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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:tour-merge">
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4 <?dbhtml filename="a-tour-of-mercurial-merging-work.html"?>
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5 <title>A tour of Mercurial: merging work</title>
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6
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7 <para id="x_338">We've now covered cloning a repository, making changes in a
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8 repository, and pulling or pushing changes from one repository
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9 into another. Our next step is <emphasis>merging</emphasis>
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10 changes from separate repositories.</para>
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11
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12 <sect1>
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13 <title>Merging streams of work</title>
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14
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15 <para id="x_339">Merging is a fundamental part of working with a distributed
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16 revision control tool. Here are a few cases in which the need
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17 to merge work arises.</para>
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18 <itemizedlist>
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19 <listitem>
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20 <para id="x_33a">Alice and Bob each have a personal copy of a
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21 repository for a project they're collaborating on. Alice
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22 fixes a bug in her repository; Bob adds a new feature in
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23 his. They want the shared repository to contain both the
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24 bug fix and the new feature.</para>
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25 </listitem>
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26 <listitem>
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27 <para id="x_33b">Cynthia frequently works on several different
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28 tasks for a single project at once, each safely isolated in
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29 its own repository. Working this way means that she often
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30 needs to merge one piece of her own work with
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31 another.</para>
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32 </listitem>
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33 </itemizedlist>
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34
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35 <para id="x_33c">Because we need to merge often, Mercurial makes
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36 the process easy. Let's walk through a merge. We'll begin by
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37 cloning yet another repository (see how often they spring up?)
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38 and making a change in it.</para>
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39
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40 &interaction.tour.merge.clone;
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41
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42 <para id="x_33d">We should now have two copies of
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43 <filename>hello.c</filename> with different contents. The
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44 histories of the two repositories have also diverged, as
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45 illustrated in <xref
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46 linkend="fig:tour-merge:sep-repos"/>. Here is a copy of our
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47 file from one repository.</para>
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48
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49 &interaction.tour.merge.cat1;
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50
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51 <para id="x_722">And here is our slightly different version from the other
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52 repository.</para>
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53
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54 &interaction.tour.merge.cat2;
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55
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56 <figure id="fig:tour-merge:sep-repos">
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57 <title>Divergent recent histories of the <filename
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58 class="directory">my-hello</filename> and <filename
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59 class="directory">my-new-hello</filename>
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60 repositories</title>
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61 <mediaobject>
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62 <imageobject><imagedata fileref="figs/tour-merge-sep-repos.png"/></imageobject>
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63 <textobject><phrase>XXX add text</phrase></textobject>
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64 </mediaobject>
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65 </figure>
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66
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67 <para id="x_33f">We already know that pulling changes from our <filename
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68 class="directory">my-hello</filename> repository will have no
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69 effect on the working directory.</para>
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70
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71 &interaction.tour.merge.pull;
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72
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73 <para id="x_340">However, the <command role="hg-cmd">hg pull</command>
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74 command says something about <quote>heads</quote>.</para>
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75
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76 <sect2>
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77 <title>Head changesets</title>
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78
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79 <para id="x_341">Remember that Mercurial records what the parent
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80 of each change is. If a change has a parent, we call it a
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81 child or descendant of the parent. A head is a change that
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82 has no children. The tip revision is thus a head, because the
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83 newest revision in a repository doesn't have any children.
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84 There are times when a repository can contain more than one
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85 head.</para>
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86
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87 <figure id="fig:tour-merge:pull">
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88 <title>Repository contents after pulling from <filename
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89 class="directory">my-hello</filename> into <filename
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90 class="directory">my-new-hello</filename></title>
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91 <mediaobject>
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92 <imageobject>
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93 <imagedata fileref="figs/tour-merge-pull.png"/>
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94 </imageobject>
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95 <textobject><phrase>XXX add text</phrase></textobject>
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96 </mediaobject>
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97 </figure>
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98
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99 <para id="x_343">In <xref linkend="fig:tour-merge:pull"/>, you can
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100 see the effect of the pull from <filename
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101 class="directory">my-hello</filename> into <filename
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102 class="directory">my-new-hello</filename>. The history that
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103 was already present in <filename
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104 class="directory">my-new-hello</filename> is untouched, but
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105 a new revision has been added. By referring to <xref
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106 linkend="fig:tour-merge:sep-repos"/>, we can see that the
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107 <emphasis>changeset ID</emphasis> remains the same in the new
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108 repository, but the <emphasis>revision number</emphasis> has
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109 changed. (This, incidentally, is a fine example of why it's
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110 not safe to use revision numbers when discussing changesets.)
