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1 \chapter{File names and pattern matching}
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2 \label{chap:names}
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3
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4 Mercurial provides mechanisms that let you work with file names in a
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5 consistent and expressive way.
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6
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7 \section{Simple file naming}
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8
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9 Mercurial uses a unified piece of machinery ``under the hood'' to
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10 handle file names. Every command behaves uniformly with respect to
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11 file names. The way in which commands work with file names is as
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12 follows.
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13
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14 If you explicitly name real files on the command line, Mercurial works
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15 with exactly those files, as you would expect.
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16 \interaction{filenames.files}
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17
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18 When you provide a directory name, Mercurial will interpret this as
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19 ``operate on every file in this directory and its subdirectories''.
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20 Mercurial traverses the files and subdirectories in a directory in
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21 alphabetical order. When it encounters a subdirectory, it will
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22 traverse that subdirectory before continuing with the current
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23 directory.
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24 \interaction{filenames.dirs}
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25
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26 \section{Running commands without any file names}
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27
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28 Mercurial's commands that work with file names have useful default
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29 behaviours when you invoke them without providing any file names or
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30 patterns. What kind of behaviour you should expect depends on what
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31 the command does. Here are a few rules of thumb you can use to
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32 predict what a command is likely to do if you don't give it any names
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33 to work with.
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34 \begin{itemize}
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35 \item Most commands will operate on the entire working directory.
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36 This is what the \hgcmd{add} command does, for example.
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37 \item If the command has effects that are difficult or impossible to
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38 reverse, it will force you to explicitly provide at least one name
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39 or pattern (see below). This protects you from accidentally
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40 deleting files by running \hgcmd{remove} with no arguments, for
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41 example.
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42 \end{itemize}
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43
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44 It's easy to work around these default behaviours if they don't suit
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45 you. If a command normally operates on the whole working directory,
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46 you can invoke it on just the current directory and its subdirectories
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47 by giving it the name ``\dirname{.}''.
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48 \interaction{filenames.wdir-subdir}
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49
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50 Along the same lines, some commands normally print file names relative
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51 to the root of the repository, even if you're invoking them from a
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52 subdirectory. Such a command will print file names relative to your
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53 subdirectory if you give it explicit names. Here, we're going to run
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54 \hgcmd{status} from a subdirectory, and get it to operate on the
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55 entire working directory while printing file names relative to our
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56 subdirectory, by passing it the output of the \hgcmd{root} command.
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57 \interaction{filenames.wdir-relname}
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58
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59 \section{Telling you what's going on}
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60
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61 The \hgcmd{add} example in the preceding section illustrates something
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62 else that's helpful about Mercurial commands. If a command operates
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63 on a file that you didn't name explicitly on the command line, it will
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64 usually print the name of the file, so that you will not be surprised
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65 what's going on.
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66
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67 The principle here is of \emph{least surprise}. If you've exactly
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68 named a file on the command line, there's no point in repeating it
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69 back at you. If Mercurial is acting on a file \emph{implicitly},
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70 because you provided no names, or a directory, or a pattern (see
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71 below), it's safest to tell you what it's doing.
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72
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73 For commands that behave this way, you can silence them using the
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74 \hggopt{-q} option. You can also get them to print the name of every
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75 file, even those you've named explicitly, using the \hggopt{-v}
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76 option.
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77
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78 \section{Using patterns to identify files}
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79
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80 In addition to working with file and directory names, Mercurial lets
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81 you use \emph{patterns} to identify files. Mercurial's pattern
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82 handling is expressive.
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83
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84 On Unix-like systems (Linux, MacOS, etc.), the job of matching file
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85 names to patterns normally falls to the shell. On these systems, you
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86 must explicitly tell Mercurial that a name is a pattern. On Windows,
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87 the shell does not expand patterns, so Mercurial will automatically
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88 identify names that are patterns, and expand them for you.
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89
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90 To provide a pattern in place of a regular name on the command line,
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91 the mechanism is simple:
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92 \begin{codesample2}
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93 syntax:patternbody
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94 \end{codesample2}
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95 That is, a pattern is identified by a short text string that says what
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96 kind of pattern this is, followed by a colon, followed by the actual
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97 pattern.
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98
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99 Mercurial supports two kinds of pattern syntax. The most frequently
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100 used is called \texttt{glob}; this is the same kind of pattern
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101 matching used by the Unix shell, and should be familiar to Windows
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102 command prompt users, too.
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103
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104 When Mercurial does automatic pattern matching on Windows, it uses
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105 \texttt{glob} syntax. You can thus omit the ``\texttt{glob:}'' prefix
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106 on Windows, but it's safe to use it, too.
