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author mpm@selenic.com
date Thu, 23 Jun 2005 13:48:44 -0800
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1 Mercurial Frequently Asked Questions
2
3 Section 1: General Usage
4 ------------------------
5
6 Q. I did an 'hg pull' and my working directory is empty!
7
8 There are two parts to Mercurial: the repository and the working
9 directory. 'hg pull' pulls all new changes from a remote repository
10 into the local one but doesn't alter the working directory.
11
12 This keeps you from upsetting your work in progress, which may not be
13 ready to merge with the new changes you've pulled and also allows you
14 to manage merging more easily (see below about best practices).
15
16 To update your working directory, run 'hg update'. If you're sure you
17 want to update your working directory on a pull, you can also use 'hg
18 pull -u'. This will refuse to merge or overwrite local changes.
19
20
21 Q. What is the difference between revision numbers, changeset IDs,
22 and tags?
23
24 Mercurial will generally allow you to refer to a revision in three
25 ways: by revision number, by changeset ID, and by tag.
26
27 A revision number is a simple decimal number that corresponds with the
28 ordering of commits in the local repository. It is important to
29 understand that this ordering can change from machine to machine due
30 to Mercurial's distributed, decentralized architecture.
31
32 This is where changeset IDs come in. A changeset ID is a 160-bit
33 identifier that uniquely describes a changeset and its position in the
34 change history, regardless of which machine it's on. This is
35 represented to the user as a 40 digit hexadecimal number. As that
36 tends to be unwieldy, Mercurial will accept any unambiguous substring
37 of that number when specifying versions. It will also generally print
38 these numbers in "short form", which is the first 12 digits.
39
40 You should always use some form of changeset ID rather than the local
41 revision number when discussing revisions with other Mercurial users
42 as they may have different revision numbering on their system.
43
44 Finally, a tag is an arbitrary string that has been assigned a
45 correspondence to a changeset ID. This lets you refer to revisions
46 symbolically.
47
48
49 Q. What are branches, heads, and the tip?
50
51 The central concept of Mercurial is branching. A 'branch' is simply
52 an independent line of development. In most other version control
53 systems, all users generally commit to the same line of development
54 called 'the trunk' or 'the main branch'. In Mercurial, every developer
55 effectively works on a private branch and there is no internal concept
56 of 'the main branch'.
57
58 Thus Mercurial works hard to make repeated merging between branches
59 easy. Simply run 'hg pull' and 'hg update -m' and commit the result.
60
61 'Heads' are simply the most recent commits on a branch. Technically,
62 they are changesets which have no children. Merging is the process of
63 joining points on two branches into one, usually at their current
64 heads. Use 'hg heads' to find the heads in the current repository.
65
66 The 'tip' is the most recently changed head, and also the highest
67 numbered revision. If you have just made a commit, that commit will be
68 the head. Alternately, if you have just pulled from another
69 repository, the tip of that repository becomes the current tip.
70
71 The 'tip' is the default revision for many commands such as update,
72 and also functions as a special symbolic tag.
73
74
75 Q. How does merging work?
76
77 The merge process is simple. Usually you will want to merge the tip
78 into your working directory. Thus you run 'hg update -m' and Mercurial
79 will incorporate the changes from tip into your local changes.
80
81 The first step of this process is tracing back through the history of
82 changesets and finding the 'common ancestor' of the two versions that
83 are being merged. This is done on a project-wide and a file by file
84 basis.
85
86 For files that have been changed in both projects, a three-way merge
87 is attempted to add the changes made remotely into the changes made
88 locally. If there are conflicts between these changes, the user is
89 prompted to interactively resolve them.
90
91 Mercurial uses a helper tool for this, which is usually found by the
92 hgmerge script. Example tools include tkdiff, kdiff3, and the classic
93 RCS merge.
94
95 After you've completed the merge and you're satisfied that the results
96 are correct, it's a good idea to commit your changes. Mercurial won't
97 allow you to perform another merge until you've done this commit as
98 that would lose important history that will be needed for future
99 merges.
