(see also TipsAndTricks)

Mercurial Frequently Asked Questions

TableOfContents


1. General Questions

1.1. What is the license of the project?

The project is available under the GNU General Public License, v2. See COPYING in the release for more details.

2. Common Problems

(the content of this section is included from the subpage ["FAQ/CommonProblems"])

Include(FAQ/CommonProblems)

3. Terminology

3.1. What are revision numbers, changeset IDs, and tags?

Mercurial will generally allow you to refer to a revision in three ways: by revision number, by changeset ID, and by tag.

A revision number is a simple decimal number that corresponds with the ordering of commits in the local repository. It is important to understand that this ordering can change from machine to machine due to Mercurial's distributed, decentralized architecture.

This is where changeset IDs come in. A changeset ID is a 160-bit identifier that uniquely describes a changeset and its position in the change history, regardless of which machine it's on. This is represented to the user as a 40 digit hexadecimal number. As that tends to be unwieldy, Mercurial will accept any unambiguous substring of that number when specifying versions. It will also generally print these numbers in "short form", which is the first 12 digits.

You should always use some form of changeset ID rather than the local revision number when discussing revisions with other Mercurial users as they may have different revision numbering on their system.

Finally, a tag is an arbitrary string that has been assigned a correspondence to a changeset ID. This lets you refer to revisions symbolically.

3.2. What are branches, heads, and the tip?

The central concept of Mercurial is branching. A 'branch' is simply an independent line of development. In most other version control systems, all users generally commit to the same line of development called 'the trunk' or 'the main branch'. In Mercurial, every developer effectively works on a private branch and there is no internal concept of 'the main branch'.

Thus Mercurial works hard to make repeated merging between branches easy. Simply run hg pull, hg merge and commit the result.

'Heads' are simply the most recent commits on a branch. Technically, they are changesets which have no children. Merging is the process of joining points on two branches into one, usually at their current heads. Use "hg heads" to find the heads in the current repository.

The 'tip' is the most recently changed head, and also the highest numbered revision. If you have just made a commit, that commit will be the tip. Alternately, if you have just pulled from another repository, the tip of that repository becomes the current tip.

The 'tip' is the default revision for many commands such as update, and also functions as a special symbolic tag.

4. General Usage

4.1. How does merging work?

The merge process is simple. Usually you will want to merge the tip into your working directory. Thus you run hg merge and Mercurial will incorporate the changes from tip into your local changes.

The first step of this process is tracing back through the history of changesets and finding the 'common ancestor' of the two versions that are being merged. This is done on a project-wide and a file by file basis.

For files that have been changed in both projects, a three-way merge is attempted to add the changes made remotely into the changes made locally. If there are conflicts between these changes, the user is prompted to interactively resolve them.

Mercurial uses a helper tool for this, which is usually found by the hgmerge script. Example tools include tkdiff, kdiff3, and the classic RCS merge.

After you've completed the merge and you're satisfied that the results are correct, it's a good idea to commit your changes. Mercurial won't allow you to perform another merge until you've done this commit as that would lose important history that will be needed for future merges.

4.2. What are some best practices for distributed development with Mercurial?

First, merge often! This makes merging easier for everyone and you find out about conflicts (which are often rooted in incompatible design decisions) earlier.

Second, don't hesitate to use multiple trees locally. Mercurial makes this fast and light-weight. Typical usage is to have an incoming tree, an outgoing tree, and a separate tree for each area being worked on.

The incoming tree is best maintained as a pristine copy of the upstream repository. This works as a cache so that you don't have to pull multiple copies over the network. No need to check files out here as you won't be changing them.

The outgoing tree contains all the changes you intend for merge into upsteam. Publish this tree with hg serve or hgweb.cgi or use hg push to push it to another publicly availabe repository.

Then, for each feature you work on, create a new tree. Commit early and commit often, merge with incoming regularly, and once you're satisfied with your feature, pull the changes into your outgoing tree.

