code>git_commit_non_empty_tree</code> function in a commit filter.
 978 </p>
 979 </dd>
 980 <dt class="hdlist1">
 981 --original &lt;namespace&gt;
 982 </dt>
 983 <dd>
 984 <p>
 985         Use this option to set the namespace where the original commits
 986         will be stored. The default value is <em>refs/original</em>.
 987 </p>
 988 </dd>
 989 <dt class="hdlist1">
 990 -d &lt;directory&gt;
 991 </dt>
 992 <dd>
 993 <p>
 994         Use this option to set the path to the temporary directory used for
 995         rewriting.  When applying a tree filter, the command needs to
 996         temporarily check out the tree to some directory, which may consume
 997         considerable space in case of large projects.  By default it
 998         does this in the <code>.git-rewrite/</code> directory but you can override
 999         that choice by this parameter.
1000 </p>
1001 </dd>
1002 <dt class="hdlist1">
1003 -f
1004 </dt>
1005 <dt class="hdlist1">
1006 --force
1007 </dt>
1008 <dd>
1009 <p>
1010         <em>git filter-branch</em> refuses to start with an existing temporary
1011         directory or when there are already refs starting with
1012         <em>refs/original/</em>, unless forced.
1013 </p>
1014 </dd>
1015 <dt class="hdlist1">
1016 --state-branch &lt;branch&gt;
1017 </dt>
1018 <dd>
1019 <p>
1020         This option will cause the mapping from old to new objects to
1021         be loaded from named branch upon startup and saved as a new
1022         commit to that branch upon exit, enabling incremental of large
1023         trees. If <em>&lt;branch&gt;</em> does not exist it will be created.
1024 </p>
1025 </dd>
1026 <dt class="hdlist1">
1027 &lt;rev-list options&gt;&#8230;
1028 </dt>
1029 <dd>
1030 <p>
1031         Arguments for <em>git rev-list</em>.  All positive refs included by
1032         these options are rewritten.  You may also specify options
1033         such as <code>--all</code>, but you must use <code>--</code> to separate them from
1034         the <em>git filter-branch</em> options. Implies <a href="#Remap_to_ancestor">[Remap_to_ancestor]</a>.
1035 </p>
1036 </dd>
1037 </dl></div>
1038 <div class="sect2">
1039 <h3 id="Remap_to_ancestor">Remap to ancestor</h3>
1040 <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
1041 set of revisions which get rewritten. However, positive refs on the command
1042 line are distinguished: we don&#8217;t let them be excluded by such limiters. For
1043 this purpose, they are instead rewritten to point at the nearest ancestor that
1044 was not excluded.</p></div>
1045 </div>
1046 </div>
1047 </div>
1048 <div class="sect1">
1049 <h2 id="_exit_status">EXIT STATUS</h2>
1050 <div class="sectionbody">
1051 <div class="paragraph"><p>On success, the exit status is <code>0</code>.  If the filter can&#8217;t find any commits to
1052 rewrite, the exit status is <code>2</code>.  On any other error, the exit status may be
1053 any other non-zero value.</p></div>
1054 </div>
1055 </div>
1056 <div class="sect1">
1057 <h2 id="_examples">EXAMPLES</h2>
1058 <div class="sectionbody">
1059 <div class="paragraph"><p>Suppose you want to remove a file (containing confidential information
1060 or copyright violation) from all commits:</p></div>
1061 <div class="listingblock">
1062 <div class="content">
1063 <pre><code>git filter-branch --tree-filter 'rm filename' HEAD</code></pre>
1064 </div></div>
1065 <div class="paragraph"><p>However, if the file is absent from the tree of some commit,
1066 a simple <code>rm filename</code> will fail for that tree and commit.
