A useful behavior that Mercurial has is that if you pass the name of a directory to a command, every Mercurial command will treat this as “I want to operate on every file in this directory and its subdirectories”.

$ mkdir b
$ echo b > b/somefile.txt
$ echo c > b/source.cpp
$ mkdir b/d
$ echo d > b/d/test.h
$ hg add b
adding b/d/test.h
adding b/somefile.txt
adding b/source.cpp
$ hg commit -m 'Added all files in subdirectory'

Notice in this example that Mercurial printed the names of the files it added, whereas it didn't do so when we added the file named myfile.txt in the earlier example.

What's going on is that in the former case, we explicitly named the file to add on the command line. The assumption that Mercurial makes in such cases is that we know what we are doing, and it doesn't print any output.

However, when we imply the names of files by giving the name of a directory, Mercurial takes the extra step of printing the name of each file that it does something with. This makes it more clear what is happening, and reduces the likelihood of a silent and nasty surprise. This behavior is common to most Mercurial commands.

Mercurial does not track directory information. Instead, it tracks the path to a file. Before creating a file, it first creates any missing directory components of the path. After it deletes a file, it then deletes any empty directories that were in the deleted file's path. This sounds like a trivial distinction, but it has one minor practical consequence: it is not possible to represent a completely empty directory in Mercurial.

Empty directories are rarely useful, and there are unintrusive workarounds that you can use to achieve an appropriate effect. The developers of Mercurial thus felt that the complexity that would be required to manage empty directories was not worth the limited benefit this feature would bring.

If you need an empty directory in your repository, there are a few ways to achieve this. One is to create a directory, then hg add a “hidden” file to that directory. On Unix-like systems, any file name that begins with a period (“.”) is treated as hidden by most commands and GUI tools. This approach is illustrated below.

$ hg init hidden-example
$ cd hidden-example
$ mkdir empty
$ touch empty/.hidden
$ hg add empty/.hidden
$ hg commit -m 'Manage an empty-looking directory'
$ ls empty
$ cd ..
$ hg clone hidden-example tmp
updating working directory
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ ls tmp
empty
$ ls tmp/empty

Another way to tackle a need for an empty directory is to simply create one in your automated build scripts before they will need it.

It is important to understand that removing a file has only two effects.

Removing a file does not in any way alter the history of the file.

If you update the working directory to a changeset that was committed when it was still tracking a file that you later removed, the file will reappear in the working directory, with the contents it had when you committed that changeset. If you then update the working directory to a later changeset, in which the file had been removed, Mercurial will once again remove the file from the working directory.

Mercurial considers a file that you have deleted, but not used hg remove to delete, to be missing. A missing file is represented with “!” in the output of hg status. Mercurial commands will not generally do anything with missing files.

$ hg init missing-example
$ cd missing-example
$ echo a > a
$ hg add a
$ hg commit -m 'File about to be missing'
$ rm a
$ hg status
! a

If your repository contains a file that hg status reports as missing, and you want the file to stay gone, you can run hg remove --after at any time later on, to tell Mercurial that you really did mean to remove the file.

$ hg remove --after a
$ hg status
R a

On the other hand, if you deleted the missing file by accident, give hg revert the name of the file to recover. It will reappear, in unmodified form.

$ hg revert a
$ cat a
a
$ hg status

You might wonder why Mercurial requires you to explicitly tell it that you are deleting a file. Early during the development of Mercurial, it let you delete a file however you pleased; Mercurial would notice the absence of the file automatically when you next ran a hg commit, and stop tracking the file. In practice, this made it too easy to accidentally remove a file without noticing.

Mercurial offers a combination command, hg addremove, that adds untracked files and marks missing files as removed.

$ hg init addremove-example
$ cd addremove-example
$ echo a > a
$ echo b > b
$ hg addremove
adding a
adding b

The hg commit command also provides a -A option that performs this same add-and-remove, immediately followed by a commit.

$ echo c > c
$ hg commit -A -m 'Commit with addremove'
adding c

What happens during a merge is that changes “follow” a copy. To best illustrate what this means, let's create an example. We'll start with the usual tiny repository that contains a single file.

$ hg init my-copy
$ cd my-copy
$ echo line > file
$ hg add file
$ hg commit -m 'Added a file'

We need to do some work in parallel, so that we'll have something to merge. So let's clone our repository.

$ cd ..
$ hg clone my-copy your-copy
updating working directory
1 files updated, 0 files merged, 0 files removed, 0 files unresolved

Back in our initial repository, let's use the hg copy command to make a copy of the first file we created.

$ cd my-copy
$ hg copy file new-file

If we look at the output of the hg status command afterwards, the copied file looks just like a normal added file.

$ hg status
A new-file

But if we pass the -C option to hg status, it prints another line of output: this is the file that our newly-added file was copied from.

$ hg status -C
A new-file
  file
$ hg commit -m 'Copied file'

Now, back in the repository we cloned, let's make a change in parallel. We'll add a line of content to the original file that we created.

$ cd ../your-copy
$ echo 'new contents' >> file
$ hg commit -m 'Changed file'

Now we have a modified file in this repository. When we pull the changes from the first repository, and merge the two heads, Mercurial will propagate the changes that we made locally to file into its copy, new-file.

$ hg pull ../my-copy
pulling from ../my-copy
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg merge
merging file and new-file to new-file
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ cat new-file
line
new contents

This behavior—of changes to a file propagating out to copies of the file—might seem esoteric, but in most cases it's highly desirable.

First of all, remember that this propagation only happens when you merge. So if you hg copy a file, and subsequently modify the original file during the normal course of your work, nothing will happen.

The second thing to know is that modifications will only propagate across a copy as long as the changeset that you're merging changes from hasn't yet seen the copy.

