Mercurial provides mechanisms that let you work with file names in a consistent and expressive way.
Mercurial uses a unified piece of machinery “under the hood” to handle file names. Every command behaves uniformly with respect to file names. The way in which commands work with file names is as follows.
If you explicitly name real files on the command line, Mercurial works with exactly those files, as you would expect.
When you provide a directory name, Mercurial will interpret this as “operate on every file in this directory and its subdirectories”. Mercurial traverses the files and subdirectories in a directory in alphabetical order. When it encounters a subdirectory, it will traverse that subdirectory before continuing with the current directory.
Mercurial’s commands that work with file names have useful default behaviours when you invoke them without providing any file names or patterns. What kind of behaviour you should expect depends on what the command does. Here are a few rules of thumb you can use to predict what a command is likely to do if you don’t give it any names to work with.
It’s easy to work around these default behaviours if they don’t suit you. If a command normally operates on the whole working directory, you can invoke it on just the current directory and its subdirectories by giving it the name “.”.
Along the same lines, some commands normally print file names relative to the root of the repository, even if you’re invoking them from a subdirectory. Such a command will print file names relative to your subdirectory if you give it explicit names. Here, we’re going to run “hg status” from a subdirectory, and get it to operate on the entire working directory while printing file names relative to our subdirectory, by passing it the output of the “hg root” command.
The “hg add” example in the preceding section illustrates something else that’s helpful about Mercurial commands. If a command operates on a file that you didn’t name explicitly on the command line, it will usually print the name of the file, so that you will not be surprised what’s going on.
The principle here is of least surprise. If you’ve exactly named a file on the command line, there’s no point in repeating it back at you. If Mercurial is acting on a file implicitly, because you provided no names, or a directory, or a pattern (see below), it’s safest to tell you what it’s doing.
For commands that behave this way, you can silence them using the -q option. You can also get them to print the name of every file, even those you’ve named explicitly, using the -v option.
In addition to working with file and directory names, Mercurial lets you use patterns to identify files. Mercurial’s pattern handling is expressive.
On Unix-like systems (Linux, MacOS, etc.), the job of matching file names to patterns normally falls to the shell. On these systems, you must explicitly tell Mercurial that a name is a pattern. On Windows, the shell does not expand patterns, so Mercurial will automatically identify names that are patterns, and expand them for you.
To provide a pattern in place of a regular name on the command line, the mechanism is simple:
That is, a pattern is identified by a short text string that says what kind of pattern this is, followed by a colon, followed by the actual pattern.
Mercurial supports two kinds of pattern syntax. The most frequently used is called glob; this is the same kind of pattern matching used by the Unix shell, and should be familiar to Windows command prompt users, too.
When Mercurial does automatic pattern matching on Windows, it uses glob syntax. You can thus omit the “glob:” prefix on Windows, but it’s safe to use it, too.
The re syntax is more powerful; it lets you specify patterns using regular expressions, also known as regexps.
By the way, in the examples that follow, notice that I’m careful to wrap all of my patterns in quote characters, so that they won’t get expanded by the shell before Mercurial sees them.
This is an overview of the kinds of patterns you can use when you’re matching on glob patterns.
The “⋆” character matches any string, within a single directory.
The “⋆⋆” pattern matches any string, and crosses directory boundaries. It’s not a standard Unix glob token, but it’s accepted by several popular Unix shells, and is very useful.
The “?” pattern matches any single character.
The “[” character begins a character class. This matches any single character within the class. The class ends with a “]” character. A class may contain multiple ranges of the form “a-f”, which is shorthand for “abcdef”.
If the first character after the “[” in a character class is a “!”, it negates the class, making it match any single character not in the class.
A “{” begins a group of subpatterns, where the whole group matches if any subpattern in the group matches. The “,” character separates subpatterns, and “}” ends the group.
Don’t forget that if you want to match a pattern in any directory, you should not be using the “⋆” match-any token, as this will only match within one directory. Instead, use the “⋆⋆” token. This small example illustrates the difference between the two.