1067 Thus you may instead want to use <code>rm -f filename</code> as the script.</p></div>
1068 <div class="paragraph"><p>Using <code>--index-filter</code> with <em>git rm</em> yields a significantly faster
1069 version.  Like with using <code>rm filename</code>, <code>git rm --cached filename</code>
1070 will fail if the file is absent from the tree of a commit.  If you
1071 want to "completely forget" a file, it does not matter when it entered
1072 history, so we also add <code>--ignore-unmatch</code>:</p></div>
1073 <div class="listingblock">
1074 <div class="content">
1075 <pre><code>git filter-branch --index-filter 'git rm --cached --ignore-unmatch filename' HEAD</code></pre>
1076 </div></div>
1077 <div class="paragraph"><p>Now, you will get the rewritten history saved in HEAD.</p></div>
1078 <div class="paragraph"><p>To rewrite the repository to look as if <code>foodir/</code> had been its project
1079 root, and discard all other history:</p></div>
1080 <div class="listingblock">
1081 <div class="content">
1082 <pre><code>git filter-branch --subdirectory-filter foodir -- --all</code></pre>
1083 </div></div>
1084 <div class="paragraph"><p>Thus you can, e.g., turn a library subdirectory into a repository of
1085 its own.  Note the <code>--</code> that separates <em>filter-branch</em> options from
1086 revision options, and the <code>--all</code> to rewrite all branches and tags.</p></div>
1087 <div class="paragraph"><p>To set a commit (which typically is at the tip of another
1088 history) to be the parent of the current initial commit, in
1089 order to paste the other history behind the current history:</p></div>
1090 <div class="listingblock">
1091 <div class="content">
1092 <pre><code>git filter-branch --parent-filter 'sed "s/^\$/-p &lt;graft-id&gt;/"' HEAD</code></pre>
1093 </div></div>
1094 <div class="paragraph"><p>(if the parent string is empty - which happens when we are dealing with
1095 the initial commit - add graftcommit as a parent).  Note that this assumes
1096 history with a single root (that is, no merge without common ancestors
1097 happened).  If this is not the case, use:</p></div>
1098 <div class="listingblock">
1099 <div class="content">
1100 <pre><code>git filter-branch --parent-filter \
1101         'test $GIT_COMMIT = &lt;commit-id&gt; &amp;&amp; echo "-p &lt;graft-id&gt;" || cat' HEAD</code></pre>
1102 </div></div>
1103 <div class="paragraph"><p>or even simpler:</p></div>
1104 <div class="listingblock">
1105 <div class="content">
1106 <pre><code>git replace --graft $commit-id $graft-id
1107 git filter-branch $graft-id..HEAD</code></pre>
1108 </div></div>
1109 <div class="paragraph"><p>To remove commits authored by "Darl McBribe" from the history:</p></div>
1110 <div class="listingblock">
1111 <div class="content">
1112 <pre><code>git filter-branch --commit-filter '
1113         if [ "$GIT_AUTHOR_NAME" = "Darl McBribe" ];
1114         then
1115                 skip_commit "$@";
1116         else
1117                 git commit-tree "$@";
1118         fi' HEAD</code></pre>
1119 </div></div>
1120 <div class="paragraph"><p>The function <em>skip_commit</em> is defined as follows:</p></div>
1121 <div class="listingblock">
1122 <div class="content">
1123 <pre><code>skip_commit()
1124 {
1125         shift;
1126         while [ -n "$1" ];
1127         do
1128                 shift;
1129                 map "$1";
1130                 shift;
1131         done;
1132 }</code></pre>
1133 </div></div>
1134 <div class="paragraph"><p>The shift magic first throws away the tree id and then the -p
1135 parameters.  Note that this handles merges properly! In case Darl
1136 committed a merge between P1 and P2, it will be propagated properly
1137 and all children of the merge will become merge commits with P1,P2
1138 as their parents instead of the merge commit.</p></div>
1139 <div class="paragraph"><p><strong>NOTE</strong> the changes introduced by the commits, and which are not reverted
1140 by subsequent commits, will still be in the rewritten branch. If you want
1141 to throw out <em>changes</em> together with the commits, you should use the
1142 interactive mode of <em>git rebase</em>.</p></div>
