Pro Git Chapter 2: Git Basics

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Git Basics

Getting a Git Repository

You can get a Git project using two main approaches. The first takes an existing project or directory and imports it into Git. The second clones an existing Git repository from another server.

Initializing a Repository in an Existing Directory

If you’re starting to track an existing project in Git, you need to go to the project’s directory and type

$ git init 

This creates a new sub directory named .git that contains all of your necessary repository files — a Git repository skeleton. At this point, nothing in your project is tracked yet. (See Chapter 9 for more information about exactly what files are contained in the .git directory you just created.)

If you want to start version-controlling existing files (as opposed to an empty directory), you should probably begin tracking those files and do an initial commit. You can accomplish that with a few git add commands that specify the files you want to track, followed by a commit:

$ git add *.c

$ git add README

$ git commit m ‘initial project version'

We’ll go over what these commands do in just a minute. At this point, you have a Git repository with tracked files and an initial commit.

Cloning an Existing Repository

If you want to get a copy of an existing Git repository — for example, a project you’d like to contribute to — the command you need is git clone. If you’re familiar with other VCS systems such as Subversion, you’ll notice that the command is clone and not checkout. This is an important distinction — Git receives a copy of nearly all data that the server has. Every version of every file for the history of the project is pulled down when you run git clone. In fact, if your server disk gets corrupted, you can use any of the clones on any client to set the server back to the state it was in when it was cloned (you may lose some server-side hooks and such, but all the versioned data would be there—see Chapter 4 for more details). You clone a repository with git clone [url]. For example, if you want to clone the Ruby Git library called Grit, you can do so like this:

$ git clone git://github.com/wingadium1/grit.git 

That creates a directory named grit, initializes a .git directory inside it, pulls down all the data for that repository, and checks out a working copy of the latest version. If you go into the new grit directory, you’ll see the project files in there, ready to be worked on or used. If you want to clone the repository into a directory named something other than grit, you can specify that as the next command-line option:

$ git clone git://github.com/wingadium1/grit.git mygrit

That command does the same thing as the previous one, but the target directory is called mygrit. Git has a number of different transfer protocols you can use. The previous example uses the git:// protocol, but you may also see http(s):// or [email protected]:/path.git, which uses the SSH transfer protocol. Chapter 4 will introduce all of the available options the server can set up to access your Git repository and the pros and cons of each.

Recording Changes to the Repository

You have a bona fide Git repository and a checkout or working copy of the files for that project. You need to make some changes and commit snapshots of those changes into your repository each time the project reaches a state you want to record.

Remember that each file in your working directory can be in one of two states: tracked or untracked. Tracked files are files that were in the last snapshot; they can be unmodified, modified, or staged. Untracked files are everything else – any files in your working directory that were not in your last snapshot and are not in your staging area. When you first clone a repository, all of your files will be tracked and unmodified because you just checked them out and haven’t edited anything.

As you edit files, Git sees them as modified, because you’ve changed them since your last commit. You stage these modified files and then commit all your staged changes, and the cycle repeats. This lifecycle is illustrated in Figure 2.1.

Figure 2.1 The lifecycle of the status of your files

Checking the Status of Your Files

The main tool you use to determine which files are in which state is the git status command. If you run this command directly after a clone, you should see something like this:

$ git status

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

This means you have a clean working directory—in other words, there are no tracked and modified files. Git also doesn’t see any untracked files, or they would be listed here. Finally, the command tells you which branch you’re on. For now, that is always master, which is the default; you won’t worry about it there. Then next chapter will go over branches and references in detail. Let’s say you add a new file to your project, a simple README file. If the file didn’t exist before, and you run git status, you see your untracked file like so:

$ vim README

$ git status
# On branch master
# Untracked files:
# (use "git add <file>..." to include in what will be committed)
#
# README
nothing added to commit but untracked files present (use "git add" to track)

You can see that your new README file is untracked, because it’s under the "Untracked files" heading in your status output. Untracked basically means that Git sees a file you didn’t have in the previous snapshot (commit); Git won’t start including it in your commit snapshots until you explicitly tell it to do so. It does this so you don’t accidentally begin including generated binary files or other files that you did not mean to include. You do want to start including README, so let’s start tracking the file.

Tracking New Files

In order to begin tracking a new file, you use the command git add. To begin tracking the README file, you can run this:

$ git add README

If you run your status command again, you can see that your README file is now tracked and staged:

$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# new file: README
#

You can tell that it’s staged because it’s under the "Changes to be committed" heading. If you commit at this point, the version of the file at the time you ran git add is what will be in the historical snapshot. You may recall that when you ran git init earlier, you then ran git add (files) — that was to begin tracking files in your directory. The git add command takes a path name for either a file or a directory; if it’s a directory, the command adds all the files in that directory recursively.