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111 We can view the heads in a repository using the <command
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112 role="hg-cmd">hg heads</command> command.</para>
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113
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114 &interaction.tour.merge.heads;
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115 </sect2>
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116
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117 <sect2>
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118 <title>Performing the merge</title>
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119
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120 <para id="x_344">What happens if we try to use the normal <command
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121 role="hg-cmd">hg update</command> command to update to the
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122 new tip?</para>
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123
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124 &interaction.tour.merge.update;
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125
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126 <para id="x_345">Mercurial is telling us that the <command
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127 role="hg-cmd">hg update</command> command won't do a merge;
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128 it won't update the working directory when it thinks we might
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129 want to do a merge, unless we force it to do so.
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130 (Incidentally, forcing the update with <command>hg update
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131 -C</command> would revert any uncommitted changes in the
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132 working directory.)</para>
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133
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134 <para id="x_723">To start a merge between the two heads, we use the
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135 <command role="hg-cmd">hg merge</command> command.</para>
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136
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137 &interaction.tour.merge.merge;
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138
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139 <para id="x_347">We resolve the contents of <filename>hello.c</filename>
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140
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141 This updates the working directory so that it
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142 contains changes from <emphasis>both</emphasis> heads, which
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143 is reflected in both the output of <command role="hg-cmd">hg
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144 parents</command> and the contents of
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145 <filename>hello.c</filename>.</para>
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146
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147 &interaction.tour.merge.parents;
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148 </sect2>
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149 <sect2>
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150 <title>Committing the results of the merge</title>
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151
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152 <para id="x_348">Whenever we've done a merge, <command role="hg-cmd">hg
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153 parents</command> will display two parents until we <command
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154 role="hg-cmd">hg commit</command> the results of the
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155 merge.</para>
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156
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157 &interaction.tour.merge.commit;
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158
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159 <para id="x_349">We now have a new tip revision; notice that it has
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160 <emphasis>both</emphasis> of our former heads as its parents.
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161 These are the same revisions that were previously displayed by
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162 <command role="hg-cmd">hg parents</command>.</para>
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163
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164 &interaction.tour.merge.tip;
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165
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166 <para id="x_34a">In <xref
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167 linkend="fig:tour-merge:merge"/>, you can see a
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168 representation of what happens to the working directory during
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169 the merge, and how this affects the repository when the commit
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170 happens. During the merge, the working directory has two
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171 parent changesets, and these become the parents of the new
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172 changeset.</para>
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173
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174 <figure id="fig:tour-merge:merge">
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175 <title>Working directory and repository during merge, and
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176 following commit</title>
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177 <mediaobject>
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178 <imageobject>
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179 <imagedata fileref="figs/tour-merge-merge.png"/>
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180 </imageobject>
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181 <textobject><phrase>XXX add text</phrase></textobject>
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182 </mediaobject>
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183 </figure>
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184
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185 <para id="x_69c">We sometimes talk about a merge having
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186 <emphasis>sides</emphasis>: the left side is the first parent
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187 in the output of <command role="hg-cmd">hg parents</command>,
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188 and the right side is the second. If the working directory
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189 was at e.g. revision 5 before we began a merge, that revision
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190 will become the left side of the merge.</para>
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191 </sect2>
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192 </sect1>
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193
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194 <sect1>
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195 <title>Merging conflicting changes</title>
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196
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197 <para id="x_34b">Most merges are simple affairs, but sometimes you'll find
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198 yourself merging changes where each side modifies the same portions
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199 of the same files. Unless both modifications are identical,
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200 this results in a <emphasis>conflict</emphasis>, where you have
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201 to decide how to reconcile the different changes into something
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202 coherent.</para>
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203
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204 <figure id="fig:tour-merge:conflict">
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205 <title>Conflicting changes to a document</title>
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206 <mediaobject>
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207 <imageobject><imagedata fileref="figs/tour-merge-conflict.png"/></imageobject>
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208 <textobject><phrase>XXX add text</phrase></textobject>
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209 </mediaobject>
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210 </figure>
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211
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212 <para id="x_34d"><xref linkend="fig:tour-merge:conflict"/> illustrates
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213 an instance of two conflicting changes to a document. We
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214 started with a single version of the file; then we made some
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215 changes; while someone else made different changes to the same
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216 text. Our task in resolving the conflicting changes is to
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217 decide what the file should look like.</para>
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218
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219 <para id="x_34e">Mercurial doesn't have a built-in facility for handling
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220 conflicts. Instead, it runs an external program, usually one
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221 that displays some kind of graphical conflict resolution
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222 interface. By default, Mercurial tries to find one of several
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223 different merging tools that are likely to be installed on your
