It is often extremely useful to compare two project trees (usually for the same project) and figure out exactly what has changed between them. A record of such changes is called a changeset or a delta .
Changesets are a very central concept to
-- much of
defined in terms of operations performed with changesets.
If you have a changeset between an "old tree" and a "new tree", you can "apply the changeset" to the old tree to get the new tree -- in other words, you can automatically make the editing changes described by a changeset. If you have some third tree, you can apply the patch to get an approximation of making the same changes to that third tree.
includes sophisticated tools for creating and applying
computes a changeset describing the differences between two
trees. The basic command syntax is:
% tla mkpatch ORIGINAL MODIFIED DESTINATION
which compares the trees
creates a new directory,
, and stores the
compares trees, it uses inventory ids. For example, it
considers two directories or two files to be "the same directory (or
file)" if they have the same id -- regardless of where each is located
in its respective tree. (See Inventory Ids for Source.)
A changeset produced by
describes what files and directories
have been added or removed, which have been renamed, which files have
been changed (and how they have been changed), and what file
permissions have changed (and how). When regular text files are
produces a context diff describing the
can compare binary files (saving complete
copies of the old and new versions if they differ) and symbolic links
(saving the old and new link targets, if they differ).
A detailed description of the format of a changeset is provided in an appendix (see The arch Changeset Format).
is used to apply a changeset to tree:
% tla dopatch PATCH-SET TREE
is exactly the same as the the "original" tree seen by
, then the effect is to modify
so that it is exactly
the same as the the "modified" tree seen by
, with one
exception (explained below).
"Exactly the same" means that the directory structure is the same, symbolic link targets are the same, the contents of regular files are the same, and file permissions are the same. Modification times, files with multiple (hard) links, and file ownership are not reliably preserved.
The exception to the "exactly the same" rule is that if the patch
requires that files or directories be removed from
, those files
and directories will be saved in a subdirectory of
eye-splitting name matching the pattern:
is the name of the patch-set directory and
What if a tree patched by
is not exactly the same as the
original tree seen by
Below is a brief description of what to expect. Complete
documentation of the
process is included with the source
takes an inventory of the tree being patched. It uses
inventory ids to decide which files and directories expected by the
changeset are present or missing from the tree, and to figure out
where each file and directory is located in the tree.
Simple Patches If the changeset contains an ordinary patch or
metadata patch for a link, directory or file, and that file is present
in the tree,
applies the patch in the ordinary way. If the
patch applies cleanly, the modified file, link, or directory is left
If a simple patch fails to apply cleanly,
will always leave
file (the file originally in the tree being patched,
without any changes) and a
file (the part of the patch that
could not be applied).
If the patch was a context diff,
will also leave behind the
file itself -- partially patched.
If an (unsuccessful) patch was for a binary file, no partially-patched file will be left. Instead, there will be:
.orig -- the file originally in the tree being patched, without modifications. .rej -- a complete copy of the file from the modified tree, with permissions copied from `.orig'. .patch-orig -- a complete copy of the file from the original tree seen by `mkpatch', with permissions retained from that original -or- the symbolic link from the original tree seen by `mkpatch' with permissions as in the original tree.
If an (unsuccessful) patch was for a symbolic link, no partially patched file will be left. Instead there will be:
.orig -- the unmodified file from the original tree .rej -- a symbolic link with the target intended by the patch and permissions copied from .orig .patch-orig -- a complete copy of the file from the original tree seen by `mkpatch', with permissions retained from that original -or- the symbolic link from the original tree seen by `mkpatch' with permissions as in the original tree.
Patches for Missing Files
All patches for missing files and directories are stored in a subdirectory of the root of the tree being patched called
is the basename of the changeset directory and
Directory Rearrangements and New Directories
Directories are added, deleted, and rearranged much as you would expect, even if you don't know it's what you'd expect.
Suppose that when
was called the
Directory or file: Id: a/x.c id_1 a/bar.c id_2
a/x.c id_1 a/y.c id_2
with changes to both files. The patch will want to rename the file
, and change the contents of the files with
Suppose, for example, that you have a tree with:
a/foo.c id_1 a/zip.c id_2
and the you apply the patch to that tree. After the patch, you'll be left with:
a/foo.c id_1 a/y.c (was zip.c) id_2
with patches made to the contents of both files.
Here's a sample of some subtleties and ways of handling conflicts:
Suppose that the original tree seen by mkpatch has:
Directory or file: Id: ./a id_a ./a/b id_b ./a/b/c id_c
and that the modified directory has:
./a id_a ./a/c id_c ./a/c/b id_b
Finally, suppose that the tree has:
./x id_a ./x/b id_b ./x/c id_new_directory ./x/c/b id_different_file_named_b ./x/c/q id_c
When patch gets done with the tree, it will have:
./x id_a Since the patch doesn't do anything to change the directory with id_a. ./x/c.orig id_new_directory ./x/c.rej id_c Since the patch wants to make the directory with id_c a subdirectory named "c" of the directory with id_a, but the tree already had a different directory there, with the id id_new_directory. ./x/c.rej/b id_b Since the patch wants to rename the directory with id_b to be a subdirectory named "b" of the directory with id_c. ./x/c.orig/b id_different_file_named_b Since the patch made new changes to this file, it stayed with its parent directory.arch Meets hello-world: A Tutorial Introduction to The arch Revision Control System