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  <title>Git Magic - Chapter&nbsp;5.&nbsp;Lessons of History</title>
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    <div class="toc">
      <ul class="toc">
<li><b>Git Magic</b></li>
        <li>
          <span class="preface"><a href=
          "index.html">Preface</a></span>

          <ul>
            <li><span class="section"><a href=
            "index.html#thanks">Thanks!</a></span></li>

            <li><span class="section"><a href=
            "index.html#license">License</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch01.html">1.
          Introduction</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch01.html#work_is_play">Work is Play</a></span></li>

            <li><span class="section"><a href=
            "ch01.html#version_control">Version
            Control</a></span></li>

            <li><span class="section"><a href=
            "ch01.html#distributed_control">Distributed
            Control</a></span></li>

            <li><span class="section"><a href=
            "ch01.html#a_silly_superstition">A Silly
            Superstition</a></span></li>

            <li><span class="section"><a href=
            "ch01.html#merge_conflicts">Merge
            Conflicts</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch02.html">2. Basic
          Tricks</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch02.html#saving_state">Saving State</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#add_delete_rename">Add, Delete,
            Rename</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#advanced_undo_redo">Advanced
            Undo/Redo</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#reverting">Reverting</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#changelog_generation">Changelog
            Generation</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#downloading_files">Downloading
            Files</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#the_bleeding_edge">The Bleeding
            Edge</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#instant_publishing">Instant
            Publishing</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#what_have_i_done">What Have I
            Done?</a></span></li>

            <li><span class="section"><a href=
            "ch02.html#exercise">Exercise</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch03.html">3. Cloning
          Around</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch03.html#sync_computers">Sync
            Computers</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#classic_source_control">Classic Source
            Control</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#secret_source">Secret Source</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#bare_repositories">Bare
            repositories</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#push_versus_pull">Push versus
            pull</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#forking_a_project">Forking a
            Project</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#ultimate_backups">Ultimate
            Backups</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#light_speed_multitask">Light-Speed
            Multitask</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#guerilla_version_control">Guerilla Version
            Control</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#mercurial">Mercurial</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#bazaar">Bazaar</a></span></li>

            <li><span class="section"><a href=
            "ch03.html#why_i_use_git">Why I use Git</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch04.html">4. Branch
          Wizardry</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch04.html#the_boss_key">The Boss Key</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#dirty_work">Dirty Work</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#quick_fixes">Quick Fixes</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#merging">Merging</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#uninterrupted_workflow">Uninterrupted
            Workflow</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#reorganizing_a_medley">Reorganizing a
            Medley</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#managing_branches">Managing
            Branches</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#temporary_branches">Temporary
            Branches</a></span></li>

            <li><span class="section"><a href=
            "ch04.html#work_how_you_want">Work How You
            Want</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch05.html">5. Lessons of
          History</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch05.html#i_stand_corrected">I Stand
            Corrected</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#and_then_some">… And Then
            Some</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#local_changes_last">Local Changes
            Last</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#rewriting_history">Rewriting
            History</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#making_history">Making
            History</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#where_did_it_all_go_wrong">Where Did It All
            Go Wrong?</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#who_made_it_all_go_wrong">Who Made It All Go
            Wrong?</a></span></li>

            <li><span class="section"><a href=
            "ch05.html#personal_experience">Personal
            Experience</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch06.html">6. Multiplayer
          Git</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch06.html#who_am_i">Who Am I?</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#git_over_ssh_http">Git Over SSH,
            HTTP</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#git_over_anything">Git Over
            Anything</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#patches_the_global_currency">Patches: The
            Global Currency</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#sorry_we_8217_ve_moved">Sorry, We’ve
            Moved</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#remote_branches">Remote
            Branches</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#multiple_remotes">Multiple
            Remotes</a></span></li>