1143 <div class="paragraph"><p>You can rewrite the commit log messages using <code>--msg-filter</code>.  For
1144 example, <em>git svn-id</em> strings in a repository created by <em>git svn</em> can
1145 be removed this way:</p></div>
1146 <div class="listingblock">
1147 <div class="content">
1148 <pre><code>git filter-branch --msg-filter '
1149         sed -e "/^git-svn-id:/d"
1150 '</code></pre>
1151 </div></div>
1152 <div class="paragraph"><p>If you need to add <em>Acked-by</em> lines to, say, the last 10 commits (none
1153 of which is a merge), use this command:</p></div>
1154 <div class="listingblock">
1155 <div class="content">
1156 <pre><code>git filter-branch --msg-filter '
1157         cat &amp;&amp;
1158         echo "Acked-by: Bugs Bunny &lt;bunny@bugzilla.org&gt;"
1159 ' HEAD~10..HEAD</code></pre>
1160 </div></div>
1161 <div class="paragraph"><p>The <code>--env-filter</code> option can be used to modify committer and/or author
1162 identity.  For example, if you found out that your commits have the wrong
1163 identity due to a misconfigured user.email, you can make a correction,
1164 before publishing the project, like this:</p></div>
1165 <div class="listingblock">
1166 <div class="content">
1167 <pre><code>git filter-branch --env-filter '
1168         if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
1169         then
1170                 GIT_AUTHOR_EMAIL=john@example.com
1171         fi
1172         if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
1173         then
1174                 GIT_COMMITTER_EMAIL=john@example.com
1175         fi
1176 ' -- --all</code></pre>
1177 </div></div>
1178 <div class="paragraph"><p>To restrict rewriting to only part of the history, specify a revision
1179 range in addition to the new branch name.  The new branch name will
1180 point to the top-most revision that a <em>git rev-list</em> of this range
1181 will print.</p></div>
1182 <div class="paragraph"><p>Consider this history:</p></div>
1183 <div class="listingblock">
1184 <div class="content">
1185 <pre><code>     D--E--F--G--H
1186     /     /
1187 A--B-----C</code></pre>
1188 </div></div>
1189 <div class="paragraph"><p>To rewrite only commits D,E,F,G,H, but leave A, B and C alone, use:</p></div>
1190 <div class="listingblock">
1191 <div class="content">
1192 <pre><code>git filter-branch ... C..H</code></pre>
1193 </div></div>
1194 <div class="paragraph"><p>To rewrite commits E,F,G,H, use one of these:</p></div>
1195 <div class="listingblock">
1196 <div class="content">
1197 <pre><code>git filter-branch ... C..H --not D
1198 git filter-branch ... D..H --not C</code></pre>
1199 </div></div>
1200 <div class="paragraph"><p>To move the whole tree into a subdirectory, or remove it from there:</p></div>
1201 <div class="listingblock">
1202 <div class="content">
1203 <pre><code>git filter-branch --index-filter \
1204         'git ls-files -s | sed "s-\t\"*-&amp;newsubdir/-" |
1205                 GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
1206                         git update-index --index-info &amp;&amp;
1207          mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD</code></pre>
1208 </div></div>
1209 </div>
1210 </div>
1211 <div class="sect1">
1212 <h2 id="_checklist_for_shrinking_a_repository">CHECKLIST FOR SHRINKING A REPOSITORY</h2>
1213 <div class="sectionbody">
1214 <div class="paragraph"><p>git-filter-branch can be used to get rid of a subset of files,
1215 usually with some combination of <code>--index-filter</code> and
1216 <code>--subdirectory-filter</code>.  People expect the resulting repository to
1217 be smaller than the original, but you need a few more steps to
1218 actually make it smaller, because Git tries hard not to lose your
1219 objects until you tell it to.  First make sure that:</p></div>
1220 <div class="ulist"><ul>
1221 <li>
1222 <p>
1223 You really removed all variants of a filename, if a blob was moved
1224   over its lifetime.  <code>git log --name-only --follow --all -- filename</code>
1225   can help you find renames.
1226 </p>
1227 </li>
1228 <li>
1229 <p>
1230 You really filtered all refs: use <code>--tag-name-filter cat -- --all</code>
1231   when calling git-filter-branch.