Staging Modified Files

Let’s change a file that was already tracked. If you change a previously tracked file called benchmarks.rb and then run your status command again, you get something that looks like this:

$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# new file: README
#
# Changed but not updated:
# (use "git add <file>..." to update what will be committed)
#
# modified: benchmarks.rb
#

The benchmarks.rb file appears under a section named "Changed but not updated" — which means that a file that is tracked has been modified in the working directory but not yet staged. To stage it, you run the git add command (it’s a multipurpose command—you use it to begin tracking new files, to stage files, and to do other things like marking merge-conflicted files as resolved). Let’s run git add now to stage the benchmarks.rb file, and then run git status again:

$ git add benchmarks.rb

$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# new file: README
# modified: benchmarks.rb
#

Both files are staged and will go into your next commit. At this point, suppose you remember one little change that you want to make in benchmarks.rb before you commit it. You open it again and make that change, and you’re ready to commit. However, let’s run git status one more time:

$ vim benchmarks.rb

$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# new file: README
# modified: benchmarks.rb
#
# Changed but not updated:
# (use "git add <file>..." to update what will be committed)
#
# modified: benchmarks.rb
#

What the heck? Now benchmarks.rb is listed as both staged and unstaged. How is that possible? It turns out that Git stages a file exactly as it is when you run the git add command. If you commit now, the version of benchmarks.rb as it was when you last ran the git add command is how it will go into the commit, not the version of the file as it looks in your working directory when you run git commit. If you modify a file after you run git add, you have to run git add again to stage the latest version of the file:

$ git add benchmarks.rb

$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# new file: README
# modified: benchmarks.rb
#

Ignoring Files

Often, you’ll have a class of files that you don’t want Git to automatically add or even show you as being untracked. These are generally automatically generated files such as log files or files produced by your build system. In such cases, you can create a file listing patterns to match them named .gitignore. Here is an example .gitignore file:

$ cat .gitignore *.[oa] *˜

The first line tells Git to ignore any files ending in .o or .a — object and archive files that may be the product of building your code. The second line tells Git to ignore all files that end with a tilde ( ), which is used by many text editors such as Emacs to mark temporary files. You may also include a log, tmp, or pid directory; automatically generated documentation; and soon. Setting up a .gitignore file before you get going is generally a good idea so you don’t accidentally commit files that you really don’t want in your Git repository. The rules for the patterns you can put in the .gitignore file are as follows:

  • Blank lines or lines starting with # are ignored.
  • Standard glob patterns work.
  • You can end patterns with a forward slash (/) to specify a directory.
  • You can negate a pattern by starting it with an exclamation point (!). Glob patterns are like simplified regular expressions that shells use. An asterisk (*) matches zero or more characters; [abc] matches any character inside the brackets (in this case a, b, or c); a question mark (?) matches a single character; and brackets enclosing characters seperated by a hyphen([0-9]) matches any character between them (in this case 0 through 9). Here is another example .gitignore file:
# a comment this is ignored 
*.a 
# no .a files 
!lib.a
# but do track lib.a, even though you're ignoring .a files above 
/TODO
# only ignore the root TODO file, not subdir/TODO 
build/
# ignore all files in the build/ directory 
doc/*.txt
# ignore doc/notes.txt, but not doc/server/arch.txt

Viewing Your Staged and Unstaged Changes

If the git status command is too vague for you — you want to know exactly what youchanged,notjustwhichfileswerechanged—youcanusethegit diffcommand. We’ll cover git diff in more detail later; but you’ll probably use it most often to answer these two questions: What have you changed but not yet staged? And what have you staged that you are about to commit? Although git status answers those questions very generally, git diff shows you the exact lines added and removed — the patch, as it were. Let’s say you edit and stage the README file again and then edit the benchmarks.rb file without staging it. If you run your status command, you once again see something like this:

$ git status 
# On branch master 
# Changes to be committed: 
#   (use "git reset HEAD <file>..." to unstage)
# 
# new file: README 
# 
# Changed but not updated: 
#   (use "git add <file>..." to update what will be committed) 
# 
# modified: benchmarks.rb #

To see what you’ve changed but not yet staged, type git diff with no other arguments:

$ git diff
diff --git a/benchmarks.rb b/benchmarks.rb 
index 3cb747f..da65585 100644 
--- a/benchmarks.rb 
+++ b/benchmarks.rb 
@@ -36,6 +36,10 @@ def main 
                @commit.parents[0].parents[0].parents[0] 
            end

+           run_code(x, 'commits 1') do 
+               git.commits.size 
+           end 
+           run_code(x, 'commits 2') do 
                log = git.commits('master', 15) 
                log.size

That command compares what is in your working directory with what is in your staging area. The result tells you the changes you’ve made that you haven’t yet staged. If you want to see what you’ve staged that will go into your next commit, you can use git diff -cached. (In Git versions 1.6.1 and later, you can also use git diff -staged, which may be easier to remember.) This command compares your staged changes to your last commit:

$ git diff --cached 
diff --git a/README b/README 
new file mode 100644 
index 0000000..03902a1 
--- /dev/null 
+++ b/README2 
@@ -0,0 +1,5 @@ 
+ grit 
+ by Tom Preston-Werner, Chris Wanstrath 
+ http://github.com/mojombo/grit 
+ 
+ Grit is a Ruby library for extracting information from a Git repository

It’s important to note that git diff by itself doesn’t show all changes made since your last commit — only changes that are still unstaged. This can be confusing, because if you’ve staged all of your changes, git diff will give you no output.

