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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter id="chap:daily">
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4 <title>Mercurial in daily use</title>
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5
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6 <sect1>
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7 <title>Telling Mercurial which files to track</title>
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8
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9 <para>Mercurial does not work with files in your repository unless
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10 you tell it to manage them. The <command role="hg-cmd">hg
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11 status</command> command will tell you which files Mercurial
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12 doesn't know about; it uses a
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13 <quote><literal>?</literal></quote> to display such
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14 files.</para>
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15
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16 <para>To tell Mercurial to track a file, use the <command
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17 role="hg-cmd">hg add</command> command. Once you have added a
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18 file, the entry in the output of <command role="hg-cmd">hg
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19 status</command> for that file changes from
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20 <quote><literal>?</literal></quote> to
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21 <quote><literal>A</literal></quote>. <!--
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22 &interaction.daily.files.add; --></para>
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23
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24 <para>After you run a <command role="hg-cmd">hg commit</command>,
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25 the files that you added before the commit will no longer be
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26 listed in the output of <command role="hg-cmd">hg
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27 status</command>. The reason for this is that <command
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28 role="hg-cmd">hg status</command> only tells you about
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29 <quote>interesting</quote> files&emdash;those that you have
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30 modified or told Mercurial to do something with&emdash;by
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31 default. If you have a repository that contains thousands of
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32 files, you will rarely want to know about files that Mercurial
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33 is tracking, but that have not changed. (You can still get this
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34 information; we'll return to this later.)</para>
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35
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36 <para>Once you add a file, Mercurial doesn't do anything with it
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37 immediately. Instead, it will take a snapshot of the file's
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38 state the next time you perform a commit. It will then continue
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39 to track the changes you make to the file every time you commit,
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40 until you remove the file.</para>
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41
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42 <sect2>
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43 <title>Explicit versus implicit file naming</title>
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44
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45 <para>A useful behaviour that Mercurial has is that if you pass
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46 the name of a directory to a command, every Mercurial command
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47 will treat this as <quote>I want to operate on every file in
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48 this directory and its subdirectories</quote>. <!--
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49 &interaction.daily.files.add-dir; --> Notice in this example
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50 that Mercurial printed the names of the files it added,
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51 whereas it didn't do so when we added the file named
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52 <filename>a</filename> in the earlier example.</para>
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53
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54 <para>What's going on is that in the former case, we explicitly
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55 named the file to add on the command line, so the assumption
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56 that Mercurial makes in such cases is that you know what you
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57 were doing, and it doesn't print any output.</para>
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58
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59 <para>However, when we <emphasis>imply</emphasis> the names of
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60 files by giving the name of a directory, Mercurial takes the
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61 extra step of printing the name of each file that it does
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62 something with. This makes it more clear what is happening,
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63 and reduces the likelihood of a silent and nasty surprise.
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64 This behaviour is common to most Mercurial commands.</para>
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65
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66 </sect2>
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67 <sect2>
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68 <title>Aside: Mercurial tracks files, not directories</title>
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69
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70 <para>Mercurial does not track directory information. Instead,
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71 it tracks the path to a file. Before creating a file, it
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72 first creates any missing directory components of the path.