Mercurial accepts the same regular expression syntax as the Python programming language (it uses Python’s regexp engine internally). This is based on the Perl language’s regexp syntax, which is the most popular dialect in use (it’s also used in Java, for example).
I won’t discuss Mercurial’s regexp dialect in any detail here, as regexps are not often used. Perl-style regexps are in any case already exhaustively documented on a multitude of web sites, and in many books. Instead, I will focus here on a few things you should know if you find yourself needing to use regexps with Mercurial.
A regexp is matched against an entire file name, relative to the root of the repository. In other words, even if you’re already in subbdirectory foo, if you want to match files under this directory, your pattern must start with “foo/”.
One thing to note, if you’re familiar with Perl-style regexps, is that Mercurial’s are rooted. That is, a regexp starts matching against the beginning of a string; it doesn’t look for a match anywhere within the string. To match anywhere in a string, start your pattern with “.⋆”.
Not only does Mercurial give you a variety of ways to specify files; it lets you further winnow those files using filters. Commands that work with file names accept two filtering options.
You can provide multiple -I and -X options on the command line, and intermix them as you please. Mercurial interprets the patterns you provide using glob syntax by default (but you can use regexps if you need to).
You can read a -I filter as “process only the files that match this filter”.
The -X filter is best read as “process only the files that don’t match this pattern”.
XXX.
If you’re working in a mixed development environment that contains both Linux (or other Unix) systems and Macs or Windows systems, you should keep in the back of your mind the knowledge that they treat the case (“N” versus “n”) of file names in incompatible ways. This is not very likely to affect you, and it’s easy to deal with if it does, but it could surprise you if you don’t know about it.
Operating systems and filesystems differ in the way they handle the case of characters in file and directory names. There are three common ways to handle case in names.
On Unix-like systems, it is possible to have any or all of the above ways of handling case in action at once. For example, if you use a USB thumb drive formatted with a FAT32 filesystem on a Linux system, Linux will handle names on that filesystem in a case preserving, but insensitive, way.
Mercurial’s repository storage mechanism is case safe. It translates file names so that they can be safely stored on both case sensitive and case insensitive filesystems. This means that you can use normal file copying tools to transfer a Mercurial repository onto, for example, a USB thumb drive, and safely move that drive and repository back and forth between a Mac, a PC running Windows, and a Linux box.
When operating in the working directory, Mercurial honours the naming policy of the filesystem where the working directory is located. If the filesystem is case preserving, but insensitive, Mercurial will treat names that differ only in case as the same.
An important aspect of this approach is that it is possible to commit a changeset on a case sensitive (typically Linux or Unix) filesystem that will cause trouble for users on case insensitive (usually Windows and MacOS) users. If a Linux user commits changes to two files, one named myfile.c and the other named MyFile.C, they will be stored correctly in the repository. And in the working directories of other Linux users, they will be correctly represented as separate files.
If a Windows or Mac user pulls this change, they will not initially have a problem, because Mercurial’s repository storage mechanism is case safe. However, once they try to “hg update” the working directory to that changeset, or “hg merge” with that changeset, Mercurial will spot the conflict between the two file names that the filesystem would treat as the same, and forbid the update or merge from occurring.
If you are using Windows or a Mac in a mixed environment where some of your collaborators are using Linux or Unix, and Mercurial reports a case folding conflict when you try to “hg update” or “hg merge”, the procedure to fix the problem is simple.
Just find a nearby Linux or Unix box, clone the problem repository onto it, and use Mercurial’s “hg rename” command to change the names of any offending files or directories so that they will no longer cause case folding conflicts. Commit this change, “hg pull” or “hg push” it across to your Windows or MacOS system, and “hg update” to the revision with the non-conflicting names.
The changeset with case-conflicting names will remain in your project’s history, and you still won’t be able to “hg update” your working directory to that changeset on a Windows or MacOS system, but you can continue development unimpeded.
| Note: Prior to version 0.9.3, Mercurial did not use a case safe repository storage mechanism, and did not detect case folding conflicts. If you are using an older version of Mercurial on Windows or MacOS, I strongly recommend that you upgrade. |