Diff for "TutorialFirstChange"

Tutorial - Making our first change

(This page is part 4 of 9 of the Tutorial series. Previous part is TutorialHistory, next part is TutorialShareChange)

Carrying forward from TutorialHistory, we are inside our my-hello repository that we cloned in TutorialClone.

A good Mercurial development practice is to isolate related changes in a separate repository (see also WorkingPractices). This prevents unrelated code from getting mixed up, and makes it easier to test individual chunks of work one by one. Let's start out by following that model.

Our silly goal is to get the "hello, world" program to print another line of output. First, we create a new repository called my-hello-new-output, by cloning from my-hello, for our little project (using Mercurial 1.0):

$ cd ..
$ hg clone my-hello my-hello-new-output
updating working directory
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

Notice that we have given our new repository a descriptive name, basically identifying the purpose of the repository. Since making a clone of a repository in Mercurial is cheap, we will quickly accumulate many slightly different repositories. If we do not give these repositories descriptive names, we will rapidly lose the ability to tell them apart (see RepositoryNaming).

Now it's time to make a change in the new repository. Let's go into the repository's working directory, which is simply our name for the directory where all the files are, and modify the source code with our favorite editor:

$ cd my-hello-new-output
$ vi hello.c

The contents of hello.c initially look like this:

/*
 * hello.c
 *
 * Placed in the public domain by Bryan O'Sullivan
 *
 * This program is not covered by patents in the United States or other
 * countries.
 */

#include <stdio.h>

int main(int argc, char **argv)
{
        printf("hello, world!\n");
        return 0;
}

Let's edit main so that it prints an extra line of output:

(...)

int main(int argc, char **argv)
{
        printf("hello, world!\n");
        printf("sure am glad I'm using Mercurial!\n");
        return 0;
}

Once we're done, we quit out of our favorite editor, and we're done. That's it. The edit is now ready for us to create a changeset.

But what if we're interrupted, and we've forgotten what changes are going to make it into the changeset once we create it? For this, we use the status command.

$ hg status
M hello.c

The output is terse, but prefix M is simply telling us that hello.c has been modified, so our change is ready to go into a changeset.

Instead of the long-winded hg status we can alternatively just type hg st, as Mercurial allows us to abbreviate commands as long as the entered character sequence is not ambiguous.

$ hg st
M hello.c

We may also examine the actual changes we have made to the files using the diff command:

$ hg diff
diff -r 82e55d328c8c hello.c
--- a/hello.c   Fri Aug 26 01:21:28 2005 -0700
+++ b/hello.c   Mon May 05 00:27:56 2008 +0200
@@ -12,5 +12,6 @@
 int main(int argc, char **argv)
 {
        printf("hello, world!\n");
+       printf("sure am glad I'm using Mercurial!\n");
        return 0;
 }

In case we wish to discard our changes and start over, we may use the revert command to restore hello.c to its unmodified state (or use the --all option to revert all files). Just make sure you know this is what you really want (see Revert).

$ hg revert hello.c

revert renames the modified file hello.c to hello.c.orig and restores hello.c to its unmodified state.

status now lists hello.c.orig as not tracked (prefix "?").

$ hg st
? hello.c.orig

If we change our mind again and want to reuse the modification we have made, we just remove the unmodified state of hello.c and rename the modified hello.c.orig to hello.c

$ rm hello.c
$ mv hello.c.orig hello.c
$ hg st
M hello.c

The act of creating a changeset is called committing it. We perform a commit using the commit command. The commit command has a nice short alias: ci ("check in"), so we can use that:

$ hg ci

This drops us into an editor, and presents us with a few cryptic lines of text.

Note: The default editor is vi. This can be changed using the EDITOR or HGEDITOR environment variables.

HG: Enter commit message.  Lines beginning with 'HG:' are removed.
HG: --
HG: user: mpm@selenic.com
HG: branch 'default'
HG: changed hello.c

The first line is empty and the lines that follow identify the user, branch name and the files that will go into this changeset.

The default branch name is "default" (see NamedBranches). The default value for "user" is taken from the ~/.hgrc configuration file from value "username" of section "ui" (see hgrc(5)). Alternatively, it can also be specified on the command line with option -u (see hg help ci or hg.1.html#commit).

To commit the changeset, we must describe the reason for it (see ChangeSetComments). Let's type something like this:

Express great joy at existence of Mercurial
HG: Enter commit message.  Lines beginning with 'HG:' are removed.
HG: --
HG: user: mpm@selenic.com
HG: branch 'default'
HG: changed hello.c

Next, we save the text and quit the editor, and, if all goes well, the commit command will exit and prints no output.

If you quit the editor without saving the text or enter no text, commit will abort the operation, so you may change your mind before committing.

Let's see what the status command will tell us now?

$ hg st

Nothing! Our change has been committed to a changeset, so there's no modified file in need of a commit. Our tip now matches our working directory contents.

The parents command shows us that our repository's working directory is now synced (see Update) to the newly committed changeset (here we have only one parent revision, which is always the case after a commit. We will see two parents in TutorialMerge):

$ hg par
changeset:   2:86794f718fb1
tag:         tip
user:        mpm@selenic.com
date:        Mon May 05 01:20:46 2008 +0200
summary:     Express great joy at existence of Mercurial

There it is! We've committed a new changeset.

We can now examine the change history for our new work:

$ hg log
changeset:   2:86794f718fb1
tag:         tip
user:        mpm@selenic.com
date:        Mon May 05 01:20:46 2008 +0200
summary:     Express great joy at existence of Mercurial

(...)

Note: The user, date, and changeset ID will of course vary.

As we discussed in TutorialClone, the new changeset only exists in this repository. This is a critical part of the way Mercurial works.

To share changes, we must continue to TutorialShareChange.

CategoryTutorial