version control - Difference between Revert and Update in Mercurial

Question

I'm just getting started with Mercurial, and I've come across something which I don't understand.

I made changes to several files, and now I want to undo all the changes I made to one of them (i.e. go back to my last commit for one specific file).

As far as I can see, the command I want is revert.

In the page I linked to, there is the following statement:

This operation however does not change the parent revision of the working directory (or revisions in case of an uncommitted merge). To undo an uncomitted merge, you can use "hg update -C -r." which will reset the parents to the first parent.

I don't understand the difference between the two (hg revert vs. hg update -C -r). Can anyone enlighten me as to the difference? And in my case, do I really want the revert or the update to go get rid of the changes I made to the file?

Answer 1

The first difference is revert can work on a subset of the working copy while update works on the whole working copy. the other difference is in what happens when you want to go back to a version other than the last committed one.

if we have revisions (caps are committed, lower case are changes in the working copy, parent revision is C )

A-B-C-d

update -C -r B will give you

A-B-C

with your working copy set to B, any changes will result in branching from B (parent revision set to B)

A-B-C
  e

revert -r B will give you

A-B-C-b'

where b' is a set of changes which undoes everything in the intermediate committed changes, in this case it undoes all of C. any changes now just join the b' set (parent revision left unchanged at C)