book Basic Snapshotting

Git is all about composing and saving snapshots of your project and then working with and comparing those snapshots. This section will explain the commands needed to compose and commit snapshots of your project.

An important concept here is that Git has an 'index', which acts as sort of a staging area for your snapshot. This allows you to build up a series of well composed snapshots from changed files in your working directory, rather than having to commit all of the file changes at once.

In a nutshell, you will use git add to start tracking new files and also to stage changes to already tracked files, then git status and git diff to see what has been modified and staged and finally git commit to record your snapshot into your history. This will be the basic workflow that you use most of the time.

docs book git add adds file contents to the staging area

In Git, you have to add file contents to your staging area before you can commit them. If the file is new, you can run git add to initially add the file to your staging area, but even if the file is already "tracked" - ie, it was in your last commit - you still need to call git add to add new modifications to your staging area. Let's see a few examples of this.

Going back to our Hello World example, once we've initiated the project, we would now start adding our files to it and we would do that with git add. We can use git status to see what the state of our project is.

$ git status -s
?? README
?? hello.rb

So right now we have two untracked files. We can now add them.

$ git add README hello.rb

Now if we run git status again, we'll see that they've been added.

$ git status -s
A  README
A  hello.rb

It is also common to recusively add all files in a new project by specifying the current working directory like this: git add .. Since Git will recursively add all files under a directory you give it, if you give it the current working directory, it will simply start tracking every file there. In this case, a git add . would have done the same thing as a git add README hello.rb, or for that matter so would git add *, but that's only because we don't have subdirectories which the * would not recurse into.

OK, so now if we edit one of these files and run git status again, we will see something odd.

$ vim README
$ git status -s
AM README
A  hello.rb

The 'AM' status means that the file has been modified on disk since we last added it. This means that if we commit our snapshot right now, we will be recording the version of the file when we last ran git add, not the version that is on our disk. Git does not assume that what the file looks like on disk is neccesarily what you want to snapshot - you have to tell Git with the git add command.

In a nutshell, you run git add on a file when you want to include whatever changes you've made to it in your next commit snapshot. Anything you've changed that is not added will not be included - this means you can craft your snapshots with a bit more precision than most other SCM systems.

For a very interesting example of using this flexibility to stage only parts of modified files at a time, see the '-p' option to git add in the Pro Git book.

docs book git status view the status of your files in the working directory and staging area

As you saw in the git add section, in order to see what the status of your staging area is compared to the code in your working directory, you can run the git status command. I demonstrated using it with the -s option, which gives you short output. Without that flag, the git status command will give you more context and hints. Here is the same status output with and without the -s. The short output looks like this:

$ git status -s
AM README
A  hello.rb

Where the same status with the long output looks like this:

$ git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached ..." to unstage)
#
# new file:   README
# new file:   hello.rb
#
# Changed but not updated:
#   (use "git add ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
# modified:   README
#

You can easily see how much more compact the short output is, but the long output has useful tips and hints as to what commands you may want to use next.

Git will also tell you about files that were deleted since your last commit or files that were modified or staged since your last commit.

$ git status -s
M  README
 D hello.rb

You can see there are two columns in the short status output. The first column is for the staging area, the second is for the working directory. So for example, if you have the README file staged and then you modify it again without running git add a second time, you'll see this:

$ git status -s
MM README
 D hello.rb

In a nutshell, you run git status to see if anything has been modified and/or staged since your last commit so you can decide if you want to commit a new snapshot and what will be recorded in it.

docs book git diff shows diff of what is staged and what is modified but unstaged

There are two main uses of the git diff command. One use we will describe here, the other we will describe later in the "Inspection and Comparison" section. The way we're going to use it here is to describe the changes that are staged or modified on disk but unstaged.

git diff show diff of unstaged changes

Without any extra arguments, a simple git diff will display in unified diff format (a patch) what code or content you've changed in your project since the last commit that are not yet staged for the next commit snapshot.

$ vim hello.rb
$ git status -s
 M hello.rb
$ git diff
diff --git a/hello.rb b/hello.rb
index d62ac43..8d15d50 100644
--- a/hello.rb
+++ b/hello.rb
@@ -1,7 +1,7 @@
 class HelloWorld
   
   def self.hello
-    puts "hello world"
+    puts "hola mundo"
   end
 
 end

So where git status will show you what files have changed and/or been staged since your last commit, git diff will show you what those changes actually are, line by line. It's generally a good follow-up command to git status

git diff --cached show diff of staged changes

The git diff --cached command will show you what contents have been staged. That is, this will show you the changes that will currently go into the next commit snapshot. So, if you were to stage the change to hello.rb in the example above, git diff by itself won't show you any output because it will only show you what is not yet staged.