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73 After it deletes a file, it then deletes any empty directories
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74 that were in the deleted file's path. This sounds like a
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75 trivial distinction, but it has one minor practical
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76 consequence: it is not possible to represent a completely
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77 empty directory in Mercurial.</para>
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78
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79 <para>Empty directories are rarely useful, and there are
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80 unintrusive workarounds that you can use to achieve an
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81 appropriate effect. The developers of Mercurial thus felt
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82 that the complexity that would be required to manage empty
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83 directories was not worth the limited benefit this feature
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84 would bring.</para>
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85
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86 <para>If you need an empty directory in your repository, there
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87 are a few ways to achieve this. One is to create a directory,
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88 then <command role="hg-cmd">hg add</command> a
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89 <quote>hidden</quote> file to that directory. On Unix-like
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90 systems, any file name that begins with a period
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91 (<quote><literal>.</literal></quote>) is treated as hidden by
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92 most commands and GUI tools. This approach is illustrated
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93 below.</para>
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94
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95 <!-- &interaction.daily.files.hidden; -->
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96
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97 <para>Another way to tackle a need for an empty directory is to
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98 simply create one in your automated build scripts before they
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99 will need it.</para>
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100
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101 </sect2>
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102 </sect1>
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103 <sect1>
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104 <title>How to stop tracking a file</title>
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105
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106 <para>Once you decide that a file no longer belongs in your
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107 repository, use the <command role="hg-cmd">hg remove</command>
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108 command; this deletes the file, and tells Mercurial to stop
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109 tracking it. A removed file is represented in the output of
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110 <command role="hg-cmd">hg status</command> with a
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111 <quote><literal>R</literal></quote>. <!--
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112 &interaction.daily.files.remove; --></para>
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113
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114 <para>After you <command role="hg-cmd">hg remove</command> a file,
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115 Mercurial will no longer track changes to that file, even if you
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116 recreate a file with the same name in your working directory.
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117 If you do recreate a file with the same name and want Mercurial
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118 to track the new file, simply <command role="hg-cmd">hg
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119 add</command> it. Mercurial will know that the newly added
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120 file is not related to the old file of the same name.</para>
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121
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122 <sect2>
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123 <title>Removing a file does not affect its history</title>
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124
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125 <para>It is important to understand that removing a file has
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126 only two effects.</para>
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127 <itemizedlist>
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128 <listitem><para>It removes the current version of the file
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129 from the working directory.</para>
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130 </listitem>
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131 <listitem><para>It stops Mercurial from tracking changes to
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132 the file, from the time of the next commit.</para>
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133 </listitem></itemizedlist>
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134 <para>Removing a file <emphasis>does not</emphasis> in any way
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135 alter the <emphasis>history</emphasis> of the file.</para>
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136
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137 <para>If you update the working directory to a changeset in
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138 which a file that you have removed was still tracked, it will
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139 reappear in the working directory, with the contents it had
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140 when you committed that changeset. If you then update the
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141 working directory to a later changeset, in which the file had
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142 been removed, Mercurial will once again remove the file from
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143 the working directory.</para>
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144
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145 </sect2>
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146 <sect2>
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147 <title>Missing files</title>
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148
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149 <para>Mercurial considers a file that you have deleted, but not
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150 used <command role="hg-cmd">hg remove</command> to delete, to
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151 be <emphasis>missing</emphasis>. A missing file is
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152 represented with <quote><literal>!</literal></quote> in the
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153 output of <command role="hg-cmd">hg status</command>.
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154 Mercurial commands will not generally do anything with missing
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155 files. <!-- &interaction.daily.files.missing; --></para>
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156
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157 <para>If your repository contains a file that <command
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158 role="hg-cmd">hg status</command> reports as missing, and
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159 you want the file to stay gone, you can run <command
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160 role="hg-cmd">hg remove <option
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161 role="hg-opt-remove">--after</option></command> at any
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162 time later on, to tell Mercurial that you really did mean to
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163 remove the file. <!-- &interaction.daily.files.remove-after;
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164 --></para>
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165
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166 <para>On the other hand, if you deleted the missing file by
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167 accident, give <command role="hg-cmd">hg revert</command> the
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168 name of the file to recover. It will reappear, in unmodified
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169 form.</para>
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170
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171 <!-- &interaction.daily.files.recover-missing; -->
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172
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173 </sect2>
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174 <sect2>
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175 <title>Aside: why tell Mercurial explicitly to remove a
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176 file?</title>
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177
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178 <para>You might wonder why Mercurial requires you to explicitly
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179 tell it that you are deleting a file. Early during the
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180 development of Mercurial, it let you delete a file however you
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181 pleased; Mercurial would notice the absence of the file
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182 automatically when you next ran a <command role="hg-cmd">hg
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183 commit</command>, and stop tracking the file. In practice,
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184 this made it too easy to accidentally remove a file without
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185 noticing.</para>
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186
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187 </sect2>
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188 <sect2>
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189 <title>Useful shorthand&emdash;adding and removing files in one
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190 step</title>
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191
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192 <para>Mercurial offers a combination command, <command
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193 role="hg-cmd">hg addremove</command>, that adds untracked
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194 files and marks missing files as removed. <!--
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195 &interaction.daily.files.addremove; --> The <command
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196 role="hg-cmd">hg commit</command> command also provides a
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197 <option role="hg-opt-commit">-A</option> option that performs
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198 this same add-and-remove, immediately followed by a commit.
