hg-website / original / text / from-svn-to-hg.txt

Notes: 
* svn info -> hg paths
* pull and push
* svn up -r REV -> hg up REV

= Basic concepts of Mercurial for Subversion users =

//If you're interested in the concepts behind Mercurial and already know Subversion, please join us and listen to a great explanation from Martin Geisler: //

Let me try to make some of the basic concepts clear:

* Like in Subversion, history consists of a number of commits. They're
  called changesets in Mercurial.

* Subversion requires a strict linear ordering of the commits and
  gives nice linear revision numbers to them. So revision N has only
  one child revision, rN+1.

  This is simple, but it requires a central server to make sure that
  everybody agrees on the revision numbers.

* Mercurial generalizes this by letting each changeset have multiple
  children. If I work alone and make commits I'll make

    C1 --> C2 --> C3

  by making three commits. The commit C3 with no children is a "head".
  It is also the newest changeset in the repository -- called "tip".

  If I shared C1 with you and you started your work from that, your
  commits will build a repository like this:

    C1 --> C2' --> C3'

  Here C3' is a head in your repository and I don't know anything
  about C2' and C3' yet.

* If I pull from you, or you push to me, the two repositories are
  compared. By default, all missing changesets are transferred. This
  is all there is to push/pull: compare two graphs of changesets and
  transfer the missing ones.

  After a pull from you my repository will look like this:

         /-> C2 --> C3
    C1 -<
         \-> C2' --> C3'

  Here C1 has two child changesets, and the repository has two heads
  since the development has diverged.

  The changeset C3' will be the new tip since it is the newest
  changeset in the repository. Note that tip is always a head, but a
  head need not be the tip.

* Having two heads suggest that someone should merge them -- otherwise
  the changes from one will never be combined with the changes made in
  the other head.

  When merging with 'hg merge' the task is to figure out the canonical
  way to combine the changesets. If the changes do not overlap this is
  usually trivial, otherwise you have to do a three-way merge. The
  merge must be committed and this creates a changeset which explains
  to the world how you think the two heads should be combined:

         /-> C2 --> C3   -\
    C1 -<                  >-> M
         \-> C2' --> C3' -/

  Note that the merge changeset M has two parents.

  If you do not merge C3 and C3' and try to push you get the 'new
  remote head' message and push aborts. It aborts since it is a little
  "impolite" to leave the job of merging to someone else -- he who
  created the two heads by pulling in some code should also normally
  do the merging.
  

> Sometimes it's hard to keep the several DVCS workings in my mind
  
  It helped me a lot to think in terms of the changeset graph. Remember
that:

  * "hg commit" adds a new node. The parent changesets of the new node
    is given by "hg parents"

  * "hg push" and "hg pull" transfer nodes in the graph between two
    repositories.

  * "hg update" updates the working copy to reflect a given node in
    the history graph. This also changes the parent changeset of the
    next commit, see "hg parents".

> Is there not a simple Mercurial cheat sheet somewhere?

There are some here:

http://www.selenic.com/mercurial/wiki/index.cgi/QuickReferenceCardsAndCheatSheets

- Martin Geisler

PS: These descriptions were written on the [Mercurial mailinglist](http://selenic.com/mailman/listinfo/mercurial). 
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