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git-filter-repo/README.md
Elijah Newren e1d126a1ea Reference package managers in installation instructions 
Signed-off-by: Elijah Newren <newren@gmail.com>
2019-11-25 10:38:46 -08:00

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git filter-repo is a versatile tool for rewriting history, which includes
[capabilities I have not found anywhere
else](#design-rationale-behind-filter-repo). It roughly falls into the
same space of tool as [git
filter-branch](https://git-scm.com/docs/git-filter-branch) but without the
capitulation-inducing poor
[performance](https://public-inbox.org/git/CABPp-BGOz8nks0+Tdw5GyGqxeYR-3FF6FT5JcgVqZDYVRQ6qog@mail.gmail.com/),
with far more capabilities, and with a design that scales usability-wise
beyond trivial rewriting cases.
While most users will probably just use filter-repo as a simple command
line tool (and likely only use a few of its flags), at its core filter-repo
contains a library for creating history rewriting tools. As such, users
with specialized needs can leverage it to quickly create [entirely new
history rewriting tools](contrib/filter-repo-demos).
# Table of Contents
* [Prerequisites](#prerequisites)
* [How do I install it?](#how-do-i-install-it)
* [How do I use it?](#how-do-i-use-it)
* [Why filter-repo instead of other alternatives?](#why-filter-repo-instead-of-other-alternatives)
* [filter-branch](#filter-branch)
* [BFG Repo Cleaner](#bfg-repo-cleaner)
* [Simple example, with comparisons](#simple-example-with-comparisons)
* [Solving this with filter-repo](#solving-this-with-filter-repo)
* [Solving this with other filtering tools](#solving-this-with-other-filtering-tools)
* [Design rationale behind filter-repo](#design-rationale-behind-filter-repo)
* [How do I contribute?](#how-do-i-contribute)
* [Is there a Code of Conduct?](#is-there-a-code-of-conduct)
* [Upstream Improvements](#upstream-improvements)
# Prerequisites
filter-repo requires:
* a fairly [recent version of git](#upstream-improvements) (git >=
2.22.0 at a minimum, newer versions are needed for some features)
* python3 (python >= 3.5)
# How do I install it?
git-filter-repo is a single-file python script, which was done to make
installation for basic use trivial: just copy it into your $PATH.
See [INSTALL.md](INSTALL.md) for things beyond basic usage or special
cases. The more involved instructions are needed if you
* are working with a python3 executable named something other than "python3"
* want to install documentation (beyond the builtin docs shown with -h)
* want to run some of the [contrib](contrib/filter-repo-demos/) examples
* want to create your own python filtering scripts using filter-repo as a
module/library
# How do I use it?
See the [user
manual](https://htmlpreview.github.io/?https://github.com/newren/git-filter-repo/blob/docs/html/git-filter-repo.html).
If you prefer learning from examples, the [simple
example](#simple-example-with-comparisons) below may be of interest,
and the user manual has an extensive [examples
section](https://htmlpreview.github.io/?https://github.com/newren/git-filter-repo/blob/docs/html/git-filter-repo.html#EXAMPLES).
# Why filter-repo instead of other alternatives?
This was covered in more detail in a [Git Rev News article on
filter-repo](https://git.github.io/rev_news/2019/08/21/edition-54/#an-introduction-to-git-filter-repo--written-by-elijah-newren),
but some highlights for the main competitors:
## filter-branch
* filter-branch is [extremely to unusably
slow](https://public-inbox.org/git/CABPp-BGOz8nks0+Tdw5GyGqxeYR-3FF6FT5JcgVqZDYVRQ6qog@mail.gmail.com/)
([multiple orders of magnitude slower than it should
be](https://htmlpreview.github.io/?https://raw.githubusercontent.com/newren/git-filter-repo/docs/html/git-filter-branch.html#PERFORMANCE))
for non-trivial repositories.