1232 </p>
1233 </li>
1234 </ul></div>
1235 <div class="paragraph"><p>Then there are two ways to get a smaller repository.  A safer way is
1236 to clone, that keeps your original intact.</p></div>
1237 <div class="ulist"><ul>
1238 <li>
1239 <p>
1240 Clone it with <code>git clone file:///path/to/repo</code>.  The clone
1241   will not have the removed objects.  See <a href="git-clone.html">git-clone(1)</a>.  (Note
1242   that cloning with a plain path just hardlinks everything!)
1243 </p>
1244 </li>
1245 </ul></div>
1246 <div class="paragraph"><p>If you really don&#8217;t want to clone it, for whatever reasons, check the
1247 following points instead (in this order).  This is a very destructive
1248 approach, so <strong>make a backup</strong> or go back to cloning it.  You have been
1249 warned.</p></div>
1250 <div class="ulist"><ul>
1251 <li>
1252 <p>
1253 Remove the original refs backed up by git-filter-branch: say <code>git
1254   for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git
1255   update-ref -d</code>.
1256 </p>
1257 </li>
1258 <li>
1259 <p>
1260 Expire all reflogs with <code>git reflog expire --expire=now --all</code>.
1261 </p>
1262 </li>
1263 <li>
1264 <p>
1265 Garbage collect all unreferenced objects with <code>git gc --prune=now</code>
1266   (or if your git-gc is not new enough to support arguments to
1267   <code>--prune</code>, use <code>git repack -ad; git prune</code> instead).
1268 </p>
1269 </li>
1270 </ul></div>
1271 </div>
1272 </div>
1273 <div class="sect1">
1274 <h2 id="PERFORMANCE">PERFORMANCE</h2>
1275 <div class="sectionbody">
1276 <div class="paragraph"><p>The performance of git-filter-branch is glacially slow; its design makes it
1277 impossible for a backward-compatible implementation to ever be fast:</p></div>
1278 <div class="ulist"><ul>
1279 <li>
1280 <p>
1281 In editing files, git-filter-branch by design checks out each and
1282   every commit as it existed in the original repo.  If your repo has
1283   <code>10^5</code> files and <code>10^5</code> commits, but each commit only modifies five
1284   files, then git-filter-branch will make you do <code>10^10</code> modifications,
1285   despite only having (at most) <code>5*10^5</code> unique blobs.
1286 </p>
1287 </li>
1288 <li>
1289 <p>
1290 If you try and cheat and try to make git-filter-branch only work on
1291   files modified in a commit, then two things happen
1292 </p>
1293 <div class="ulist"><ul>
1294 <li>
1295 <p>
1296 you run into problems with deletions whenever the user is simply
1297      trying to rename files (because attempting to delete files that
1298      don&#8217;t exist looks like a no-op; it takes some chicanery to remap
1299      deletes across file renames when the renames happen via arbitrary
1300      user-provided shell)
1301 </p>
1302 </li>
1303 <li>
1304 <p>
1305 even if you succeed at the map-deletes-for-renames chicanery, you
1306      still technically violate backward compatibility because users
1307      are allowed to filter files in ways that depend upon topology of
1308      commits instead of filtering solely based on file contents or
1309      names (though this has not been observed in the wild).
1310 </p>
1311 </li>
1312 </ul></div>
1313 </li>
1314 <li>
1315 <p>
1316 Even if you don&#8217;t need to edit files but only want to e.g. rename or
1317   remove some and thus can avoid checking out each file (i.e. you can
1318   use --index-filter), you still are passing shell snippets for your
1319   filters.  This means that for every commit, you have to have a
1320   prepared git repo where those filters can be run.  That&#8217;s a
1321   significant setup.
1322 </p>
1323 </li>
1324 <li>
1325 <p>
1326 Further, several additional files are created or updated per commit
1327   by git-filter-branch.  Some of these are for supporting the
1328   convenience functions provided by git-filter-branch (such as map()),
1329   while others are for keeping track of internal state (but could have
1330   also been accessed by user filters; one of git-filter-branch&#8217;s
1331   regression tests does so).  This essentially amounts to using the
1332   filesystem as an IPC mechanism between git-filter-branch and the
1333   user-provided filters.  Disks tend to be a slow IPC mechanism, and
1334   writing these files also effectively represents a forced
1335   synchronization point between separate processes that we hit with
1336   every commit.