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224 system. It first tries a few fully automatic merging tools; if
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225 these don't succeed (because the resolution process requires
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226 human guidance) or aren't present, it tries a few
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227 different graphical merging tools.</para>
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228
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229 <para id="x_34f">It's also possible to get Mercurial to run a
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230 specific program or script, by setting the
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231 <envar>HGMERGE</envar> environment variable to the name of your
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232 preferred program.</para>
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233
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234 <sect2>
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235 <title>Using a graphical merge tool</title>
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236
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237 <para id="x_350">My preferred graphical merge tool is
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238 <command>kdiff3</command>, which I'll use to describe the
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239 features that are common to graphical file merging tools. You
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240 can see a screenshot of <command>kdiff3</command> in action in
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241 <xref linkend="fig:tour-merge:kdiff3"/>. The kind of
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242 merge it is performing is called a <emphasis>three-way
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243 merge</emphasis>, because there are three different versions
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244 of the file of interest to us. The tool thus splits the upper
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245 portion of the window into three panes:</para>
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246 <itemizedlist>
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247 <listitem><para id="x_351">At the left is the <emphasis>base</emphasis>
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248 version of the file, i.e. the most recent version from
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249 which the two versions we're trying to merge are
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250 descended.</para>
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251 </listitem>
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252 <listitem><para id="x_352">In the middle is <quote>our</quote> version of
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253 the file, with the contents that we modified.</para>
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254 </listitem>
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255 <listitem><para id="x_353">On the right is <quote>their</quote> version
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256 of the file, the one that from the changeset that we're
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257 trying to merge with.</para>
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258 </listitem></itemizedlist>
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259 <para id="x_354">In the pane below these is the current
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260 <emphasis>result</emphasis> of the merge. Our task is to
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261 replace all of the red text, which indicates unresolved
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262 conflicts, with some sensible merger of the
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263 <quote>ours</quote> and <quote>theirs</quote> versions of the
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264 file.</para>
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265
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266 <para id="x_355">All four of these panes are <emphasis>locked
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267 together</emphasis>; if we scroll vertically or horizontally
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268 in any of them, the others are updated to display the
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269 corresponding sections of their respective files.</para>
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270
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271 <figure id="fig:tour-merge:kdiff3">
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272 <title>Using <command>kdiff3</command> to merge versions of a
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273 file</title>
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274 <mediaobject>
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275 <imageobject>
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276 <imagedata width="100%" fileref="figs/kdiff3.png"/></imageobject>
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277 <textobject>
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278 <phrase>XXX add text</phrase>
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279 </textobject>
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280 </mediaobject>
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281 </figure>
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282
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283 <para id="x_357">For each conflicting portion of the file, we can choose to
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284 resolve the conflict using some combination of text from the
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285 base version, ours, or theirs. We can also manually edit the
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286 merged file at any time, in case we need to make further
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287 modifications.</para>
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288
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289 <para id="x_358">There are <emphasis>many</emphasis> file merging tools
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290 available, too many to cover here. They vary in which
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291 platforms they are available for, and in their particular
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292 strengths and weaknesses. Most are tuned for merging files
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293 containing plain text, while a few are aimed at specialised
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294 file formats (generally XML).</para>
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295
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296 </sect2>
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297 <sect2>
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298 <title>A worked example</title>
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299
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300 <para id="x_359">In this example, we will reproduce the file modification
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301 history of <xref linkend="fig:tour-merge:conflict"/>
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302 above. Let's begin by creating a repository with a base
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303 version of our document.</para>
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304
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305 &interaction.tour-merge-conflict.wife;
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306
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307 <para id="x_35a">We'll clone the repository and make a change to the
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308 file.</para>
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309
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310 &interaction.tour-merge-conflict.cousin;
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311
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312 <para id="x_35b">And another clone, to simulate someone else making a
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313 change to the file. (This hints at the idea that it's not all
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314 that unusual to merge with yourself when you isolate tasks in
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315 separate repositories, and indeed to find and resolve
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316 conflicts while doing so.)</para>
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317
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318 &interaction.tour-merge-conflict.son;
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319
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320 <para id="x_35c">Having created two
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321 different versions of the file, we'll set up an environment
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322 suitable for running our merge.</para>
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323
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324 &interaction.tour-merge-conflict.pull;
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325
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326 <para id="x_35d">In this example, I'll set
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327 <envar>HGMERGE</envar> to tell Mercurial to use the
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328 non-interactive <command>merge</command> command. This is
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329 bundled with many Unix-like systems. (If you're following this
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330 example on your computer, don't bother setting
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331 <envar>HGMERGE</envar>. You'll get dropped into a GUI file
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332 merge tool instead, which is much preferable.)</para>
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333
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334 &interaction.tour-merge-conflict.merge;