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199 <!-- &interaction.daily.files.commit-addremove; --></para>
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200
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201 </sect2>
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202 </sect1>
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203 <sect1>
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204 <title>Copying files</title>
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205
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206 <para>Mercurial provides a <command role="hg-cmd">hg
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207 copy</command> command that lets you make a new copy of a
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208 file. When you copy a file using this command, Mercurial makes
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209 a record of the fact that the new file is a copy of the original
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210 file. It treats these copied files specially when you merge
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211 your work with someone else's.</para>
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212
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213 <sect2>
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214 <title>The results of copying during a merge</title>
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215
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216 <para>What happens during a merge is that changes
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217 <quote>follow</quote> a copy. To best illustrate what this
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218 means, let's create an example. We'll start with the usual
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219 tiny repository that contains a single file. <!--
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220 &interaction.daily.copy.init; --> We need to do some work in
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221 parallel, so that we'll have something to merge. So let's
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222 clone our repository. <!-- &interaction.daily.copy.clone; -->
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223 Back in our initial repository, let's use the <command
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224 role="hg-cmd">hg copy</command> command to make a copy of
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225 the first file we created. <!-- &interaction.daily.copy.copy;
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226 --></para>
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227
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228 <para>If we look at the output of the <command role="hg-cmd">hg
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229 status</command> command afterwards, the copied file looks
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230 just like a normal added file. <!--
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231 &interaction.daily.copy.status; --> But if we pass the <option
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232 role="hg-opt-status">-C</option> option to <command
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233 role="hg-cmd">hg status</command>, it prints another line of
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234 output: this is the file that our newly-added file was copied
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235 <emphasis>from</emphasis>. <!--
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236 &interaction.daily.copy.status-copy; --></para>
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237
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238 <para>Now, back in the repository we cloned, let's make a change
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239 in parallel. We'll add a line of content to the original file
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240 that we created. <!-- &interaction.daily.copy.other; --> Now
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241 we have a modified <filename>file</filename> in this
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242 repository. When we pull the changes from the first
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243 repository, and merge the two heads, Mercurial will propagate
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244 the changes that we made locally to <filename>file</filename>
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245 into its copy, <filename>new-file</filename>. <!--
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246 &interaction.daily.copy.merge; --></para>
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247
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248 </sect2>
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249 <sect2 id="sec:daily:why-copy">
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250 <title>Why should changes follow copies?</title>
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251
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252 <para>This behaviour, of changes to a file propagating out to
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253 copies of the file, might seem esoteric, but in most cases
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254 it's highly desirable.</para>
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255
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256 <para>First of all, remember that this propagation
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257 <emphasis>only</emphasis> happens when you merge. So if you
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258 <command role="hg-cmd">hg copy</command> a file, and
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259 subsequently modify the original file during the normal course
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260 of your work, nothing will happen.</para>
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261
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262 <para>The second thing to know is that modifications will only
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263 propagate across a copy as long as the repository that you're
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264 pulling changes from <emphasis>doesn't know</emphasis> about
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265 the copy.</para>
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266
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267 <para>The reason that Mercurial does this is as follows. Let's
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268 say I make an important bug fix in a source file, and commit
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269 my changes. Meanwhile, you've decided to <command
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270 role="hg-cmd">hg copy</command> the file in your repository,
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271 without knowing about the bug or having seen the fix, and you