4.3. How do I import from a repository created in a different SCM?

See ConvertingRepositories for various tips.

4.4. What about Windows support?

See WindowsInstall for getting started using Windows.

4.5. Is there a GUI front-end?

See ["GUIClients"] for information on graphical merge tools and other front-ends.

5. Tags

5.1. How do tags work in Mercurial?

Tags work slightly differently in Mercurial than most revision systems. The design attempts to meet the following requirements:

Thus Mercurial stores tags as a file in the working dir. This file is called .hgtags and consists of a list of changeset IDs and their corresponding tags. To add a tag to the system, simply add a line to this file and then commit it for it to take effect. The hg tag command will do this for you and hg tags will show the currently effective tags.

Note that because tags refer to changeset IDs and the changeset ID is effectively the sum of all the contents of the repository for that change, it is impossible in Mercurial to simultaneously commit and add a tag. Thus tagging a revision must be done as a second step.

5.2. What if I want to just keep local tags?

You can use "hg tag" command with an option -l or --local. This will store the tag in the file .hg/localtags, which will not be distributed or versioned. The format of this file is identical to the one of .hgtags and the tags stored there are handled the same.

5.3. How do tags work with multiple heads?

The tags that are in effect at any given time are the tags specified in each head, with heads closer to the tip taking precedence. Local tags override all other tags.

5.4. What if multiple lines with different revisions use the same tag name in .hgtags?

Only the last line where the tag appears is taken into account. The behavior is identical when this happens in .hg/localtags.

6. Bugs and Features

6.1. I found a bug, what do I do?

Report it to the mercurial mailing list, mercurial@selenic.com or in the bug tracker http://www.selenic.com/mercurial/bts/

6.2. What should I include in my bug report?

Enough information to reproduce or diagnose the bug. If you can, try using the hg -v and hg -d switches to figure out exactly what Mercurial is doing.

If you can reproduce the bug in a simple repository, that is very helpful. The best is to create a simple shell script to automate this process, which can then be added to our test suite.

6.3. Can Mercurial do <x>?

If you'd like to request a feature, send your request to mercurial@selenic.com. As Mercurial is still very new, there are certainly features it is missing and you can give us feedback on how best to implement them.

Be sure to see ToDo and MissingFeatures to see what's already planned and where we need help.

7. Web Interface

Find the URL for the file and then replace the changeset identifier with tip.

7.2. How do I change the style of the web interface to the visually more attractive gitweb?

In [http://www.selenic.com/mercurial/hgrc.5.html hgrc] set

[web]
style = gitweb

To switch back to the default style specify "style = default" (see [http://hgbook.red-bean.com/hgbookch6.html#x10-1390006.6.5 hgbook]).

8. Technical Details

8.1. What limits does Mercurial have?

Mercurial currently assumes that single files, indices, and manifests can fit in memory for efficiency.

There should otherwise be no limits on file name length, file size, file contents, number of files, or number of revisions.

The network protocol is big-endian.

File names cannot contain the null character or newlines. Committer addresses cannot contain newlines.

Mercurial is primarily developed for UNIX systems, so some UNIXisms may be present in ports.

8.2. How does Mercurial store its data?

The fundamental storage type in Mercurial is a "revlog". A revlog is the set of all revisions of a named object. Each revision is either stored compressed in its entirety or as a compressed binary delta against the previous version. The decision of when to store a full version is made based on how much data would be needed to reconstruct the file. This lets us ensure that we never need to read huge amounts of data to reconstruct a object, regardless of how many revisions of it we store.

In fact, we should always be able to do it with a single read, provided we know when and where to read. This is where the index comes in. Each revlog has an index containing a special hash (nodeid) of the text, hashes for its parents, and where and how much of the revlog data we need to read to reconstruct it. Thus, with one read of the index and one read of the data, we can reconstruct any version in time proportional to the object size.

Similarly, revlogs and their indices are append-only. This means that adding a new version is also O(1) seeks.