* [filter-branch is riddled with
gotchas](https://htmlpreview.github.io/?https://raw.githubusercontent.com/newren/git-filter-repo/docs/html/git-filter-branch.html#SAFETY)
that can silently corrupt your rewrite or at least thwart your
"cleanup" efforts by giving you something more problematic and messy
than what you started with.
* filter-branch is [very onerous](#simple-example-with-comparisons)
[to
use](https://github.com/newren/git-filter-repo/blob/a6a6a1b0f62d365bbe2e76f823e1621857ec4dbd/contrib/filter-repo-demos/filter-lamely#L9-L61)
for any rewrite which is even slightly non-trivial.
* die-hard fans of filter-branch may be interested in
[filter-lamely](contrib/filter-repo-demos/filter-lamely)
(a.k.a. [filter-branch-ish](contrib/filter-repo-demos/filter-branch-ish)),
a reimplementation of filter-branch based on filter-repo which is
more performant (though not nearly as fast or safe as
filter-repo).
## BFG Repo Cleaner
* great tool for its time, but while it makes some things simple, it
is limited to a few kinds of rewrites.
* its architecture is not amenable to handling more types of
rewrites.
* its architecture presents some shortcomings and bugs even for its
intended usecase.
* fans of bfg may be interested in
[bfg-ish](contrib/filter-repo-demos/bfg-ish), a reimplementation of bfg
based on filter-repo which includes several new features and bugfixes
relative to bfg.
# Simple example, with comparisons
Let's say that we want to extract a piece of a repository, with the intent
on merging just that piece into some other bigger repo. For extraction, we
want to:
* extract the history of a single directory, src/. This means that only
paths under src/ remain in the repo, and any commits that only touched
paths outside this directory will be removed.
* rename all files to have a new leading directory, my-module/ (e.g. so that
src/foo.c becomes my-module/src/foo.c)
* rename any tags in the extracted repository to have a 'my-module-'
prefix (to avoid any conflicts when we later merge this repo into
something else)
## Solving this with filter-repo
Doing this with filter-repo is as simple as the following command:
```shell
git filter-repo --path src/ --to-subdirectory-filter my-module --tag-rename '':'my-module-'
```
(the single quotes are unnecessary, but make it clearer to a human that we
are replacing the empty string as a prefix with `my-module-`)
## Solving this with other filtering tools
By contrast, BFG Repo Cleaner is not capable of this kind of rewrite,
it would take considerable effort to do this safely with
fast-export/fast-import (especially if you wanted empty commits pruned
or commit hashes rewritten), and filter-branch comes with a pile of
caveats (more on that below) even once you figure out the necessary
invocation(s):
```shell
git filter-branch \
--tree-filter 'mkdir -p my-module && \
git ls-files \
| grep -v ^src/ \
| xargs git rm -f -q && \
ls -d * \
| grep -v my-module \
| xargs -I files mv files my-module/' \
--tag-name-filter 'echo "my-module-$(cat)"' \
--prune-empty -- --all
git clone file://$(pwd) newcopy
cd newcopy
git for-each-ref --format="delete %(refname)" refs/tags/ \
| grep -v refs/tags/my-module- \
| git update-ref --stdin
git gc --prune=now
```
Some might notice that the above filter-branch invocation will be really
slow due to using --tree-filter; you could alternatively use the
--index-filter option of filter-branch, changing the above commands to:
```shell
git filter-branch \
--index-filter 'git ls-files \
| grep -v ^src/ \
| xargs git rm -q --cached;
git ls-files -s \
| sed "s%$(printf \\t)%&my-module/%" \
| git update-index --index-info;
git ls-files \
| grep -v ^my-module/ \
| xargs git rm -q --cached' \
--tag-name-filter 'echo "my-module-$(cat)"' \
--prune-empty -- --all
git clone file://$(pwd) newcopy
cd newcopy
git for-each-ref --format="delete %(refname)" refs/tags/ \
| grep -v refs/tags/my-module- \
| git update-ref --stdin
git gc --prune=now
```
However, for either filter-branch command there are a pile of caveats.