For another example, if you stage the benchmarks.rb file and then edit it, you can use git diff to see the changes in the file that are staged and the changes that are unstaged:

$ git add benchmarks.rb 
$ echo '# test line' >> benchmarks.rb 
$ git status 
# On branch master 
# 
# Changes to be committed: 
# 
# modified: benchmarks.rb 
# 
# Changed but not updated: 
# 
# modified: benchmarks.rb 
#

Now you can use git diff to see what is still unstaged

$ git diff 
diff --git a/benchmarks.rb b/benchmarks.rb 
index e445e28..86b2f7c 100644 
--- a/benchmarks.rb 
+++ b/benchmarks.rb 
@@ -127,3 +127,4 @@ end 
    main()

    ##pp Grit::GitRuby.cache_client.stats 
+  
# test line

and git diff –cached to see what youve staged so far:

$ git diff --cached 
diff --git a/benchmarks.rb b/benchmarks.rb 
index 3cb747f..e445e28 100644 
--- a/benchmarks.rb 
+++ b/benchmarks.rb 
@@ -36,6 +36,10 @@ def main 
                @commit.parents[0].parents[0].parents[0] 
            end

+           run_code(x, 'commits 1') do 
+               git.commits.size 
+           end 
+           run_code(x, 'commits 2') 
                do log = git.commits('master', 15) 
                log.size

Committing Your Changes

Now that your staging area is setup the way you want it, you can commit your changes. Remember that any thing that is still unstaged — any files you have created or modified that you haven’t run git add on since you edited them — won’t go into this commit.

They will stay as modified files on your disk. In this case, the last time you ran git status, you saw that everything was staged, so you’re ready to commit your changes. The simplest way to commit is to type git commit:

$ git commit

Doing so launches your editor of choice. (This is set by your shell’s $EDITOR environment variable — usually vim or emacs, although you can configure it with whatever you want using the git config --global core.editor command as you saw in Chapter 1.

The editor displays the following text (this example is a Vim screen):

# 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 <file>..." to unstage)
# 
#       new file: README
#       modified: benchmarks.rb 
˜ 
˜ 
˜ 
".git/COMMIT_EDITMSG" 10L, 283C

You can see that the default commit message contains the latest output of the git status command commented out and one empty line on top. You can remove these comments and type your commit message, or you can leave them there to help you remember what you’re committing. (For a nevenmore explicit reminder of what you’ve modified, you can pass the -v option to git commit. Doing so also puts the diff of your change in the editor so you can see exactly what you did.) When you exit the editor, Git creates your commit with that commit message (with the comments and diff stripped out). Alternatively, you can type your commit message inline with the commit command by specifying it after a -m flag, like this:

$ git commit -m "Story 182: Fix benchmarks for speed" 
[master]: created 463dc4f: "Fix benchmarks for speed" 
 2 files changed, 3 insertions(+), 0 deletions(-) 
 create mode 100644 README

Now you’ve created your first commit! You can see that the commit has given you some output about itself: which branch you committed to (master), what SHA–1 checksumthecommithas(463dc4f),howmanyfileswerechanged,andstatisticsabout lines added and removed in the commit. Remember that the commit records the snapshot you set up in your staging area. Anything you didn’t stage is still sitting there modified; you can do another commit to add it to your history. Every time you perform a commit, you’re recording a snapshot of your project that you can revert to or compare to later.

Skipping the Staging Area

Although it can be amazingly useful for crafting commits exactly how you want them, the staging area is sometimes a bit more complex than you need in your workflow. If you want to skip the staging area, Git provides a simple shortcut. Providing the -a option to the git commit command makes Git automatically stage every file that is already tracked before doing the commit, letting you skip the git add part:

$ git status
# On branch master
# 
# Changed but not updated:
# 
# modified: benchmarks.rb
# 
$ git commit -a -m 'added new benchmarks' 
[master 83e38c7] added new benchmarks
 1 files changed, 5 insertions(+),
 0 deletions(-)

Notice how you don’t have to run git add on the benchmarks.rb file in this case before you commit.

RemovingFiles

To remove a file from Git, you have to remove it from your tracked files (more accurately, remove it from your staging area) and then commit. The git rm command does that and also removes the file from your working directory so you don’t see it as an untracked file next time around.