            <li><span class="section"><a href=
            "ch06.html#my_preferences">My
            Preferences</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch07.html">7. Git
          Grandmastery</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch07.html#source_releases">Source
            Releases</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#commit_what_changed">Commit What
            Changed</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#my_commit_is_too_big">My Commit Is Too
            Big!</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#the_index_git_8217_s_staging_area">The
            Index: Git’s Staging Area</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#don_8217_t_lose_your_head">Don’t Lose Your
            HEAD</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#head_hunting">HEAD-hunting</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#building_on_git">Building On
            Git</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#daring_stunts">Daring Stunts</a></span></li>

            <li><span class="section"><a href=
            "ch07.html#preventing_bad_commits">Preventing Bad
            Commits</a></span></li>
          </ul>
        </li>

        <li>
          <span class="chapter"><a href="ch08.html">8. Secrets
          Revealed</a></span>

          <ul>
            <li><span class="section"><a href=
            "ch08.html#invisibility">Invisibility</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#integrity">Integrity</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#intelligence">Intelligence</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#indexing">Indexing</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#git_8217_s_origins">Git’s
            Origins</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#the_object_database">The Object
            Database</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#blobs">Blobs</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#trees">Trees</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#commits">Commits</a></span></li>

            <li><span class="section"><a href=
            "ch08.html#indistinguishable_from_magic">Indistinguishable
            From Magic</a></span></li>
          </ul>
        </li>

        <li>
          <span class="appendix"><a href="apa.html">A. Git
          Shortcomings</a></span>

          <ul>
            <li><span class="section"><a href=
            "apa.html#sha1_weaknesses">SHA1
            Weaknesses</a></span></li>

            <li><span class="section"><a href=
            "apa.html#microsoft_windows">Microsoft
            Windows</a></span></li>

            <li><span class="section"><a href=
            "apa.html#unrelated_files">Unrelated
            Files</a></span></li>

            <li><span class="section"><a href=
            "apa.html#who_8217_s_editing_what">Who’s Editing
            What?</a></span></li>

            <li><span class="section"><a href=
            "apa.html#file_history">File History</a></span></li>

            <li><span class="section"><a href=
            "apa.html#initial_clone">Initial Clone</a></span></li>

            <li><span class="section"><a href=
            "apa.html#volatile_projects">Volatile
            Projects</a></span></li>

            <li><span class="section"><a href=
            "apa.html#global_counter">Global
            Counter</a></span></li>

            <li><span class="section"><a href=
            "apa.html#empty_subdirectories">Empty
            Subdirectories</a></span></li>

            <li><span class="section"><a href=
            "apa.html#initial_commit">Initial
            Commit</a></span></li>

            <li><span class="section"><a href=
            "apa.html#interface_quirks">Interface
            Quirks</a></span></li>
          </ul>
        </li>

        <li><span class="appendix"><a href="apb.html">B.
        Translating This Guide</a></span></li>
      </ul>
    </div>
<div class="content">
  <div class="chapter">
    <div class="titlepage">
      <div>
        <div>
          <h1 class="title"><a id="lessons_of_history" name=
          "lessons_of_history"></a>Chapter&nbsp;5.&nbsp;Lessons of
          History</h1>
        </div>
      </div>
    </div>

    <p>A consequence of Git’s distributed nature is that history
    can be edited easily. But if you tamper with the past, take
    care: only rewrite that part of history which you alone
    possess. Just as nations forever argue over who committed what
    atrocity, if someone else has a clone whose version of history
    differs to yours, you will have trouble reconciling when your
    trees interact.</p>

    <p>Some developers strongly feel history should be immutable,
    warts and all. Others feel trees should be made presentable
    before they are unleashed in public. Git accommodates both
    viewpoints. Like cloning, branching, and merging, rewriting
    history is simply another power Git gives you. It is up to you
    to use it wisely.</p>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="i_stand_corrected" name=
            "i_stand_corrected"></a>I Stand Corrected</h2>
          </div>
        </div>
      </div>

      <p>Did you just commit, but wish you had typed a different
      message? Then run:</p>
      <pre class="literallayout">
$ git commit --amend
</pre>

      <p>to change the last message. Realized you forgot to add a
      file? Run <span class="strong"><strong>git
      add</strong></span> to add it, and then run the above
      command.</p>