First, some may be wondering why I list five commands here for
filter-branch. Despite the use of --all and --tag-name-filter, and
filter-branch's manpage claiming that a clone is enough to get rid of
old objects, the extra steps to delete the other tags and do another
gc are still required to clean out the old objects and avoid mixing
new and old history before pushing somewhere. Other caveats:
* Commit messages are not rewritten; so if some of your commit
messages refer to prior commits by (abbreviated) sha1, after the
rewrite those messages will now refer to commits that are no longer
part of the history. It would be better to rewrite those
(abbreviated) sha1 references to refer to the new commit ids.
* The --prune-empty flag sometimes misses commits that should be
pruned, and it will also prune commits that *started* empty rather
than just ended empty due to filtering. For repositories that
intentionally use empty commits for versioning and publishing
related purposes, this can be detrimental.
* The commands above are OS-specific. GNU vs. BSD issues for sed,
xargs, and other commands often trip up users; I think I failed to
get most folks to use --index-filter since the only example in the
filter-branch manpage that both uses it and shows how to move
everything into a subdirectory is linux-specific, and it is not
obvious to the reader that it has a portability issue since it
silently misbehaves rather than failing loudly.
* The --index-filter version of the filter-branch command may be two to
three times faster than the --tree-filter version, but both
filter-branch commands are going to be multiple orders of magnitude
slower than filter-repo.
* Both commands assume all filenames are composed entirely of ascii
characters (even special ascii characters such as tabs or double
quotes will wreak havoc and likely result in missing files or
misnamed files)
# Design rationale behind filter-repo
None of the existing repository filtering tools did what I wanted;
they all came up short for my needs. No tool provided any of the
first eight traits below I wanted, and all failed to provide at least
one of the last four traits as well:
1. [Starting report] Provide user an analysis of their repo to help
them get started on what to prune or rename, instead of expecting
them to guess or find other tools to figure it out. (Triggered, e.g.
by running the first time with a special flag, such as --analyze.)
1. [Keep vs. remove] Instead of just providing a way for users to
easily remove selected paths, also provide flags for users to
only *keep* certain paths. Sure, users could workaround this by
specifying to remove all paths other than the ones they want to
keep, but the need to specify all paths that *ever* existed in
**any** version of the repository could sometimes be quite
painful. For filter-branch, using pipelines like `git ls-files |
grep -v ... | xargs -r git rm` might be a reasonable workaround
but can get unwieldy and isn't as straightforward for users; plus
those commands are often operating-system specific (can you spot
the GNUism in the snippet I provided?).
1. [Renaming] It should be easy to rename paths. For example, in
addition to allowing one to treat some subdirectory as the root
of the repository, also provide options for users to make the
root of the repository just become a subdirectory. And more
generally allow files and directories to be easily renamed.
Provide sanity checks if renaming causes multiple files to exist
at the same path. (And add special handling so that if a commit
merely copied oldname->newname without modification, then
filtering oldname->newname doesn't trigger the sanity check and
die on that commit.)
1. [More intelligent safety] Writing copies of the original refs to
a special namespace within the repo does not provide a
user-friendly recovery mechanism. Many would struggle to recover
using that. Almost everyone I've ever seen do a repository
filtering operation has done so with a fresh clone, because
wiping out the clone in case of error is a vastly easier recovery
mechanism. Strongly encourage that workflow by detecting and
bailing if we're not in a fresh clone, unless the user overrides
with --force.
1. [Auto shrink] Automatically remove old cruft and repack the
repository for the user after filtering (unless overridden); this
simplifies things for the user, helps avoid mixing old and new
history together, and avoids problems where the multi-step
process for shrinking the repo documented in the manpage doesn't
actually work in some cases. (I'm looking at you,
filter-branch.)