If you simply remove the file from your working directory, it shows up under the "Changed but not updated" (that is, unstaged) area of your git status output:

$ rm grit.gemspec $ git status
# On branch master
# 
# Changed but not updated:
# (use "git add/rm <file>..." to update what will be committed)
# 
# deleted: grit.gemspec 
#

Then, if you run git rm, it stages the file’s removal:

$ git rm grit.gemspec rm 'grit.gemspec' $ git status
# On branch master
# 
# Changes to be committed:
#    (use "git reset HEAD <file>..." to unstage)
# 
#       deleted: grit.gemspec 
#

The next time you commit, the file will be gone and no longer tracked. If you modified the file and added it to the index already,you must force the removal with the -f option. This is a safety feature to prevent accidental removal of data that hasn’t yet been recorded in a snapshot and that can’t be recovered from Git.

Another useful thing you may want to do is to keep the file in your working tree but remove it from your staging area. In other words, you may want to keep the file on your hard drive but not have Git track it anymore. This is particularly useful if you forgot to add something to your .gitignore file and accidentally added it, like a large log file or a bunch of .a compiled files. To do this, use the --cached option:

$ git rm --cached readme.txt

You can pass files, directories, and file-glob patterns to the git rm command. That means you can do things such as

$ git rm log/\*.log 

Note the backslash () in front of the *. This is necessary because Git does its own file name expansion in addition to your shell’s file name expansion. This command removes all files that have the .log extension in the log/ directory. Or, you can do something like this:

$ git rm \*.txt

This command removes all files that end with .txt

Moving Files

Unlike many other VCS systems, Git doesn’t explicitly track file movement. If you rename a file in Git, no metadata is stored in Git that tells it you renamed the file. However, Git is pretty smart about figuring that out after the fact — we’ll deal with detecting file movement a bit later. Thus it’s a bit confusing that Git has a mv command. If you want to rename a file in Git, you can run something like

$ git mv file_from file_to

and it works fine. In fact, if you run something like this and look at the status, you’ll see that Git considers it a renamed file:

$ git mv README.txt README $ git status
# On branch master
# Your branch is ahead of 'origin/master' by 1 commit.
# 
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
# 
# renamed: README.txt -> README #

However, this is equivalent to running something like this:

$ mv README.txt README
$ git rm README.txt
$ git add README

Git figures out that it’s are name implicitly, so it doesn’t matter if you rename a file that way or with the mv command. The only real difference is that mv is one command instead of three — it’s a convenience function. More important, you can use any tool you like to rename a file, and address the add/rm later, before you commit.

Viewing the Commit History

After you have created several commits, or if you have cloned a repository with an existing commit history, you’ll probably want to look back to see what has happened. The most basic and powerful tool to do this is the git log command.

These examples use a very simple project called simplegit that I often use for demonstrations. To get the project, run

git clone git://github.com/wingadium1/simplegit-progit.git

When you run git log in this project, you should get output that looks something like this:

$ git log 
commit ca82a6dff817ec66f44342007202690a93763949
Author: Hoang Thanh Son <[email protected]>
Date: Mon Mar 17 21:52:11 2008 -0700

    changed the verison number

commit 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
Author: Hoang Thanh Son <[email protected]>
Date: Sat Mar 15 16:40:33 2008 -0700

    removed unnecessary test code

commit a11bef06a3f659402fe7563abf99ad00de2209e6
Author: Hoang Thanh Son <[email protected]>
Date: Sat Mar 15 10:31:28 2008 -0700

    first commit

By default, with no arguments, git log lists the commits made in that repository in reverse chronological order. That is, the most recent commits show up first. As you cansee, this command lists each commit with its SHA–1 checksum, the author’s name and e-mail, the date written, and the commit message. A huge number and variety of options to the git log command are available to show you exactly what you’re looking for. Here, we’ll show you some of the mostused options. One of the more helpful options is -p, which shows the diff introduced in each commit. You can also use -2, which limits the output to only the last two entries:

$ git log p -2 
commit ca82a6dff817ec66f44342007202690a93763949 
Author: Hoang Thanh Son <[email protected]> 
Date: Mon Mar 17 21:52:11 2008 -0700

    changed the verison number

diff --git a/Rakefile b/Rakefile
index a874b73..8f94139 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require 'rake/gempackagetask'
  spec = Gem::Specification.new do |s|
-   s.version = "0.1.0"
+   s.version = "0.1.1"
    s.author = "Hoang Thanh Son"

commit 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
Author: Hoang Thanh Son <[email protected]>
Date: Sat Mar 15 16:40:33 2008 -0700

    removed unnecessary test code

diff --git a/lib/simplegit.rb b/lib/simplegit.rb
index a0a60ae..47c6340 100644
--- a/lib/simplegit.rb
+++ b/lib/simplegit.rb @@ -18,8 +18,3 @@ class SimpleGit
    end

end

-if $0 == __FILE__ 
-  git = SimpleGit.new 
-  puts git.show 
-end 
\ No newline at end of file