1337 </p>
1338 </li>
1339 <li>
1340 <p>
1341 The user-provided shell commands will likely involve a pipeline of
1342   commands, resulting in the creation of many processes per commit.
1343   Creating and running another process takes a widely varying amount
1344   of time between operating systems, but on any platform it is very
1345   slow relative to invoking a function.
1346 </p>
1347 </li>
1348 <li>
1349 <p>
1350 git-filter-branch itself is written in shell, which is kind of slow.
1351   This is the one performance issue that could be backward-compatibly
1352   fixed, but compared to the above problems that are intrinsic to the
1353   design of git-filter-branch, the language of the tool itself is a
1354   relatively minor issue.
1355 </p>
1356 <div class="ulist"><ul>
1357 <li>
1358 <p>
1359 Side note: Unfortunately, people tend to fixate on the
1360      written-in-shell aspect and periodically ask if git-filter-branch
1361      could be rewritten in another language to fix the performance
1362      issues.  Not only does that ignore the bigger intrinsic problems
1363      with the design, it&#8217;d help less than you&#8217;d expect: if
1364      git-filter-branch itself were not shell, then the convenience
1365      functions (map(), skip_commit(), etc) and the <code>--setup</code> argument
1366      could no longer be executed once at the beginning of the program
1367      but would instead need to be prepended to every user filter (and
1368      thus re-executed with every commit).
1369 </p>
1370 </li>
1371 </ul></div>
1372 </li>
1373 </ul></div>
1374 <div class="paragraph"><p>The <a href="https://github.com/newren/git-filter-repo/">git filter-repo</a> tool is
1375 an alternative to git-filter-branch which does not suffer from these
1376 performance problems or the safety problems (mentioned below). For those
1377 with existing tooling which relies upon git-filter-branch, <em>git
1378 filter-repo</em> also provides
1379 <a href="https://github.com/newren/git-filter-repo/blob/master/contrib/filter-repo-demos/filter-lamely">filter-lamely</a>,
1380 a drop-in git-filter-branch replacement (with a few caveats).  While
1381 filter-lamely suffers from all the same safety issues as
1382 git-filter-branch, it at least ameliorates the performance issues a
1383 little.</p></div>
1384 </div>
1385 </div>
1386 <div class="sect1">
1387 <h2 id="SAFETY">SAFETY</h2>
1388 <div class="sectionbody">
1389 <div class="paragraph"><p>git-filter-branch is riddled with gotchas resulting in various ways to
1390 easily corrupt repos or end up with a mess worse than what you started
1391 with:</p></div>
1392 <div class="ulist"><ul>
1393 <li>
1394 <p>
1395 Someone can have a set of "working and tested filters" which they
1396   document or provide to a coworker, who then runs them on a different
1397   OS where the same commands are not working/tested (some examples in
1398   the git-filter-branch manpage are also affected by this).
1399   BSD vs. GNU userland differences can really bite.  If lucky, error
1400   messages are spewed.  But just as likely, the commands either don&#8217;t
1401   do the filtering requested, or silently corrupt by making some
1402   unwanted change.  The unwanted change may only affect a few commits,
1403   so it&#8217;s not necessarily obvious either.  (The fact that problems
1404   won&#8217;t necessarily be obvious means they are likely to go unnoticed
1405   until the rewritten history is in use for quite a while, at which
1406   point it&#8217;s really hard to justify another flag-day for another
1407   rewrite.)
1408 </p>
1409 </li>
1410 <li>
1411 <p>
1412 Filenames with spaces are often mishandled by shell snippets since
1413   they cause problems for shell pipelines.  Not everyone is familiar
1414   with find -print0, xargs -0, git-ls-files -z, etc.  Even people who
1415   are familiar with these may assume such flags are not relevant
1416   because someone else renamed any such files in their repo back
1417   before the person doing the filtering joined the project.  And
1418   often, even those familiar with handling arguments with spaces may
1419   not do so just because they aren&#8217;t in the mindset of thinking about
1420   everything that could possibly go wrong.