100
101
102 Q. How do tags work in Mercurial?
103
104 Tags work slightly differently in Mercurial than most revision
105 systems. The design attempts to meet the following requirements:
106
107 - be version controlled and mergeable just like any other file
108 - allow signing of tags
109 - allow adding a tag to an already committed changeset
110 - allow changing tags in the future
111
112 Thus Mercurial stores tags as a file in the working dir. This file is
113 called .hgtags and consists of a list of changeset IDs and their
114 corresponding tags. To add a tag to the system, simply add a line to
115 this file and then commit it for it to take effect. The 'hg tag'
116 command will do this for you and 'hg tags' will show the currently
117 effective tags.
118
119 Note that because tags refer to changeset IDs and the changeset ID is
120 effectively the sum of all the contents of the repository for that
121 change, it is impossible in Mercurial to simultaneously commit and add
122 a tag. Thus tagging a revision must be done as a second step.
123
124 Q. How do tags work with multiple heads?
125
126 The tags that are in effect at any given time are the tags specified
127 in each head, with heads closer to the tip taking precedence.
128
129
130 Q. What are some best practices for distributed development with Mercurial?
131
132 First, merge often! This makes merging easier for everyone and you
133 find out about conflicts (which are often rooted in incompatible
134 design decisions) earlier.
135
136 Second, don't hesitate to use multiple trees locally. Mercurial makes
137 this fast and light-weight. Typical usage is to have an incoming tree,
138 an outgoing tree, and a separate tree for each area being worked on.
139
140 The incoming tree is best maintained as a pristine copy of the
141 upstream repository. This works as a cache so that you don't have to
142 pull multiple copies over the network. No need to check files out here
143 as you won't be changing them.
144
145 The outgoing tree contains all the changes you intend for merger into
146 upsteam. Publish this tree with 'hg serve' or hgweb.cgi or use 'hg
147 push' to push it to another publicly availabe repository.
148
149 Then, for each feature you work on, create a new tree. Commit early
150 and commit often, merge with incoming regularly, and once you're
151 satisfied with your feature, pull the changes into your outgoing tree.
152
153
154 Q. How do I import from a repository created in a different SCM?
155
156 Take a look at contrib/convert-repo. This is an extensible
157 framework for converting between repository types.
158
159
160 Q. What about Windows support?
161
162 Patches to support Windows are being actively integrated, a fully
163 working Windows version is probably not far off
164
165
166 Section 2: Technical
167 --------------------
168
169 Q. What limits does Mercurial have?
170
171 Mercurial currently assumes that single files, indices, and manifests
172 can fit in memory for efficiency.
173
174 Offsets in revlogs are currently tracked with 32 bits, so a revlog for
175 a single file can currently not grow beyond 4G.
176
177 There should otherwise be no limits on file name length, file size,
178 file contents, number of files, or number of revisions.
179
180 The network protocol is big-endian.
181
182 File names cannot contain the null character. Committer addresses
183 cannot contain newlines.
184
185 Mercurial is primarily developed for UNIX systems, so some UNIXisms
186 may be present in ports.
187
188
189 Q. How does signing work?
190
191 Take a look at the hgeditor script for an example. The basic idea
192 is to sign the manifest ID inside that changelog entry. The manifest
193 ID is a recursive hash of all of the files in the system and their
194 complete history, and thus signing the manifest hash signs the entire
195 project to that point.
196
197 More precisely: each file hash is an SHA1 hash of the contents of that
198 file and the hashes of its parent revisions. The manifest contains a
199 list of each file in the project along with its current file hash.
200 This manifest is hashed similarly to the file hashes, incorporating
201 the hashes of the parent revisions.
202
203
204 Q. What about hash collisions? What about weaknesses in SHA1?
205
206 The SHA1 hashes are large enough that the odds of accidental hash collision
207 are negligible for projects that could be handled by the human race.
208 The known weaknesses in SHA1 are currently still not practical to
209 attack, and Mercurial will switch to SHA256 hashing before that
210 becomes a realistic concern.
211
212 Collisions with the "short hashes" are not a concern as they're always
213 checked for ambiguity and are still long enough that they're not
214 likely to happen for reasonably-sized projects (< 1M changes).