This option displays the same information but with a diff directly following each entry. This is very helpful for code review or to quickly browse what happened during a series of commits that a collaborator has added. You can also use a series of summarizing options with git log. For example, if you want to see some abbreviated stats for each commit, you can use the --stat option:

$ git log --stat
commit ca82a6dff817ec66f44342007202690a93763949 
Author: Hoang Thanh Son <[email protected]> 
Date: Mon Mar 17 21:52:11 2008 -0700

    changed the verison number

Rakefile | 2 +1 
files changed, 1 insertions(+), 1 deletions(-)

commit 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
Author: Hoang Thanh Son <[email protected]>
Date: Sat Mar 15 16:40:33 2008 -0700

    removed unnecessary test code

lib/simplegit.rb | 5 ----
1 files changed, 0 insertions(+), 5 deletions(-)

commit a11bef06a3f659402fe7563abf99ad00de2209e6
Author: Hoang Thanh Son <[email protected]>
Date: Sat Mar 15 10:31:28 2008 -0700

    first commit

README           | 6 ++++++ 
Rakefile         | 23 +++++++++++++++++++++++ 
lib/simplegit.rb | 25 +++++++++++++++++++++++++
3 files changed, 54 insertions(+), 0 deletions(-)

As you can see,the –stat option prints below each commit entry a list of modified files, how many files were changed, and how many lines in those files were added and removed. It also puts a summary of the information at the end. Another really useful option is –pretty. This option changes the log output to formats other than the default. A few prebuilt options are available for you to use. The oneline option prints each commit on a single line, which is useful if you’re looking at a lot of commits. In addition, the short, full, and fuller options show the output in roughly the same format but with less or more information, respectively:

$ git log --pretty=oneline
ca82a6dff817ec66f44342007202690a93763949 changed the verison number
085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7 removed unnecessary test code
a11bef06a3f659402fe7563abf99ad00de2209e6 first commit

The most interesting option is format, which allows you to specify your own log output format. This is especially useful when you’re generating output for machine parsing — because you specify the format explicitly, you know it won’t change with updates to Git:

$ git log --pretty=format:"%h - %an, %ar : %s" 
ca82a6d - Hoang Thanh Son, 11 months ago : changed the verison number
085bb3b - Hoang Thanh Son, 11 months ago : removed unnecessary test code
a11bef0 - Hoang Thanh Son, 11 months ago : first commit
Option Description of Output
%H Commit hash
%h Abbreviated commit hash
%T Tree hash
%t Abbreviated tree hash
%P Parent hashes
%p Abbreviated parent hashes
%an Author name
%ae Author e-mail
%ad Author date (format respects the date= option)
%ar Author date, relative
%cn Committer name
%ce Committer email
%cd Committer date
%cr Committer date, relative
%s Subject

Table 2.1 Lists some of the more useful options that format takes.

You may be wondering what the difference is between author and committer. The author is the person who originally wrote the work,where as the committer is the person who last applied the work. So, if you send in a patch to a project and one of the core members applies the patch, both of you get credit — you as the author and the core member as the committer. We’ll cover this distinction a bit more in Chapter 5.

The oneline and format options are particularly useful with another log option called –graph. This option adds a nice little ASCII graph showing your branch and merge history, which we can see our copy of the Grit project repository:

$ git log --pretty=format:"%h %s" --graph
* 2d3acf9 ignore errors from SIGCHLD on trap
* 5e3ee11 Merge branch 'master' of git://github.com/dustin/grit
|\ 
| * 420eac9 Added a method for getting the current branch.
* | 30e367c timeout code and tests
* | 5a09431 add timeout protection to grit
* | e1193f8 support for heads with slashes in them
|/
* d6016bc require time for xmlschema
* 11d191e Merge branch 'defunkt' into local

Those are only some simple output-formatting options to git log — there are many more.

Option Description
-p Show the patch introduced with each commit.
–stat Show statistics for files modified in each commit.
–shortstat Display only the changed /insertions/deletions line from the –stat command.
–name-only Show the list of files modified after the commit information.
–name-status Show the list of files affected with added/modified/deleted information as well.
–abbrev-commit Show only the first few characters of the SHA-1 checksum instead of all 40.
–relative-date Display the date in a relative format (forexample,"2weeks ago") instead of using the full date format.
–graph Display an ASCII graph of the branch and merge history beside the log output.
–pretty Show commits in an alternate format. Options include one line, short, full, fuller, and format (where you specify your own format).

Table 2.2 Lists the options we’ve covereds of a randsome other common formatting options that may be useful, along with how they change the output of the log command.