$ git status -s
 M hello.rb
$ git add hello.rb 
$ git status -s
M  hello.rb
$ git diff
$

If you want to see the staged changes, you can run git diff --cached instead.

$ git status -s
M  hello.rb
$ git diff
$ 
$ git diff --cached
diff --git a/hello.rb b/hello.rb
index d62ac43..8d15d50 100644
--- a/hello.rb
+++ b/hello.rb
@@ -1,7 +1,7 @@
 class HelloWorld
   
   def self.hello
-    puts "hello world"
+    puts "hola mundo"
   end
 
 end

git diff HEAD show diff of all staged or unstaged changes

If you want to see both staged and unstaged changes together, you can run git diff HEAD - this basically means you want to see the difference between your working directory and the last commit, ignoring the staging area. If we make another change to our hello.rb file then we'll have some changes staged and some changes unstaged. Here are what all three diff commands will show you:

$ vim hello.rb 
$ git diff
diff --git a/hello.rb b/hello.rb
index 4f40006..2ae9ba4 100644
--- a/hello.rb
+++ b/hello.rb
@@ -1,7 +1,7 @@
 class HelloWorld
   
+  # says hello
   def self.hello
     puts "hola mundo"
   end
 
 end
$ git diff --cached
diff --git a/hello.rb b/hello.rb
index 2aabb6e..4f40006 100644
--- a/hello.rb
+++ b/hello.rb
@@ -1,7 +1,7 @@
 class HelloWorld
 
   def self.hello
-    puts "hello world"
+    puts "hola mundo"
   end
 
 end
$ git diff HEAD
diff --git a/hello.rb b/hello.rb
index 2aabb6e..2ae9ba4 100644
--- a/hello.rb
+++ b/hello.rb
@@ -1,7 +1,8 @@
 class HelloWorld
 
+  # says hello
   def self.hello
-    puts "hello world"
+    puts "hola mundo"
   end
 
 end

git diff --stat show summary of changes instead of a full diff

If we don't want the full diff output, but we want more than the git status output, we can use the --stat option, which will give us a summary of changes instead. Here is the same example as above, but using the --stat option instead.

$ git status -s
MM hello.rb
$ git diff --stat
 hello.rb |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
$ git diff --cached --stat
 hello.rb |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
$ git diff HEAD --stat
 hello.rb |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

You can also provide a file path at the end of any of these options to limit the diff output to a specific file or subdirectory.

In a nutshell, you run git diff to see details of the git status command - how files have been modified or staged on a line by line basis.

docs book git commit records a snapshot of the staging area

Now that you have staged the content you want to snapshot with the git add command, you run git commit to actually record the snapshot. Let's stage and commit all the changes to our hello.rb file. In this first example, we'll use the -m option to provide the commit message on the command line.

$ git add hello.rb 
$ git status -s
M  hello.rb
$ git commit -m 'my hola mundo changes'
[master 68aa034] my hola mundo changes
 1 files changed, 2 insertions(+), 1 deletions(-)

Now we have recorded the snapshot. If we run git status again, we will see that we have a "clean working directory", which means that we have not made any changes since our last commit - there is no un-snapshotted work in our checkout.

$ git status
# On branch master
nothing to commit (working directory clean)

If you leave off the -m option, Git will try to open a text editor for you to write your commit message. In vim, which it will default to if it can find nothing else in your settings, the screen might look something like this:


# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
# modified:   hello.rb
#
~
~
".git/COMMIT_EDITMSG" 9L, 257C

At this point you add your actual commit message at the top of the document. Any lines starting with '#' will be ignored - Git will put the output of the git status command in there for you as a reminder of what you have modified and staged.

In general, it's very important to write a good commit message. For open source projects, it's generally a rule to write your message more or less in this format:

Short (50 chars or less) summary of changes

More detailed explanatory text, if necessary.  Wrap it to about 72
characters or so.  In some contexts, the first line is treated as the
subject of an email and the rest of the text as the body.  The blank
line separating the summary from the body is critical (unless you omit
the body entirely); some git tools can get confused if you run the
two together.