Revlogs are used to represent all revisions of files, manifests, and changesets. Compression for typical objects with lots of revisions can range from 100 to 1 for things like project makefiles to over 2000 to 1 for objects like the manifest.

8.3. How does Mercurial handle binary files?

See BinaryFiles.

8.4. What about Windows line endings vs. Unix line endings?

See EncodeDecodeFilter.

8.5. What about keyword replacement (i.e. $Id$)?

See KeywordPlan and KeywordExpansionExtension.

8.6. How are Mercurial diffs and deltas calculated?

Mercurial diffs are calculated rather differently than those generated by the traditional diff algorithm (but with output that's completely compatible with patch of course). The algorithm is an optimized C implementation based on Python's [http://python.org/doc/2.4.1/lib/module-difflib.html difflib], which is intended to generate diffs that are easier for humans to read rather than be 'minimal'. This same algorithm is also used for the internal delta compression.

In the course of investigating delta compression algorithms, we discovered that this implementation was simpler and faster than the competition in our benchmarks and also generated smaller deltas than the theoretically 'minimal' diffs of the traditional diff algorithms. This is because the traditional algorithm assumes the same cost for insertions, deletions, and unchanged elements.

8.7. How are manifests and changesets stored?

A manifest is simply a list of all files in a given revision of a project along with the nodeids of the corresponding file revisions. So grabbing a given version of the project means simply looking up its manifest and reconstructing all the file revisions pointed to by it.

A changeset is a list of all files changed in a check-in along with a change description and some metadata like user and date. It also contains a nodeid to the relevant revision of the manifest.

8.8. How do Mercurial hashes get calculated?

Mercurial hashes both the contents of an object and the hash of its parents to create an identifier that uniquely identifies an object's contents and history. This greatly simplifies merging of histories because it avoid graph cycles that can occur when a object is reverted to an earlier state.

All file revisions have an associated hash value. These are listed in the manifest of a given project revision, and the manifest hash is listed in the changeset. The changeset hash is again a hash of the changeset contents and its parents, so it uniquely identifies the entire history of the project to that point.

8.9. What checks are there on repository integrity?

Every time a revlog object is retrieved, it is checked against its hash for integrity. It is also incidentally doublechecked by the Adler32 checksum used by the underlying zlib compression.

Running 'hg verify' decompresses and reconstitutes each revision of each object in the repository and cross-checks all of the index metadata with those contents.

But this alone is not enough to ensure that someone hasn't tampered with a repository. For that, you need cryptographic signing.

8.10. How does signing work with Mercurial?

Take a look at the ["hgeditor"] script for an example. The basic idea is to use GPG to sign the manifest ID inside that changelog entry. The manifest ID is a recursive hash of all of the files in the system and their complete history, and thus signing the manifest hash signs the entire project contents.

8.11. What about hash collisions? What about weaknesses in SHA1?

The SHA1 hashes are large enough that the odds of accidental hash collision are negligible for projects that could be handled by the human race. The known weaknesses in SHA1 are currently still not practical to attack, and Mercurial will switch to SHA256 hashing before that becomes a realistic concern.

Collisions with the "short hashes" are not a concern as they're always checked for ambiguity and are still long enough that they're not likely to happen for reasonably-sized projects (< 1M changes).

8.12. How does "hg commit" determine which files have changed?

If "hg commit" (see [http://www.selenic.com/mercurial/hg.1.html manual]) is called without file arguments, it commits all files that have "changed". Note however, that Mercurial doesn't detect changes that do not change the file time or size (This is by design. See also [http://www.selenic.com/mercurial/bts/msg3438 msg3438] in [http://www.selenic.com/mercurial/bts/issue618 issue618]). If you are repeatedly calling "hg commit" in a script, you might need to wait until the next second before changing files in the working directory in preparation for the next commit. Otherwise the file(s) might have the exact same modification time as the last commit time and Mercurial will not include them in the next commit (if the file sizes didn't change).

9. Mercurial Book

See ["/MercurialBook"].