Limiting Log Output

In addition to output-formatting options, git log takes a number of useful limiting options — that is, options that let you show only a subset of commits. You’ve seen one such option already — the -2 option, which show only the last two commits. In fact, you can do -<n>, where n is any integer to show the last n commits. In reality, you’re unlikely to use that often, because Git by default pipes all output through a pager so you see only one page of log output at a time.

However, the time-limiting options such as --since and --until are very useful. For example, this command gets the list of commits made in the last two weeks:

$ git log --since=2.weeks

This command works with lots of formats — you can specify a specific date ("2008– 01–15") or a relative date such as "2 years 1 day 3 minutes ago". You can also filter the list to commits that match some search criteria. The --author option allows you to filter on a specific author, and the --grep option lets you search for keywords in the commit messages. (Note that if you want to specify both author and grep options, you have to add --all-match or the command will match commits with either.) The last really useful option to pass to git log as a filter is a path. If you specify a directoryorfilename,you can limit the log output to commits that introduced a change to those files. This is always the last option an disgenerally preceded by double dashes (--) to separate the paths from the options. In Table 2.3 we’ll list these and a few other common options for your reference.

Option Description
-(n) Show only the last n commits
–since, –after Limit the commits to those made after the specified date.
–until, –before Limit the commits to those made before the specified date.
–author Only show commits in which the author entry matches the specified string.
–committer Only show commits in which the committer entry matches the specified string.

For example,if you want to see which commits modifying test files in the Git source code history were committed by Junio Hamano and were not merges in the month of October 2008, you can run something like this:

$ git log --pretty="%h:%s" --author=gitster --since="2008-10-01" \ 
    --before="2008-11-01" --no-merges -- t/ 
5610e3b - Fix testcase failure when extended attribute
acd3b9e - Enhance hold_lock_file_for_{update,append}()
f563754 - demonstrate breakage of detached checkout wi
d1a43f2 - reset --hard/read-tree --reset -u: remove un
51a94af - Fix "checkout --track -b newbranch" on detac
b0ad11e - pull: allow "git pull origin $something:$cur

Of the nearly 20,000 commits in the Git source code history, this command shows the 6 that match those criteria.

Using a GUI to Visualize History

If you like to use a more graphical tool to visualize your commit history, you may want to take a look at a Tcl/Tk program called gitk that is distributed with Git. Gitk is basically a visual git log tool, and it accepts nearly all the filtering options that git log does. If you type gitk on the command line in your project, you should see something like Figure 2.2.

Figure 2.2: The gitk history visualizer

You can see the commit history in the top half of the window along with a nice ancestry graph. The diff viewer in the bottom half of the window shows you the changes introduced at any commit you click.

UndoingThings

At any stage, you may want to undo something. Here,we’ll review a few basic tools for undoing changes that you’ve made. Be careful, because you can’t always undo some of these undos. This is one of the few areas in Git where you may lose some work if you do it wrong.

Changing Your Last Commit

One of the common undos takes place when you commit too early and possibly forget to add some files, or you messup your commitmessage. If you want to try that commit again, you can run commit with the –amend option:

$ git commit --amend

This command takes your staging area and uses it for the commit. If you’ve have made no changes since your last commit (for instance, you run this command it im../mediately after your previous commit), then your snapshot will look exactly the same and all you’ll change is your commit message.

The same commit-message editor fires up, but it already contains the message of your previous commit. You can edit the message the same as always, but it overwrites your previous commit.

As an example, if you commit and then realize you forgot to stage the changes in a file you wanted to add to this commit, you can do something like this:

$ git commit -m 'initial commit'
$ git add forgotten_file
$ git commit --amend

All three of these commands end up with a single commit — the second command replaces the results of the first.

Unstaginga Staged File

The next two sections demonstrate how to wrangle your staging area and working directory changes. The nice part is that the command you use to determine the state of those two areas also reminds you how to undo changes to them. For example, let’s say you’ve changed two files and want to commit them as two separate changes, but you accidentally type git add * and stage them both. How can you unstage one of the two? The git status command reminds you:

$ git add . $ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
# 
# modified: README.txt
# modified: benchmarks.rb #

Right below the "Changes to be committed" text, it says use git reset HEAD <file>... to unstage. So, let’s use that advice to unstage the benchmarks.rb file:

$ git reset HEAD benchmarks.rb benchmarks.rb: locally modified $ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
# 
# modified: README.txt
# 
# Changed but not updated:
# (use "git add <file>..." to update what will be committed)
# (use "git checkout -- <file>..." to discard changes in working directory)
# 
# modified: benchmarks.rb #

The command is a bit strange, but it works. The benchmarks.rb file is modified but once again unstaged.