      <p>Want to include a few more edits in that last commit? Then
      make those edits and run:</p>
      <pre class="literallayout">
$ git commit --amend -a
</pre>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="and_then_some" name=
            "and_then_some"></a>… And Then Some</h2>
          </div>
        </div>
      </div>

      <p>Suppose the previous problem is ten times worse. After a
      lengthy session you’ve made a bunch of commits. But you’re
      not quite happy with the way they’re organized, and some of
      those commit messages could use rewording. Then type:</p>
      <pre class="literallayout">
$ git rebase -i HEAD~10
</pre>

      <p>and the last 10 commits will appear in your favourite
      $EDITOR. A sample excerpt:</p>
      <pre class="literallayout">
pick 5c6eb73 Added repo.or.cz link
pick a311a64 Reordered analogies in "Work How You Want"
pick 100834f Added push target to Makefile
</pre>

      <p>Older commits precede newer commits in this list, unlike
      the <code class="literal">log</code> command. Here, 5c6eb73
      is the oldest commit, and 100834f is the newest. Then:</p>

      <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
          <li class="listitem">Remove commits by deleting lines.
          Like the revert command, but off the record: it will be
          as if the commit never existed.</li>

          <li class="listitem">Reorder commits by reordering
          lines.</li>

          <li class="listitem">
            <p class="simpara">Replace <code class=
            "literal">pick</code> with:</p>

            <div class="itemizedlist">
              <ul class="itemizedlist" type="circle">
                <li class="listitem"><code class=
                "literal">edit</code> to mark a commit for
                amending.</li>

                <li class="listitem"><code class=
                "literal">reword</code> to change the log
                message.</li>

                <li class="listitem"><code class=
                "literal">squash</code> to merge a commit with the
                previous one.</li>

                <li class="listitem"><code class=
                "literal">fixup</code> to merge a commit with the
                previous one and discard the log message.</li>
              </ul>
            </div>
          </li>
        </ul>
      </div>

      <p>For example, we might replace the second <code class=
      "literal">pick</code> with <code class=
      "literal">squash</code>:</p>
      <pre class="literallayout">
pick 5c6eb73 Added repo.or.cz link
squash a311a64 Reordered analogies in "Work How You Want"
pick 100834f Added push target to Makefile
</pre>

      <p>After we save and quit, Git merges a311a64 into 5c6eb73.
      Thus <span class="strong"><strong>squash</strong></span>
      merges into the next commit up: think “squash up”.</p>

      <p>Git then combines their log messages and presents them for
      editing. The command <span class=
      "strong"><strong>fixup</strong></span> skips this step; the
      squashed log message is simply discarded.</p>

      <p>If you marked a commit with <span class=
      "strong"><strong>edit</strong></span>, Git returns you to the
      past, to the oldest such commit. You can amend the old commit
      as described in the previous section, and even create new
      commits that belong here. Once you’re pleased with the
      “retcon”, go forward in time by running:</p>
      <pre class="literallayout">
$ git rebase --continue
</pre>

      <p>Git replays commits until the next <span class=
      "strong"><strong>edit</strong></span>, or to the present if
      none remain.</p>

      <p>You can also abandon the rebase with:</p>
      <pre class="literallayout">
$ git rebase --abort
</pre>

      <p>So commit early and commit often: you can tidy up later
      with rebase.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="local_changes_last" name=
            "local_changes_last"></a>Local Changes Last</h2>
          </div>
        </div>
      </div>

      <p>You’re working on an active project. You make some local
      commits over time, and then you sync with the official tree
      with a merge. This cycle repeats itself a few times before
      you’re ready to push to the central tree.</p>

      <p>But now the history in your local Git clone is a messy
      jumble of your changes and the official changes. You’d prefer
      to see all your changes in one contiguous section, and after
      all the official changes.</p>

      <p>This is a job for <span class="strong"><strong>git
      rebase</strong></span> as described above. In many cases you
      can use the <span class=
      "strong"><strong>--onto</strong></span> flag and avoid
      interaction.</p>