The reason that Mercurial does this is as follows. Let's say I make an important bug fix in a source file, and commit my changes. Meanwhile, you've decided to hg copy the file in your repository, without knowing about the bug or having seen the fix, and you have started hacking on your copy of the file.

If you pulled and merged my changes, and Mercurial didn't propagate changes across copies, your new source file would now contain the bug, and unless you knew to propagate the bug fix by hand, the bug would remain in your copy of the file.

By automatically propagating the change that fixed the bug from the original file to the copy, Mercurial prevents this class of problem. To my knowledge, Mercurial is the only revision control system that propagates changes across copies like this.

Once your change history has a record that the copy and subsequent merge occurred, there's usually no further need to propagate changes from the original file to the copied file, and that's why Mercurial only propagates changes across copies at the first merge, and not afterwards.

If, for some reason, you decide that this business of automatically propagating changes across copies is not for you, simply use your system's normal file copy command (on Unix-like systems, that's cp) to make a copy of a file, then hg add the new copy by hand. Before you do so, though, please do reread the section called “Why should changes follow copies?”, and make an informed decision that this behavior is not appropriate to your specific case.

When you use the hg copy command, Mercurial makes a copy of each source file as it currently stands in the working directory. This means that if you make some modifications to a file, then hg copy it without first having committed those changes, the new copy will also contain the modifications you have made up until that point. (I find this behavior a little counterintuitive, which is why I mention it here.)

The hg copy command acts similarly to the Unix cp command (you can use the hg cp alias if you prefer). We must supply two or more arguments, of which the last is treated as the destination, and all others are sources.

If you pass hg copy a single file as the source, and the destination does not exist, it creates a new file with that name.

$ mkdir k
$ hg copy a k
$ ls k
a

If the destination is a directory, Mercurial copies its sources into that directory.

$ mkdir d
$ hg copy a b d
$ ls d
a  b

Copying a directory is recursive, and preserves the directory structure of the source.

$ hg copy z e
copying z/a/c to e/a/c

If the source and destination are both directories, the source tree is recreated in the destination directory.

$ hg copy z d
copying z/a/c to d/z/a/c

As with the hg remove command, if you copy a file manually and then want Mercurial to know that you've copied the file, simply use the --after option to hg copy.

$ cp a n
$ hg copy --after a n

Since Mercurial's rename is implemented as copy-and-remove, the same propagation of changes happens when you merge after a rename as after a copy.

If I modify a file, and you rename it to a new name, and then we merge our respective changes, my modifications to the file under its original name will be propagated into the file under its new name. (This is something you might expect to “simply work,” but not all revision control systems actually do this.)

Whereas having changes follow a copy is a feature where you can perhaps nod and say “yes, that might be useful,” it should be clear that having them follow a rename is definitely important. Without this facility, it would simply be too easy for changes to become orphaned when files are renamed.

The case of diverging names occurs when two developers start with a file—let's call it foo—in their respective repositories.

$ hg clone orig anne
updating working directory
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg clone orig bob
updating working directory
1 files updated, 0 files merged, 0 files removed, 0 files unresolved

Anne renames the file to bar.

$ cd anne
$ hg rename foo bar
$ hg ci -m 'Rename foo to bar'

Meanwhile, Bob renames it to quux. (Remember that hg mv is an alias for hg rename.)

$ cd ../bob
$ hg mv foo quux
$ hg ci -m 'Rename foo to quux'

I like to think of this as a conflict because each developer has expressed different intentions about what the file ought to be named.

What do you think should happen when they merge their work? Mercurial's actual behavior is that it always preserves both names when it merges changesets that contain divergent renames.

# See http://www.selenic.com/mercurial/bts/issue455
$ cd ../orig
$ hg pull -u ../anne
pulling from ../anne
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg pull ../bob
pulling from ../bob
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg merge
warning: detected divergent renames of foo to:
 bar
 quux
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ ls
bar  quux

Notice that while Mercurial warns about the divergent renames, it leaves it up to you to do something about the divergence after the merge.

Another kind of rename conflict occurs when two people choose to rename different source files to the same destination. In this case, Mercurial runs its normal merge machinery, and lets you guide it to a suitable resolution.

Mercurial has a longstanding bug in which it fails to handle a merge where one side has a file with a given name, while another has a directory with the same name. This is documented as issue 29.

$ hg init issue29
$ cd issue29
$ echo a > a
$ hg ci -Ama
adding a
$ echo b > b
$ hg ci -Amb
adding b
$ hg up 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ mkdir b
$ echo b > b/b
$ hg ci -Amc
adding b/b
created new head
$ hg merge
abort: Is a directory: /tmp/issue29ozeAHx/issue29/b

When a merge occurs, most files will usually remain unmodified. For each file where Mercurial has to do something, it tracks the state of the file.

If Mercurial sees any file in the unresolved state after a merge, it considers the merge to have failed. Fortunately, we do not need to restart the entire merge from scratch.

The --list or -l option to hg resolve prints out the state of each merged file.

$ hg resolve -l
U myfile.txt

In the output from hg resolve, a resolved file is marked with R, while an unresolved file is marked with U. If any files are listed with U, we know that an attempt to commit the results of the merge will fail.

We have several options to move a file from the unresolved into the resolved state. By far the most common is to rerun hg resolve. If we pass the names of individual files or directories, it will retry the merges of any unresolved files present in those locations. We can also pass the --all or -a option, which will retry the merges of all unresolved files.

Mercurial also lets us modify the resolution state of a file directly. We can manually mark a file as resolved using the --mark option, or as unresolved using the --unmark option. This allows us to clean up a particularly messy merge by hand, and to keep track of our progress with each file as we go.