Unmodifying a Modified File

What if you realize that you don’t want to keep your changes to the benchmarks.rb file? How can you easily unmodify it — revert it back to what it looked like when you last committed (or initially cloned, or however you got it into your working directory)? Luckily, git status tells you how to do that, too. In the last example output, the unstaged area looks like this:

# Changed but not updated:
# (use "git add <file>..." to update what will be committed)
# (use "git checkout -- <file>..." to discard changes in working directory)
# 
# modified: benchmarks.rb #

It tells you pretty explicitly how to discard the changes you’ve made (at least, the newer versions of Git, 1.6.1 and later, do this — if you have an older version, we highly recommend upgrading it to get some of these nicer usability features). Let’s do what it says:

$ git checkout -- benchmarks.rb $ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
# 
# modified: README.txt #

You can see that the changes have been reverted. You should also realize that this is a dangerous command: any changes you made to that file are gone — you just copied another file over it. Don’t ever use this command unless you absolutely know that you don’t want the file. If you just need to get it out of the way, we’ll go over stashing and branching in the next chapter; these are generally better ways to go. Remember,any thing that is committed in Git can almost always be recovered. Even commits that were on branches that were deleted or commits that were overwritten with an --amend commit can be recovered (see Chapter 9 for data recovery). However, anything you lose that was never committed is likely never to be seen again.

WorkingwithRemotes

To be able to collaborate on any Git project, you need to know how to manage your remote repositories. Remote repositories are versions of your project that are hosted on the Internet or network somewhere. You can have several of them, each of which generally is either read-only or read/write for you. Collaborating with others involves managing these remote repositories and pushing and pulling data to and from them when you need to share work. Managing remote repositories includes knowing how to add remote repositories, remove remotes that are no longer valid, manage various remote branches and define them as being tracked or not, and more. In this section, we’ll cover these remote-management skills.

Showing Your Remotes

To see which remote servers you have configured, you can run the git remote command. It lists the shortnames of each remote handle you’ve specified. If you’ve cloned your repository, you should at least see origin — that is the default name Git gives to the server you cloned from:

$ git clone git://github.com/wingadium1/ticgit.git
Initialized empty Git repository in /private/tmp/ticgit/.git/
remote: Counting objects: 595, done.
remote: Compressing objects: 100% (269/269), done.
remote: Total 595 (delta 255), reused 589 (delta 253)
Receiving objects: 100% (595/595), 73.31 KiB | 1 KiB/s, done.
Resolving deltas: 100% (255/255), done.
$ cd ticgit
$ git remote origin

You can also specify -v, which shows you the URL that Git has stored for the shortname to be expanded to:

$ git remote -v origin git://github.com/wingadium1/ticgit.git

If you have more than one remote, the command lists them all. For example, my Grit repository looks something like this.

$ cd grit
$ git remote -v
bakkdoor    git://github.com/bakkdoor/grit.git
cho45       git://github.com/cho45/grit.git
defunkt     git://github.com/defunkt/grit.git
koke        git://github.com/koke/grit.git
origin      [email protected]:mojombo/grit.git

This means we can pull contributions from any of these users pretty easily. But notice that only the origin remote is an SSH URL, so it’s the only one I can push to (we’ll cover why this is in Chapter 4).

Adding Remote Repositories

I’ve mentioned and given some demonstrations of adding remote repositories in previous sections, but here is how to do it explicitly. To add a new remote Git repository as a shortname you can reference easily, run git remote add [shortname] [url]:

$ git remote origin
$ git remote add pb git://github.com/paulboone/ticgit.git
$ git remote -v 
origin git://github.com/wingadium1/ticgit.git
pb git://github.com/paulboone/ticgit.git

Now you can use the string pb on the command line in lieu of the whole URL. For example, if you want to fetch all the information that Paul has but that you don’t yet have in your repository, you can run git fetch pb:

$ git fetch pb
remote: Counting objects: 58, done.
remote: Compressing objects: 100% (41/41), done.
remote: Total 44 (delta 24), reused 1 (delta 0)
Unpacking objects: 100% (44/44), done.
From git://github.com/paulboone/ticgit
  * [new branch] master -> pb/master
  * [new branch] ticgit -> pb/ticgit

Paul’s master branch is accessible locally as pb/master — you can merge it into one of your branches, or you can check out a local branch at that point if you want to inspect it.