1421 </p>
1422 </li>
1423 <li>
1424 <p>
1425 Non-ascii filenames can be silently removed despite being in a
1426   desired directory.  Keeping only wanted paths is often done using
1427   pipelines like <code>git ls-files | grep -v ^WANTED_DIR/ | xargs git rm</code>.
1428   ls-files will only quote filenames if needed, so folks may not
1429   notice that one of the files didn&#8217;t match the regex (at least not
1430   until it&#8217;s much too late).  Yes, someone who knows about
1431   core.quotePath can avoid this (unless they have other special
1432   characters like \t, \n, or "), and people who use ls-files -z with
1433   something other than grep can avoid this, but that doesn&#8217;t mean they
1434   will.
1435 </p>
1436 </li>
1437 <li>
1438 <p>
1439 Similarly, when moving files around, one can find that filenames
1440   with non-ascii or special characters end up in a different
1441   directory, one that includes a double quote character.  (This is
1442   technically the same issue as above with quoting, but perhaps an
1443   interesting different way that it can and has manifested as a
1444   problem.)
1445 </p>
1446 </li>
1447 <li>
1448 <p>
1449 It&#8217;s far too easy to accidentally mix up old and new history.  It&#8217;s
1450   still possible with any tool, but git-filter-branch almost
1451   invites it.  If lucky, the only downside is users getting frustrated
1452   that they don&#8217;t know how to shrink their repo and remove the old
1453   stuff.  If unlucky, they merge old and new history and end up with
1454   multiple "copies" of each commit, some of which have unwanted or
1455   sensitive files and others which don&#8217;t.  This comes about in
1456   multiple different ways:
1457 </p>
1458 <div class="ulist"><ul>
1459 <li>
1460 <p>
1461 the default to only doing a partial history rewrite (<em>--all</em> is not
1462      the default and few examples show it)
1463 </p>
1464 </li>
1465 <li>
1466 <p>
1467 the fact that there&#8217;s no automatic post-run cleanup
1468 </p>
1469 </li>
1470 <li>
1471 <p>
1472 the fact that --tag-name-filter (when used to rename tags) doesn&#8217;t
1473      remove the old tags but just adds new ones with the new name
1474 </p>
1475 </li>
1476 <li>
1477 <p>
1478 the fact that little educational information is provided to inform
1479      users of the ramifications of a rewrite and how to avoid mixing old
1480      and new history.  For example, this man page discusses how users
1481      need to understand that they need to rebase their changes for all
1482      their branches on top of new history (or delete and reclone), but
1483      that&#8217;s only one of multiple concerns to consider.  See the
1484      "DISCUSSION" section of the git filter-repo manual page for more
1485      details.
1486 </p>
1487 </li>
1488 </ul></div>
1489 </li>
1490 <li>
1491 <p>
1492 Annotated tags can be accidentally converted to lightweight tags,
1493   due to either of two issues:
1494 </p>
1495 <div class="ulist"><ul>
1496 <li>
1497 <p>
1498 Someone can do a history rewrite, realize they messed up, restore
1499      from the backups in refs/original/, and then redo their
1500      git-filter-branch command.  (The backup in refs/original/ is not a
1501      real backup; it dereferences tags first.)
1502 </p>
1503 </li>
1504 <li>
1505 <p>
1506 Running git-filter-branch with either --tags or --all in your
1507      &lt;rev-list options&gt;.  In order to retain annotated tags as
1508      annotated, you must use --tag-name-filter (and must not have
1509      restored from refs/original/ in a previously botched rewrite).
1510 </p>
1511 </li>
1512 </ul></div>
1513 </li>
1514 <li>
1515 <p>
1516 Any commit messages that specify an encoding will become corrupted
1517   by the rewrite; git-filter-branch ignores the encoding, takes the
1518   original bytes, and feeds it to commit-tree without telling it the
1519   proper encoding.  (This happens whether or not --msg-filter is
1520   used.)
1521 </p>
1522 </li>
1523 <li>
1524 <p>
1525 Commit messages (even if they are all UTF-8) by default become
1526   corrupted due to not being updated&#8201;&#8212;&#8201;any references to other commit
1527   hashes in commit messages will now refer to no-longer-extant
1528   commits.