1. [Clean separation] Avoid confusing users (and prevent accidental
re-pushing of old stuff) due to mixing old repo and rewritten
repo together. (This is particularly a problem with filter-branch
when using the --tag-name-filter option, and sometimes also an
issue when only filtering a subset of branches.)
1. [Versatility] Provide the user the ability to extend the tool or
even write new tools that leverage existing capabilities, and
provide this extensibility in a way that (a) avoids the need to
fork separate processes (which would destroy performance), (b)
avoids making the user specify OS-dependent shell commands (which
would prevent users from sharing commands with each other), (c)
takes advantage of rich data structures (because hashes, dicts,
lists, and arrays are prohibitively difficult in shell) and (d)
provides reasonable string manipulation capabilities (which are
sorely lacking in shell).
1. [Old commit references] Provide a way for users to use old commit
IDs with the new repository (in particular via mapping from old to
new hashes with refs/replace/ references).
1. [Commit message consistency] If commit messages refer to other
commits by ID (e.g. "this reverts commit 01234567890abcdef", "In
commit 0013deadbeef9a..."), those commit messages should be
rewritten to refer to the new commit IDs.
1. [Become-empty pruning] Commits which become empty due to filtering
should be pruned. If the parent of a commit is pruned, the first
non-pruned ancestor needs to become the new parent. If no
non-pruned ancestor exists and the commit was not a merge, then it
becomes a new root commit. If no non-pruned ancestor exists and
the commit was a merge, then the merge will have one less parent
(and thus make it likely to become a non-merge commit which would
itself be pruned if it had no file changes of its own). One
special thing to note here is that we prune commits which become
empty, NOT commits which start empty. Some projects intentionally
create empty commits for versioning or publishing reasons, and
these should not be removed. (As a special case, commits which
started empty but whose parent was pruned away will also be
considered to have "become empty".)
1. [Become-degenerate pruning] Pruning of commits which become empty
can potentially cause topology changes, and there are lots of
special cases. Normally, merge commits are not removed since they
are needed to preserve the graph topology, but the pruning of
parents and other ancestors can ultimately result in the loss of
one or more parents. A simple case was already noted above: if a
merge commit loses enough parents to become a non-merge commit and
it has no file changes, then it too can be pruned. Merge commits
can also have a topology that becomes degenerate: it could end up
with the merge_base serving as both parents (if all intervening
commits from the original repo were pruned), or it could end up
with one parent which is an ancestor of its other parent. In such
cases, if the merge has no file changes of its own, then the merge
commit can also be pruned. However, much as we do with empty
pruning we do not prune merge commits that started degenerate
(which indicates it may have been intentional, such as with --no-ff
merges) but only merge commits that become degenerate and have no
file changes of their own.
1. [Speed] Filtering should be reasonably fast
# How do I contribute?
See the [contributing guidelines](Documentation/Contributing.md).
# Is there a Code of Conduct?
Participants in the filter-repo community are expected to adhere to
the same standards as for the git project, so the [git Code of
Conduct](https://git.kernel.org/pub/scm/git/git.git/tree/CODE_OF_CONDUCT.md)
applies.