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335
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336 <para id="x_35f">Because <command>merge</command> can't resolve the
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337 conflicting changes, it leaves <emphasis>merge
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338 markers</emphasis> inside the file that has conflicts,
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339 indicating which lines have conflicts, and whether they came
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340 from our version of the file or theirs.</para>
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341
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342 <para id="x_360">Mercurial can tell from the way <command>merge</command>
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343 exits that it wasn't able to merge successfully, so it tells
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344 us what commands we'll need to run if we want to redo the
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345 merging operation. This could be useful if, for example, we
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346 were running a graphical merge tool and quit because we were
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347 confused or realised we had made a mistake.</para>
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348
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349 <para id="x_361">If automatic or manual merges fail, there's nothing to
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350 prevent us from <quote>fixing up</quote> the affected files
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351 ourselves, and committing the results of our merge:</para>
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352
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353 &interaction.tour-merge-conflict.commit;
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354
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bos@699
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355 <note>
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356 <title>Where is the <command>hg resolve</command> command?</title>
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357
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358 <para id="x_724">The <command>hg resolve</command> command was introduced
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359 in Mercurial 1.1, which was released in December 2008. If
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360 you are using an older version of Mercurial (run <command>hg
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361 version</command> to see), this command will not be
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362 present. If your version of Mercurial is older than 1.1,
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363 you should strongly consider upgrading to a newer version
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364 before trying to tackle complicated merges.</para>
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365 </note>
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366 </sect2>
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367 </sect1>
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368 <sect1 id="sec:tour-merge:fetch">
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369 <title>Simplifying the pull-merge-commit sequence</title>
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370
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371 <para id="x_362">The process of merging changes as outlined above is
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372 straightforward, but requires running three commands in
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373 sequence.</para>
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374 <programlisting>hg pull -u
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375 hg merge
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376 hg commit -m 'Merged remote changes'</programlisting>
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377 <para id="x_363">In the case of the final commit, you also need to enter a
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378 commit message, which is almost always going to be a piece of
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379 uninteresting <quote>boilerplate</quote> text.</para>
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380
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381 <para id="x_364">It would be nice to reduce the number of steps needed, if
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382 this were possible. Indeed, Mercurial is distributed with an
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383 extension called <literal role="hg-ext">fetch</literal> that
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384 does just this.</para>
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385
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386 <para id="x_365">Mercurial provides a flexible extension mechanism that lets
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387 people extend its functionality, while keeping the core of
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388 Mercurial small and easy to deal with. Some extensions add new
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389 commands that you can use from the command line, while others
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390 work <quote>behind the scenes,</quote> for example adding
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391 capabilities to Mercurial's built-in server mode.</para>
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392
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393 <para id="x_366">The <literal role="hg-ext">fetch</literal>
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394 extension adds a new command called, not surprisingly, <command
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395 role="hg-cmd">hg fetch</command>. This extension acts as a
|
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396 combination of <command role="hg-cmd">hg pull -u</command>,
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397 <command role="hg-cmd">hg merge</command> and <command
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398 role="hg-cmd">hg commit</command>. It begins by pulling
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399 changes from another repository into the current repository. If
|
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400 it finds that the changes added a new head to the repository, it
|
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bos@699
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401 updates to the new head, begins a merge, then (if the merge
|
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bos@699
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402 succeeded) commits the result of the merge with an
|
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bos@699
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403 automatically-generated commit message. If no new heads were
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404 added, it updates the working directory to the new tip
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405 changeset.</para>
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406
|
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bos@619
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407 <para id="x_367">Enabling the <literal
|
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bos@619
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408 role="hg-ext">fetch</literal> extension is easy. Edit the
|
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bos@619
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409 <filename role="special">.hgrc</filename> file in your home
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bos@619
|
410 directory, and either go to the <literal
|
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bos@559
|
411 role="rc-extensions">extensions</literal> section or create an
|
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bos@559
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412 <literal role="rc-extensions">extensions</literal> section. Then
|
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bos@619
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413 add a line that simply reads
|
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bos@619
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414 <quote><literal>fetch=</literal></quote>.</para>
|
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415
|
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416 <programlisting>[extensions]
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|
417 fetch =</programlisting>
|
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418
|
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bos@619
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419 <para id="x_368">(Normally, the right-hand side of the
|
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bos@619
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420 <quote><literal>=</literal></quote> would indicate where to find
|
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|
421 the extension, but since the <literal
|
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bos@559
|
422 role="hg-ext">fetch</literal> extension is in the standard
|
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bos@559
|
423 distribution, Mercurial knows where to search for it.)</para>
|
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bos@559
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424
|
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bos@559
|
425 </sect1>
|
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bos@559
|
426 </chapter>
|
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427
|
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428 <!--
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429 local variables:
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430 sgml-parent-document: ("00book.xml" "book" "chapter")
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431 end:
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432 -->
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