1529 </p>
1530 </li>
1531 <li>
1532 <p>
1533 There are no facilities for helping users find what unwanted crud
1534   they should delete, which means they are much more likely to have
1535   incomplete or partial cleanups that sometimes result in confusion
1536   and people wasting time trying to understand.  (For example, folks
1537   tend to just look for big files to delete instead of big directories
1538   or extensions, and once they do so, then sometime later folks using
1539   the new repository who are going through history will notice a build
1540   artifact directory that has some files but not others, or a cache of
1541   dependencies (node_modules or similar) which couldn&#8217;t have ever been
1542   functional since it&#8217;s missing some files.)
1543 </p>
1544 </li>
1545 <li>
1546 <p>
1547 If --prune-empty isn&#8217;t specified, then the filtering process can
1548   create hoards of confusing empty commits
1549 </p>
1550 </li>
1551 <li>
1552 <p>
1553 If --prune-empty is specified, then intentionally placed empty
1554   commits from before the filtering operation are also pruned instead
1555   of just pruning commits that became empty due to filtering rules.
1556 </p>
1557 </li>
1558 <li>
1559 <p>
1560 If --prune-empty is specified, sometimes empty commits are missed
1561   and left around anyway (a somewhat rare bug, but it happens&#8230;)
1562 </p>
1563 </li>
1564 <li>
1565 <p>
1566 A minor issue, but users who have a goal to update all names and
1567   emails in a repository may be led to --env-filter which will only
1568   update authors and committers, missing taggers.
1569 </p>
1570 </li>
1571 <li>
1572 <p>
1573 If the user provides a --tag-name-filter that maps multiple tags to
1574   the same name, no warning or error is provided; git-filter-branch
1575   simply overwrites each tag in some undocumented pre-defined order
1576   resulting in only one tag at the end.  (A git-filter-branch
1577   regression test requires this surprising behavior.)
1578 </p>
1579 </li>
1580 </ul></div>
1581 <div class="paragraph"><p>Also, the poor performance of git-filter-branch often leads to safety
1582 issues:</p></div>
1583 <div class="ulist"><ul>
1584 <li>
1585 <p>
1586 Coming up with the correct shell snippet to do the filtering you
1587   want is sometimes difficult unless you&#8217;re just doing a trivial
1588   modification such as deleting a couple files.  Unfortunately, people
1589   often learn if the snippet is right or wrong by trying it out, but
1590   the rightness or wrongness can vary depending on special
1591   circumstances (spaces in filenames, non-ascii filenames, funny
1592   author names or emails, invalid timezones, presence of grafts or
1593   replace objects, etc.), meaning they may have to wait a long time,
1594   hit an error, then restart.  The performance of git-filter-branch is
1595   so bad that this cycle is painful, reducing the time available to
1596   carefully re-check (to say nothing about what it does to the
1597   patience of the person doing the rewrite even if they do technically
1598   have more time available).  This problem is extra compounded because
1599   errors from broken filters may not be shown for a long time and/or
1600   get lost in a sea of output.  Even worse, broken filters often just
1601   result in silent incorrect rewrites.
1602 </p>
1603 </li>
1604 <li>
1605 <p>
1606 To top it all off, even when users finally find working commands,
1607   they naturally want to share them.  But they may be unaware that
1608   their repo didn&#8217;t have some special cases that someone else&#8217;s does.
1609   So, when someone else with a different repository runs the same
1610   commands, they get hit by the problems above.  Or, the user just
1611   runs commands that really were vetted for special cases, but they
1612   run it on a different OS where it doesn&#8217;t work, as noted above.
1613 </p>
1614 </li>
1615 </ul></div>
1616 </div>
1617 </div>
1618 <div class="sect1">
1619 <h2 id="_git">GIT</h2>
1620 <div class="sectionbody">
1621 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
1622 </div>
1623 </div>
1624 </div>
1625 <div id="footnotes"><hr /></div>
1626 <div id="footer">
1627 <div id="footer-text">
1628 Last updated
1629  2020-11-01 22:43:13 PST
1630 </div>
1631 </div>
1632 </body>
1633 </html>