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107
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108 The \texttt{re} syntax is more powerful; it lets you specify patterns
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109 using regular expressions, also known as regexps.
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110
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111 By the way, in the examples that follow, notice that I'm careful to
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112 wrap all of my patterns in quote characters, so that they won't get
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113 expanded by the shell before Mercurial sees them.
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114
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115 \subsection{Shell-style \texttt{glob} patterns}
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116
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117 This is an overview of the kinds of patterns you can use when you're
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118 matching on glob patterns.
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119
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120 The ``\texttt{*}'' character matches any string, within a single
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121 directory.
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122 \interaction{filenames.glob.star}
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123
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124 The ``\texttt{**}'' pattern matches any string, and crosses directory
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125 boundaries. It's not a standard Unix glob token, but it's accepted by
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126 several popular Unix shells, and is very useful.
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127 \interaction{filenames.glob.starstar}
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128
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129 The ``\texttt{?}'' pattern matches any single character.
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130 \interaction{filenames.glob.question}
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131
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132 The ``\texttt{[}'' character begins a \emph{character class}. This
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133 matches any single character within the class. The class ends with a
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134 ``\texttt{]}'' character. A class may contain multiple \emph{range}s
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135 of the form ``\texttt{a-f}'', which is shorthand for
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136 ``\texttt{abcdef}''.
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137 \interaction{filenames.glob.range}
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138 If the first character after the ``\texttt{[}'' in a character class
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139 is a ``\texttt{!}'', it \emph{negates} the class, making it match any
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140 single character not in the class.
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141
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142 A ``\texttt{\{}'' begins a group of subpatterns, where the whole group
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143 matches if any subpattern in the group matches. The ``\texttt{,}''
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144 character separates subpatterns, and ``\texttt{\}}'' ends the group.
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145 \interaction{filenames.glob.group}
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146
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147 \subsubsection{Watch out!}
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148
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149 Don't forget that if you want to match a pattern in any directory, you
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150 should not be using the ``\texttt{*}'' match-any token, as this will
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151 only match within one directory. Instead, use the ``\texttt{**}''
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152 token. This small example illustrates the difference between the two.
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153 \interaction{filenames.glob.star-starstar}
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154
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155 \subsection{Regular expression matching with \texttt{re} patterns}
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156
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157 Mercurial accepts the same regular expression syntax as the Python
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158 programming language (it uses Python's regexp engine internally).
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159 This is based on the Perl language's regexp syntax, which is the most
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160 popular dialect in use (it's also used in Java, for example).
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161
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162 I won't discuss Mercurial's regexp dialect in any detail here, as
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163 regexps are not often used. Perl-style regexps are in any case
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164 already exhaustively documented on a multitude of web sites, and in
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165 many books. Instead, I will focus here on a few things you should
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166 know if you find yourself needing to use regexps with Mercurial.
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167
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168 A regexp is matched against an entire file name, relative to the root
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169 of the repository. In other words, even if you're already in
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170 subbdirectory \dirname{foo}, if you want to match files under this
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171 directory, your pattern must start with ``\texttt{foo/}''.
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172
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173 One thing to note, if you're familiar with Perl-style regexps, is that
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174 Mercurial's are \emph{rooted}. That is, a regexp starts matching
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175 against the beginning of a string; it doesn't look for a match
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176 anywhere within the string. To match anywhere in a string, start
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177 your pattern with ``\texttt{.*}''.
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178
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179 \section{Filtering files}
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180
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181 Not only does Mercurial give you a variety of ways to specify files;
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182 it lets you further winnow those files using \emph{filters}. Commands
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183 that work with file names accept two filtering options.
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184 \begin{itemize}
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185 \item \hggopt{-I}, or \hggopt{--include}, lets you specify a pattern
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186 that file names must match in order to be processed.
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187 \item \hggopt{-X}, or \hggopt{--exclude}, gives you a way to
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188 \emph{avoid} processing files, if they match this pattern.
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189 \end{itemize}
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190 You can provide multiple \hggopt{-I} and \hggopt{-X} options on the
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191 command line, and intermix them as you please. Mercurial interprets
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192 the patterns you provide using glob syntax by default (but you can use
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193 regexps if you need to).
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194
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195 You can read a \hggopt{-I} filter as ``process only the files that
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196 match this filter''.
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197 \interaction{filenames.filter.include}
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198 The \hggopt{-X} filter is best read as ``process only the files that
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199 don't match this pattern''.
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200 \interaction{filenames.filter.exclude}
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201
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202 \section{Ignoring unwanted files and directories}
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203
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204 XXX.
