Tracking Changes to Files


Teaching: 10 min
Exercises: 5 min
  • How do I record changes in Mercurial?

  • How do I check the status of my version control repository?

  • How do I record notes about what changes I made and why?

  • Display the version control status of files in a repository and explain what those statuses mean.

  • Add files to Mercurial’s collection of tracked files.

  • Record metadata about changes to a file.

  • Display the history of changes to files in a repository and explain the metadata that is recorded with each changeset.

Let’s create a file called plan.txt in which Susan is going to write her initial ideas and notes about the Salish Sea NEMO daily forecast system. You can use any text editor you want (Notepad, TextEdit, gedit, nano, emacs, vi, …) but please don’t use a word processor like Microsoft Word or LibreOffice Write. In particular, the editor does not have to be the one that you set in your Mercurial configuration earlier. In what follows it is assumed that nano is used.

$ nano plan.txt

and type into that file:

Goal: Run NEMO everyday to forecast storm surge water levels

plan.txt has now been created and it contains a single line:

$ ls
$ cat plan.txt
Goal: Run NEMO everyday to forecast storm surge water levels

We can ask Mercurial to tell us what it knows about the files in our project with the hg status command. Mercurial tells us that it has noticed the new file:

$ hg status
? plan.txt

The ? at the beginning of the line means that Mercurial isn’t keeping track of the file. We can tell Mercurial that it should do so using hg add:

$ hg add plan.txt

and then check that the right thing happened:

$ hg status
A plan.txt

Mercurial now knows that it’s supposed to keep track of plan.txt, but it hasn’t yet recorded any changes for posterity as a commit. To get it to do that, we need to run one more command:

$ hg commit -m "Starting to plan the daily NEMO forecast system."

When we run hg commit, Mercurial takes the file we have told it about by using hg add and stores a copy permanently inside the special .hg directory. That permanent copy is called a commit (or revision).

We use the -m flag (for “message”) to record a comment that will help us remember later on what we did and why. If we just run hg commit without the -m option, Mercurial will launch nano (or whatever other editor we configured at the start) so that we can write a longer message.

Good commit messages start with a brief (<70 characters) summary of changes made in the commit. If you want to go into more detail, add a blank line between the summary line and your additional notes.

If we run hg status now:

$ hg status

we get no output because everything is up to date.

If we want to know what we’ve done recently, we can ask Mercurial to show us the project’s history using hg log:

$ hg log
changeset:   0:1320339bbcae
tag:         tip
user:        Susan Allen <>
date:        Tue Jun 09 14:41:27 2015 +0200
summary:     Starting to plan the daily NEMO forecast system.

hg log lists all changes committed to a repository, starting with the most recent. The listing for each changeset includes:

The revision number is a convenient integer shorthand for the hexadecimal identifier.

Where Are My Changes?

If we run ls at this point, we will still see just one file called plan.txt. That’s because Mercurial saves information about files’ history in the special .hg directory mentioned earlier so that our filesystem doesn’t become cluttered (and so that we can’t accidentally edit or delete an old version).

Key Points

  • hg status shows the status of a repository.

  • hg add puts files in the staging area.

  • hg commit saves the staged content as a new commit in the local repository.

  • Always write a log message when committing changes.