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272 have started hacking on your copy of the file.</para>
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273
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274 <para>If you pulled and merged my changes, and Mercurial
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275 <emphasis>didn't</emphasis> propagate changes across copies,
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276 your source file would now contain the bug, and unless you
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277 remembered to propagate the bug fix by hand, the bug would
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278 <emphasis>remain</emphasis> in your copy of the file.</para>
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279
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280 <para>By automatically propagating the change that fixed the bug
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281 from the original file to the copy, Mercurial prevents this
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282 class of problem. To my knowledge, Mercurial is the
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283 <emphasis>only</emphasis> revision control system that
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284 propagates changes across copies like this.</para>
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285
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286 <para>Once your change history has a record that the copy and
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287 subsequent merge occurred, there's usually no further need to
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288 propagate changes from the original file to the copied file,
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289 and that's why Mercurial only propagates changes across copies
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290 until this point, and no further.</para>
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291
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292 </sect2>
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293 <sect2>
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294 <title>How to make changes <emphasis>not</emphasis> follow a
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295 copy</title>
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296
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297 <para>If, for some reason, you decide that this business of
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298 automatically propagating changes across copies is not for
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299 you, simply use your system's normal file copy command (on
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300 Unix-like systems, that's <command>cp</command>) to make a
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301 copy of a file, then <command role="hg-cmd">hg add</command>
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302 the new copy by hand. Before you do so, though, please do
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303 reread section <xref linkend="sec:daily:why-copy"/>, and make
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304 an informed
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305 decision that this behaviour is not appropriate to your
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306 specific case.</para>
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307
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308 </sect2>
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309 <sect2>
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bos@559
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310 <title>Behaviour of the <command role="hg-cmd">hg copy</command>
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311 command</title>
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312
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313 <para>When you use the <command role="hg-cmd">hg copy</command>
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314 command, Mercurial makes a copy of each source file as it
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315 currently stands in the working directory. This means that if
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316 you make some modifications to a file, then <command
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317 role="hg-cmd">hg copy</command> it without first having
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318 committed those changes, the new copy will also contain the
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319 modifications you have made up until that point. (I find this
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320 behaviour a little counterintuitive, which is why I mention it
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321 here.)</para>
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322
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323 <para>The <command role="hg-cmd">hg copy</command> command acts
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324 similarly to the Unix <command>cp</command> command (you can
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325 use the <command role="hg-cmd">hg cp</command> alias if you
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326 prefer). The last argument is the
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327 <emphasis>destination</emphasis>, and all prior arguments are
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328 <emphasis>sources</emphasis>. If you pass it a single file as
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329 the source, and the destination does not exist, it creates a
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330 new file with that name. <!-- &interaction.daily.copy.simple;
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331 --> If the destination is a directory, Mercurial copies its
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332 sources into that directory. <!--
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333 &interaction.daily.copy.dir-dest; --> Copying a directory is
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334 recursive, and preserves the directory structure of the
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335 source. <!-- &interaction.daily.copy.dir-src; --> If the
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336 source and destination are both directories, the source tree
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337 is recreated in the destination directory. <!--
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338 &interaction.daily.copy.dir-src-dest; --></para>
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339
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340 <para>As with the <command role="hg-cmd">hg rename</command>
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341 command, if you copy a file manually and then want Mercurial