      <p>Also see <span class="strong"><strong>git help
      rebase</strong></span> for detailed examples of this amazing
      command. You can split commits. You can even rearrange
      branches of a tree.</p>

      <p>Take care: rebase is a powerful command. For complicated
      rebases, first make a backup with <span class=
      "strong"><strong>git clone</strong></span>.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="rewriting_history" name=
            "rewriting_history"></a>Rewriting History</h2>
          </div>
        </div>
      </div>

      <p>Occasionally, you need the source control equivalent of
      airbrushing people out of official photos, erasing them from
      history in a Stalinesque fashion. For example, suppose we
      intend to release a project, but it involves a file that
      should be kept private for some reason. Perhaps I left my
      credit card number in a text file and accidentally added it
      to the project. Deleting the file is insufficient, for the
      file can be accessed from older commits. We must remove the
      file from all commits:</p>
      <pre class="literallayout">
$ git filter-branch --tree-filter 'rm top/secret/file' HEAD
</pre>

      <p>See <span class="strong"><strong>git help
      filter-branch</strong></span>, which discusses this example
      and gives a faster method. In general, <span class=
      "strong"><strong>filter-branch</strong></span> lets you alter
      large sections of history with a single command.</p>

      <p>Afterwards, the <code class=
      "literal">.git/refs/original</code> directory describes the
      state of affairs before the operation. Check the
      filter-branch command did what you wanted, then delete this
      directory if you wish to run more filter-branch commands.</p>

      <p>Lastly, replace clones of your project with your revised
      version if you want to interact with them later.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="making_history" name=
            "making_history"></a>Making History</h2>
          </div>
        </div>
      </div>

      <p><a name="makinghistory" id="makinghistory"></a>Want to
      migrate a project to Git? If it’s managed with one of the
      more well-known systems, then chances are someone has already
      written a script to export the whole history to Git.</p>

      <p>Otherwise, look up <span class="strong"><strong>git
      fast-import</strong></span>, which reads text input in a
      specific format to create Git history from scratch. Typically
      a script using this command is hastily cobbled together and
      run once, migrating the project in a single shot.</p>

      <p>As an example, paste the following listing into temporary
      file, such as <code class="literal">/tmp/history</code>:</p>
      <pre class="screen">
commit refs/heads/master
committer Alice &lt;alice@example.com&gt; Thu, 01 Jan 1970 00:00:00 +0000
data &lt;&lt;EOT
Initial commit.
EOT

M 100644 inline hello.c
data &lt;&lt;EOT
#include &lt;stdio.h&gt;

int main() {
  printf("Hello, world!\n");
  return 0;
}
EOT


commit refs/heads/master
committer Bob &lt;bob@example.com&gt; Tue, 14 Mar 2000 01:59:26 -0800
data &lt;&lt;EOT
Replace printf() with write().
EOT

M 100644 inline hello.c
data &lt;&lt;EOT
#include &lt;unistd.h&gt;

int main() {
  write(1, "Hello, world!\n", 14);
  return 0;
}
EOT
</pre>

      <p>Then create a Git repository from this temporary file by
      typing:</p>
      <pre class="literallayout">
$ mkdir project; cd project; git init
$ git fast-import --date-format=rfc2822 &lt; /tmp/history
</pre>

      <p>You can checkout the latest version of the project
      with:</p>
      <pre class="literallayout">
$ git checkout master .
</pre>

      <p>The <span class="strong"><strong>git
      fast-export</strong></span> command converts any repository
      to the <span class="strong"><strong>git
      fast-import</strong></span> format, whose output you can
      study for writing exporters, and also to transport
      repositories in a human-readable format. Indeed, these
      commands can send repositories of text files over text-only
      channels.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="where_did_it_all_go_wrong"
            name="where_did_it_all_go_wrong"></a>Where Did It All
            Go Wrong?</h2>
          </div>
        </div>
      </div>

      <p>You’ve just discovered a broken feature in your program
      which you know for sure was working a few months ago. Argh!
      Where did this bug come from? If only you had been testing
      the feature as you developed.</p>