Further paragraphs come after blank lines.

 - Bullet points are okay, too

 - Typically a hyphen or asterisk is used for the bullet, preceded by a
   single space, with blank lines in between, but conventions vary
   here

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
# modified:   hello.rb
#
~
~
~
".git/COMMIT_EDITMSG" 25L, 884C written

The commit message is very important. Since much of the power of Git is this flexibility in carefully crafting commits locally and then sharing them later, it is very powerful to be able to write three or four commits of logically seperate changes so that your work may be more easily peer reviewed. Since there is a seperation between committing and pushing those changes, do take the time to make it easier for the people you are working with to see what you've done by putting each logically seperate change in a seperate commit with a nice commit message so it is easier for them to see what you are doing and why.

git commit -a automatically stage all tracked, modified files before the commit

If you think the git add stage of the workflow is too cumbersome, Git allows you to skip that part with the -a option. This basically tells Git to run git add on any file that is "tracked" - that is, any file that was in your last commit and has been modified. This allows you to do a more Subversion style workflow if you want, simply editing files and then running git commit -a when you want to snapshot everything that has been changed. You still need to run git add to start tracking new files, though, just like Subversion.

$ vim hello.rb
$ git status -s
 M  hello.rb
$ git commit -m 'changes to hello file'
# On branch master
# Changed but not updated:
#   (use "git add ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
# modified:   hello.rb
#
no changes added to commit (use "git add" and/or "git commit -a")
$ git commit -am 'changes to hello file'
[master 78b2670] changes to hello file
 1 files changed, 2 insertions(+), 1 deletions(-)

Notice how if you don't stage any changes and then run git commit, Git will simply give you the output of the git status command, reminding you that nothing is staged. I've highlighted the important part of that message, saying that nothing is added to be committed. If you use -a, it will add and commit everything at once.

This now lets you complete the entire snapshotting workflow - you make changes to your files, then use git add to stage files you want to change, git status and git diff to see what you've changed, and then finally git commit to actually record the snapshot forever.

In a nutshell, you run git commit to record the snapshot of your staged content. This snapshot can then be compared, shared and reverted to if you need to.

docs book git reset HEAD unstage changes that you have staged

git reset is probably the most confusing command written by humans. I've been using Git for years, even wrote a book on it and I still get confused by what it is going to do at times. So, I'll just tell you the three specific invocations of it that are generally helpful and ask you to blindly use it as I do - because it can be very useful.

In this case, we can use it to unstage something that you have accidentally staged. Let's say that you have modified two files and want to record them into two different commits. You should stage and commit one, then stage and commit the other. If you accidentally stage both of them, how do you un-stage one? You do it with git reset HEAD -- file. Technically here you don't have to add the -- - it is used to tell Git when you have stopped listing options and are now listing file paths, but it's probably good to get into the habit of using it to seperate options from paths even if you don't need to.

So, let's see what it looks like to unstage something. Here we have two files that have been modified since our last commit. We will stage both, then unstage one of them.

$ git status -s
 M README
 M hello.rb
$ git add .
$ git status -s
M  README
M  hello.rb
$ git reset HEAD -- hello.rb 
Unstaged changes after reset:
M hello.rb
$ git status -s
M  README
 M hello.rb

Now you can run a git commit which will just record the changes to the README file, not the now unstaged hello.rb.



    
    In case you're curious, what it's actually doing here is it is resetting
    the checksum of the entry for that file in the "index" to be what it was
    in the last commit.  Since git add checksums a file and adds
    it to the "index", git reset HEAD overwrites that with what
    it was before, thereby effectively unstaging it.
    

    
    If you want to be able to just run git unstage, you can easily
    setup an alias in Git.  Just run 
    git config --global alias.unstage "reset HEAD".  
    Once you have run that, you can then just run 
    git unstage [file] instead.
    

    If you forget the command to unstage something, Git is helpful in
       reminding you in the output of the normal git status
       command.  For example, if you run git status without
       the -s when you have staged files, it will tell you
       how to unstage them:

$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#   modified:   README
#   modified:   hello.rb
#


    
    In a nutshell, 
    you run git reset HEAD to unstage files that you previously
    ran git add on and wish to not include in the next commit
    snapshot