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342 to know that you've copied the file, simply use the <option
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343 role="hg-opt-copy">--after</option> option to <command
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344 role="hg-cmd">hg copy</command>. <!--
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345 &interaction.daily.copy.after; --></para>
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346
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347 </sect2>
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348 </sect1>
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349 <sect1>
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350 <title>Renaming files</title>
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351
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352 <para>It's rather more common to need to rename a file than to
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353 make a copy of it. The reason I discussed the <command
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354 role="hg-cmd">hg copy</command> command before talking about
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355 renaming files is that Mercurial treats a rename in essentially
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356 the same way as a copy. Therefore, knowing what Mercurial does
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357 when you copy a file tells you what to expect when you rename a
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358 file.</para>
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359
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360 <para>When you use the <command role="hg-cmd">hg rename</command>
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361 command, Mercurial makes a copy of each source file, then
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362 deletes it and marks the file as removed. <!--
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363 &interaction.daily.rename.rename; --> The <command
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364 role="hg-cmd">hg status</command> command shows the newly
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365 copied file as added, and the copied-from file as removed. <!--
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366 &interaction.daily.rename.status; --> As with the results of a
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367 <command role="hg-cmd">hg copy</command>, we must use the
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368 <option role="hg-opt-status">-C</option> option to <command
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369 role="hg-cmd">hg status</command> to see that the added file
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370 is really being tracked by Mercurial as a copy of the original,
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371 now removed, file. <!-- &interaction.daily.rename.status-copy;
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372 --></para>
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373
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374 <para>As with <command role="hg-cmd">hg remove</command> and
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375 <command role="hg-cmd">hg copy</command>, you can tell Mercurial
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376 about a rename after the fact using the <option
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377 role="hg-opt-rename">--after</option> option. In most other
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378 respects, the behaviour of the <command role="hg-cmd">hg
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379 rename</command> command, and the options it accepts, are
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380 similar to the <command role="hg-cmd">hg copy</command>
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381 command.</para>
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382
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383 <sect2>
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384 <title>Renaming files and merging changes</title>
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385
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386 <para>Since Mercurial's rename is implemented as
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387 copy-and-remove, the same propagation of changes happens when
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388 you merge after a rename as after a copy.</para>
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389
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390 <para>If I modify a file, and you rename it to a new name, and
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391 then we merge our respective changes, my modifications to the
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392 file under its original name will be propagated into the file
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393 under its new name. (This is something you might expect to
|
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394 <quote>simply work,</quote> but not all revision control
|
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395 systems actually do this.)</para>
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396
|
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bos@559
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397 <para>Whereas having changes follow a copy is a feature where
|
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398 you can perhaps nod and say <quote>yes, that might be
|
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399 useful,</quote> it should be clear that having them follow a
|
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400 rename is definitely important. Without this facility, it
|
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401 would simply be too easy for changes to become orphaned when
|
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402 files are renamed.</para>
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403
|
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|
404 </sect2>
|
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bos@559
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405 <sect2>
|
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bos@559
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406 <title>Divergent renames and merging</title>
|
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407
|
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408 <para>The case of diverging names occurs when two developers
|
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409 start with a file&emdash;let's call it
|
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410 <filename>foo</filename>&emdash;in their respective
|
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411 repositories.</para>
|
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412
|
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bos@559