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205
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206 \section{Case sensitivity}
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207 \label{sec:names:case}
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208
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209 If you're working in a mixed development environment that contains
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210 both Linux (or other Unix) systems and Macs or Windows systems, you
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211 should keep in the back of your mind the knowledge that they treat the
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212 case (``N'' versus ``n'') of file names in incompatible ways. This is
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213 not very likely to affect you, and it's easy to deal with if it does,
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214 but it could surprise you if you don't know about it.
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215
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216 Operating systems and filesystems differ in the way they handle the
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217 \emph{case} of characters in file and directory names. There are
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218 three common ways to handle case in names.
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219 \begin{itemize}
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220 \item Completely case insensitive. Uppercase and lowercase versions
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221 of a letter are treated as identical, both when creating a file and
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222 during subsequent accesses. This is common on older DOS-based
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223 systems.
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224 \item Case preserving, but insensitive. When a file or directory is
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225 created, the case of its name is stored, and can be retrieved and
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226 displayed by the operating system. When an existing file is being
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227 looked up, its case is ignored. This is the standard arrangement on
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228 Windows and MacOS. The names \filename{foo} and \filename{FoO}
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229 identify the same file. This treatment of uppercase and lowercase
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230 letters as interchangeable is also referred to as \emph{case
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231 folding}.
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232 \item Case sensitive. The case of a name is significant at all times.
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233 The names \filename{foo} and {FoO} identify different files. This
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234 is the way Linux and Unix systems normally work.
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235 \end{itemize}
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236
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237 On Unix-like systems, it is possible to have any or all of the above
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238 ways of handling case in action at once. For example, if you use a
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239 USB thumb drive formatted with a FAT32 filesystem on a Linux system,
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240 Linux will handle names on that filesystem in a case preserving, but
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241 insensitive, way.
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242
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243 \subsection{Safe, portable repository storage}
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244
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245 Mercurial's repository storage mechanism is \emph{case safe}. It
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246 translates file names so that they can be safely stored on both case
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247 sensitive and case insensitive filesystems. This means that you can
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248 use normal file copying tools to transfer a Mercurial repository onto,
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249 for example, a USB thumb drive, and safely move that drive and
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250 repository back and forth between a Mac, a PC running Windows, and a
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251 Linux box.
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252
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253 \subsection{Detecting case conflicts}
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254
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255 When operating in the working directory, Mercurial honours the naming
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256 policy of the filesystem where the working directory is located. If
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257 the filesystem is case preserving, but insensitive, Mercurial will
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258 treat names that differ only in case as the same.
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259
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260 An important aspect of this approach is that it is possible to commit
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261 a changeset on a case sensitive (typically Linux or Unix) filesystem
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262 that will cause trouble for users on case insensitive (usually Windows
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263 and MacOS) users. If a Linux user commits changes to two files, one
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264 named \filename{myfile.c} and the other named \filename{MyFile.C},
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265 they will be stored correctly in the repository. And in the working
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266 directories of other Linux users, they will be correctly represented
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267 as separate files.
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268
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269 If a Windows or Mac user pulls this change, they will not initially
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270 have a problem, because Mercurial's repository storage mechanism is
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271 case safe. However, once they try to \hgcmd{update} the working
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272 directory to that changeset, or \hgcmd{merge} with that changeset,
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273 Mercurial will spot the conflict between the two file names that the
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274 filesystem would treat as the same, and forbid the update or merge
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275 from occurring.
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276
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277 \subsection{Fixing a case conflict}
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278
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279 If you are using Windows or a Mac in a mixed environment where some of
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280 your collaborators are using Linux or Unix, and Mercurial reports a
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281 case folding conflict when you try to \hgcmd{update} or \hgcmd{merge},
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282 the procedure to fix the problem is simple.
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283
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284 Just find a nearby Linux or Unix box, clone the problem repository
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285 onto it, and use Mercurial's \hgcmd{rename} command to change the
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286 names of any offending files or directories so that they will no
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287 longer cause case folding conflicts. Commit this change, \hgcmd{pull}
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288 or \hgcmd{push} it across to your Windows or MacOS system, and
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289 \hgcmd{update} to the revision with the non-conflicting names.
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290
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291 The changeset with case-conflicting names will remain in your
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292 project's history, and you still won't be able to \hgcmd{update} your
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293 working directory to that changeset on a Windows or MacOS system, but
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294 you can continue development unimpeded.
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295
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296 \begin{note}
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297 Prior to version~0.9.3, Mercurial did not use a case safe repository
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298 storage mechanism, and did not detect case folding conflicts. If
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299 you are using an older version of Mercurial on Windows or MacOS, I
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300 strongly recommend that you upgrade.
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301 \end{note}
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302
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303 %%% Local Variables:
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304 %%% mode: latex
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305 %%% TeX-master: "00book"
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306 %%% End:
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