      <p>It’s too late for that now. However, provided you’ve been
      committing often, Git can pinpoint the problem:</p>
      <pre class="literallayout">
$ git bisect start
$ git bisect bad HEAD
$ git bisect good 1b6d
</pre>

      <p>Git checks out a state halfway in between. Test the
      feature, and if it’s still broken:</p>
      <pre class="literallayout">
$ git bisect bad
</pre>

      <p>If not, replace "bad" with "good". Git again transports
      you to a state halfway between the known good and bad
      versions, narrowing down the possibilities. After a few
      iterations, this binary search will lead you to the commit
      that caused the trouble. Once you’ve finished your
      investigation, return to your original state by typing:</p>
      <pre class="literallayout">
$ git bisect reset
</pre>

      <p>Instead of testing every change by hand, automate the
      search by running:</p>
      <pre class="literallayout">
$ git bisect run my_script
</pre>

      <p>Git uses the return value of the given command, typically
      a one-off script, to decide whether a change is good or bad:
      the command should exit with code 0 when good, 125 when the
      change should be skipped, and anything else between 1 and 127
      if it is bad. A negative return value aborts the bisect.</p>

      <p>You can do much more: the help page explains how to
      visualize bisects, examine or replay the bisect log, and
      eliminate known innocent changes for a speedier search.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="who_made_it_all_go_wrong"
            name="who_made_it_all_go_wrong"></a>Who Made It All Go
            Wrong?</h2>
          </div>
        </div>
      </div>

      <p>Like many other version control systems, Git has a blame
      command:</p>
      <pre class="literallayout">
$ git blame bug.c
</pre>

      <p>which annotates every line in the given file showing who
      last changed it, and when. Unlike many other version control
      systems, this operation works offline, reading only from
      local disk.</p>
    </div>

    <div class="section">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="personal_experience" name=
            "personal_experience"></a>Personal Experience</h2>
          </div>
        </div>
      </div>

      <p>In a centralized version control system, history
      modification is a difficult operation, and only available to
      administrators. Cloning, branching, and merging are
      impossible without network communication. So are basic
      operations such as browsing history, or committing a change.
      In some systems, users require network connectivity just to
      view their own changes or open a file for editing.</p>

      <p>Centralized systems preclude working offline, and need
      more expensive network infrastructure, especially as the
      number of developers grows. Most importantly, all operations
      are slower to some degree, usually to the point where users
      shun advanced commands unless absolutely necessary. In
      extreme cases this is true of even the most basic commands.
      When users must run slow commands, productivity suffers
      because of an interrupted work flow.</p>

      <p>I experienced these phenomena first-hand. Git was the
      first version control system I used. I quickly grew
      accustomed to it, taking many features for granted. I simply
      assumed other systems were similar: choosing a version
      control system ought to be no different from choosing a text
      editor or web browser.</p>

      <p>I was shocked when later forced to use a centralized
      system. A flaky internet connection matters little with Git,
      but makes development unbearable when it needs to be as
      reliable as local disk. Additionally, I found myself
      conditioned to avoid certain commands because of the
      latencies involved, which ultimately prevented me from
      following my desired work flow.</p>

      <p>When I had to run a slow command, the interruption to my
      train of thought dealt a disproportionate amount of damage.
      While waiting for server communication to complete, I’d do
      something else to pass the time, such as check email or write
      documentation. By the time I returned to the original task,
      the command had finished long ago, and I would waste more
      time trying to remember what I was doing. Humans are bad at
      context switching.</p>

      <p>There was also an interesting tragedy-of-the-commons
      effect: anticipating network congestion, individuals would
      consume more bandwidth than necessary on various operations
      in an attempt to reduce future delays. The combined efforts
      intensified congestion, encouraging individuals to consume
      even more bandwidth next time to avoid even longer
      delays.</p>
    </div>
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</div><div class="footer"><a href="/~blynn/">Ben Lynn</a></div>
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gitmagic 20140125-1 / usr / share / doc / gitmagic / html / ch05.html