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413 <para><!-- &interaction.rename.divergent.clone; --> Anne renames
|
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414 the file to <filename>bar</filename>. <!--
|
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bos@559
|
415 &interaction.rename.divergent.rename.anne; --> Meanwhile, Bob
|
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416 renames it to <filename>quux</filename>. <!--
|
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bos@559
|
417 &interaction.rename.divergent.rename.bob; --></para>
|
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418
|
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bos@559
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419 <para>I like to think of this as a conflict because each
|
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420 developer has expressed different intentions about what the
|
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|
421 file ought to be named.</para>
|
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bos@559
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422
|
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bos@559
|
423 <para>What do you think should happen when they merge their
|
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bos@559
|
424 work? Mercurial's actual behaviour is that it always preserves
|
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bos@559
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425 <emphasis>both</emphasis> names when it merges changesets that
|
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bos@559
|
426 contain divergent renames. <!--
|
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bos@559
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427 &interaction.rename.divergent.merge; --></para>
|
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bos@559
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428
|
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bos@559
|
429 <para>Notice that Mercurial does warn about the divergent
|
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bos@559
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430 renames, but it leaves it up to you to do something about the
|
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bos@559
|
431 divergence after the merge.</para>
|
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bos@559
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432
|
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bos@559
|
433 </sect2>
|
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bos@559
|
434 <sect2>
|
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bos@559
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435 <title>Convergent renames and merging</title>
|
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bos@559
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436
|
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bos@559
|
437 <para>Another kind of rename conflict occurs when two people
|
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bos@559
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438 choose to rename different <emphasis>source</emphasis> files
|
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bos@559
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439 to the same <emphasis>destination</emphasis>. In this case,
|
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bos@559
|
440 Mercurial runs its normal merge machinery, and lets you guide
|
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bos@559
|
441 it to a suitable resolution.</para>
|
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bos@559
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442
|
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bos@559
|
443 </sect2>
|
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bos@559
|
444 <sect2>
|
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bos@559
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445 <title>Other name-related corner cases</title>
|
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bos@559
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446
|
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bos@559
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447 <para>Mercurial has a longstanding bug in which it fails to
|
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bos@559
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448 handle a merge where one side has a file with a given name,
|
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|
449 while another has a directory with the same name. This is
|
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bos@559
|
450 documented as <ulink role="hg-bug"
|
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bos@559
|
451 url="http://www.selenic.com/mercurial/bts/issue29">issue
|
|
bos@559
|
452 29</ulink>. <!-- &interaction.issue29.go; --></para>
|
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bos@559
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453
|
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bos@559
|
454 </sect2>
|
|
bos@559
|
455 </sect1>
|
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bos@559
|
456 <sect1>
|
|
bos@559
|
457 <title>Recovering from mistakes</title>
|
|
bos@559
|
458
|
|
bos@559
|
459 <para>Mercurial has some useful commands that will help you to
|
|
bos@559
|
460 recover from some common mistakes.</para>
|
|
bos@559
|
461
|
|
bos@559
|
462 <para>The <command role="hg-cmd">hg revert</command> command lets
|
|
bos@559
|
463 you undo changes that you have made to your working directory.
|
|
bos@559
|
464 For example, if you <command role="hg-cmd">hg add</command> a
|
|
bos@559
|
465 file by accident, just run <command role="hg-cmd">hg
|
|
bos@559
|
466 revert</command> with the name of the file you added, and
|
|
bos@559
|
467 while the file won't be touched in any way, it won't be tracked
|
|
bos@559
|
468 for adding by Mercurial any longer, either. You can also use
|
|
bos@559
|
469 <command role="hg-cmd">hg revert</command> to get rid of
|
|
bos@559
|
470 erroneous changes to a file.</para>
|
|
bos@559
|
471
|
|
bos@559
|
472 <para>It's useful to remember that the <command role="hg-cmd">hg
|
|
bos@559
|
473 revert</command> command is useful for changes that you have
|
|
bos@559
|
474 not yet committed. Once you've committed a change, if you
|
|
bos@559
|
475 decide it was a mistake, you can still do something about it,
|
|
bos@559
|
476 though your options may be more limited.</para>
|
|
bos@559
|
477
|
|
bos@559
|
478 <para>For more information about the <command role="hg-cmd">hg
|
|
bos@559
|
479 revert</command> command, and details about how to deal with
|
|
bos@559
|
480 changes you have already committed, see chapter <xref
|
|
bos@559
|
481 linkend="chap:undo"/>.</para>
|
|
bos@559
|
482
|
|
bos@559
|
483 </sect1>
|
|
bos@559
|
484 </chapter>
|
|
bos@559
|
485
|
|
bos@559
|
486 <!--
|
|
bos@559
|
487 local variables:
|
|
bos@559
|
488 sgml-parent-document: ("00book.xml" "book" "chapter")
|
|
bos@559
|
489 end:
|
|
bos@559
|
490 -->
|