# Upstream Improvements
Work on filter-repo and [its
predecessor](https://public-inbox.org/git/51419b2c0904072035u1182b507o836a67ac308d32b9@mail.gmail.com/)
has also driven numerous improvements to fast-export and fast-import
(and occasionally other commands) in core git, based on things
filter-repo needs to do its work:
* git-2.24.0
* [fast-export: handle nested tags](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=941790d7de)
* [t9350: add tests for tags of things other than a commit](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=8d7d33c1ce)
* [fast-export: allow user to request tags be marked with --mark-tags](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=a1638cfe12)
* [fast-export: add support for --import-marks-if-exists](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=208d69246e)
* [fast-import: add support for new 'alias' command](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=b8f50e5b60)
* [fast-import: allow tags to be identified by mark labels](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=f73b2aba05)
* [fast-import: fix handling of deleted tags](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=3164e6bd24)
* [fast-export: fix exporting a tag and nothing else](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=af2abd870b)
* [git-fast-import.txt: clarify that multiple merge commits are allowed](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=d1387d3895)
* git-2.23.0
* [t9350: fix encoding test to actually test reencoding](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=32615ce762)
* [fast-import: support 'encoding' commit header](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=3edfcc65fd)
* [fast-export: avoid stripping encoding header if we cannot reencode](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=ccbfc96dc4)
* [fast-export: differentiate between explicitly UTF-8 and implicitly
UTF-8](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=57a8be2cb0)
* [fast-export: do automatic reencoding of commit messages only if
requested](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=e80001f8fd)
* git-2.22.0
* [log,diff-tree: add --combined-all-paths option](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=d76ce4f734)
* [t9300: demonstrate bug with get-mark and empty orphan commits](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=62edbec7de)
* [git-fast-import.txt: fix wording about where ls command can appear](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=a63c54a019)
* [fast-import: check most prominent commands first](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=5056bb7646)
* [fast-import: only allow cat-blob requests where it makes sense](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=7ffde293f2)
* [fast-import: fix erroneous handling of get-mark with empty orphan
commits](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=cf7b857a77)
* [Honor core.precomposeUnicode in more places](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=8e712ef6fc)
* git-2.21.0
* [fast-export: convert sha1 to oid](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=843b9e6d48)
* [git-fast-import.txt: fix documentation for --quiet option](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=f55c979b14)
* [git-fast-export.txt: clarify misleading documentation about rev-list
args](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=4532be7cba)
* [fast-export: use value from correct enum](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=b93b81e799)
* [fast-export: avoid dying when filtering by paths and old tags exist](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=1f30c904b3)
* [fast-export: move commit rewriting logic into a function for reuse](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=f129c4275c)
* [fast-export: when using paths, avoid corrupt stream with non-existent
mark](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=cd13762d8f)
* [fast-export: ensure we export requested refs](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=fdf31b6369)
* [fast-export: add --reference-excluded-parents option](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=530ca19c02)
* [fast-import: remove unmaintained duplicate documentation](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=25dd3e4889)
* [fast-export: add a --show-original-ids option to show
original names](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=a965bb3116)
* [git-show-ref.txt: fix order of flags](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=bd8d6f0def)
* git-2.20.0
* [update-ref: fix type of update_flags variable to
match its usage](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=e4c34855a2)
* [update-ref: allow --no-deref with --stdin](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=d345e9fbe7)
* git-1.7.3
* [fast-export: Fix dropping of files with --import-marks and path
limiting](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=4087a02e45)
* [fast-export: Add a --full-tree option](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=7f40ab0916)
* [fast-export: Fix output order of D/F changes](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=060df62422)
* [fast-import: Improve robustness when D->F changes provided in wrong
order](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=253fb5f889)
* git-1.6.4:
* [fast-export: Set revs.topo_order before calling setup_revisions](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=668f3aa776)
* [fast-export: Omit tags that tag trees](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=02c48cd69b)
* [fast-export: Make sure we show actual ref names instead of "(null)"](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=2374502c6c)
* [fast-export: Do parent rewriting to avoid dropping relevant commits](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=32164131db)
* [fast-export: Add a --tag-of-filtered-object option for newly
dangling tags](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=2d8ad46919)
* [Add new fast-export testcases](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=25e0ca5dd6)
* [fast-export: Document the fact that git-rev-list arguments are
accepted](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=8af15d282e)
* git-1.6.3:
* [git-filter-branch: avoid collisions with variables in eval'ed
commands](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=d5b0c97d13)
* [Correct missing SP characters in grammar comment at top of
fast-import.c](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=98e1a4186a)
* [fast-export: Avoid dropping files from commits](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=ebeec7dbc5)
* git-1.6.1.4:
* [fast-export: ensure we traverse commits in topological order](
https://git.kernel.org/pub/scm/git/git.git/commit/?id=784f8affe4)
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