Version Control with Mercurial

Recovering Old Versions

Overview

Teaching: 10 min
Exercises: 0 min
Questions

  • How can I recover old versions of files?

Objectives

  • Restore older versions of files.

  • Use configuration aliases to create custom Mercurial commands.

All right: we can save changes to files and see what we’ve changed—how can we restore older versions of things? Let’s suppose we (somehow) accidentally overwrite Salish Sea NEMO forecast planning file with our grocery list:

$ nano plan.txt
$ cat plan.txt

Ricotta
Mushroom Tortellini
Bacon

hg status now tells us that the file has been changed, but those changes haven’t been committed:

$ hg status

M plan.txt

We can put things back the way they were by using hg revert:

$ hg revert plan.txt
$ cat plan.txt

Goal: Run NEMO everyday to forecast storm surge water levels

Need daily high resolution weather forcing from EC.
Also need daily average Fraser River flow from EC.

As you might guess from its name, hg revert reverts to (i.e. restores) an old version of a file. In this case, we’re telling Mercurial that we want to recover the last committed version of the file. If we want to go back even further, we can use the --rev or -r flag and a revision number instead:

$ hg revert --rev 0 plan.txt

Mercurial really doesn’t want to cause us to lose our work, so it defaults to making a backup when we use hg revert:

$ hg status

? plan.txt.orig

The plan.txt.orig file is a copy of plan.txt as it stood before the hg revert command. It’s not tracked by Mercurial. It’s just there in case we made a mistake and really didn’t want to revert, or in case there’s some content from before the revert that we decide that we really do want to copy into plan.txt. When we’re sure that we don’t need *.orig files we can just go ahead and delete them. If we really don’t want Mercurial to create *.orig files when we use hg revert, we can use the --no-backup option, or its short version -C.

The fact that files can be reverted one by one tends to change the way people organize their work. If everything is in one large document, it’s hard (but not impossible) to undo changes to the introduction without also undoing changes made later to the conclusion. If the introduction and conclusion are stored in separate files, on the other hand, moving backward and forward in time becomes much easier.

Using Aliases to Create New Mercurial Commands

If we decide that reverting without creating .orig backup file is something that we want to do regularly we can use hg config --edit to add

[alias]
nb-revert = revert --no-backup

to our Mercurial configuration settings. After we save the configuration file and exit from the editor we have a new command, hg nb-revert (no backup revert) that is the same as typing hg revert --no-backup. Note that it is good practice when creating aliases for commands to give them your own names instead of using the alias to redefine the built-in command. If we had used the alias to change add the --no-backup flag to the revert command

[alias]
revert = revert --no-backup

it would be difficult to get hg revert to create .orig backup files if we ever wanted it to.

Key Points