Fetching and Pulling from Your Remotes

As you just saw, to get data from your remote projects, you can run

$ git fetch [remote-name]

The command goes out to that remote project and pulls down all the data from that remote project that you don’t have yet. After you do this, you should have references to all the branches from that remote, which you can merge in or inspect at any time. (We’ll go over what branches are and how to use them in much more detail in Chapter 3.) If you cloned a repository, the command automatically adds that remote repository under the name origin. So, git fetch origin fetches any new work that has been pushed to that server since you cloned (or last fetched from) it. It’s important to note that the fetch command pulls the data to your local repository — it doesn’t automatically merge it with any of your work or modify what you’re currently working on. You have to merge it manually into your work when you’re ready. If you have a branch set up to track a remote branch (see the next section and Chapter 3 for more information), you can use the git pull command to automatically fetch and then merge a remote branch into your current branch. This may be an easier or more comfortable workflow for you; and by default, the git clone command automatically sets up your local master branch to track the remote master branch on the server you cloned from (assuming the remote has a master branch). Running git pull generally fetches data from the server you originally cloned from and automatically tries to merge it into the code you’re currently working on

Pushing to Your Remotes

When you have your project at a point that you want to share, you have to push it upstream. The command for this is simple: git push [remote-name] [branch-name]. If you want to push your master branch to your origin server (again, cloning generally sets up both of those names for you automatically), then you can run this to push your work back up to the server:

$ git push origin master

This command works only if you cloned from a server to which you have write access and if nobody has pushed in the meantime. If you and someone else clone at the same time and they push upstream and then you push upstream, your push will rightly be rejected. You’ll have to pull down their work first and incorporate it into yours before you’ll be allowed to push. See Chapter 3 for more detailed information on how to push to remote servers.

Inspecting a Remote

If you want to see more information about a particular remote, you can use the git remote show [remote-name] command. If you run this command with a particular shortname, such as origin, you get something like this:

$ git remote show origin
* remote origin
  URL: git://github.com/wingadium1/ticgit.git
  Remote branch merged with 'git pull' while on branch master
    master
  Tracked remote branches
    master
    ticgit

It lists the URL for the remote repository as well as the tracking branch information. The command helpfully tells you that if you’re on the master branch and you run git pull, it will automatically merge in the master branch on the remote after it fetches all the remote references. It also lists all the remote references it has pulled down.

That is a simple example you’re likely to encounter. When you’re using Git more heavily, however, you may see much more information from git remote show:

$ git remote show origin
* remote origin
  URL: [email protected]:defunkt/github.git
  Remote branch merged with 'git pull' while on branch issues
    issues
  Remote branch merged with 'git pull' while on branch master
    master
  New remote branches (next fetch will store in remotes/origin)
    caching
  Stale tracking branches (use 'git remote prune')
    libwalker
    walker2
  Tracked remote branches
    acl
    apiv2
    dashboard2
    issues
    master
    postgres
  Local branch pushed with 'git push'
    master:master

This command shows which branch is automatically pushed when you run git push on certain branches. It also shows you which remote branches on the server you don’t yet have, which remote branches you have that have been removed from the server, and multiple branches that are automatically merged when you run git pull.

Removing and Renaming Remotes

If you want to rename a reference, in newer versions of Git you can run git remote rename to change a remote’s shortname. For instance, if you want to rename pb to paul, you can do so with git remote rename:

$ git remote rename pb paul
$ git remote
origin
paul

It’s worth mentioning that this changes your remote branch names, too. What used to be referenced at pb/master is now at paul/master. If you want to remove a reference for some reason — you’ve moved the server or are no longer using a particular mirror, or perhaps a contributor isn’t contributing anymore — you can use git remote rm:

$ git remote rm paul
$ git remote
origin

Tagging

Like most VCSs, Git has the ability to tag specific points in history as being important. Generally, people use this functionality to mark release points (v1.0, and so on). In this you’ll learn how to list the available tags, how to create new tags, and what the different types of tags are.

Listing Your Tags

Listing the available tags in Git is straightforward. Just type git tag:

$ git tag
v0.1
v1.3

This command lists the tags in alphabetical order; the order in which they appear has no real importance. You can also search for tags with a particular pattern. The Git source repo, for instance, contains more than 240 tags. If you’re only interested in looking at the 1.4.2 series, you can run this:

$ git tag -l 'v1.4.2.*'
v1.4.2.1
v1.4.2.2
v1.4.2.3
v1.4.2.4

Creating Tags

Git uses two main types of tags: lightweight and annotated. A lightweight tag is very much like a branch that doesn’t change — it’s just a pointer to a specific commit. Annotated tags, however, are stored as full objects in the Git database. They’re checksummed; contain the tagger name, e-mail, and date; have a tagging message; and can be signed and verified with GNU Privacy Guard (GPG). It’s generally recommended that you create annotated tags so you can have all this information; but if you want a temporary tag or for some reason don’t want to keep the other information, lightweight tags are available too.

Annotated Tags

Creating an annotated tag in Git is simple. The easiest way is to specify -a when you run the tag command:

$ git tag -a v1.4 -m 'my version 1.4' 
$ git tag v0.1 v1.3 v1.4

The -m specifies a tagging message, which is stored with the tag. If you don’t specify a message for an annotated tag, Git launches your editor so you can type it in. You can see the tag data along with the commit that was tagged by using the git show command:

$ git show v1.4
tag v1.4
Tagger: Hoang Thanh Son <[email protected]>
Date: Mon Feb 9 14:45:11 2009 -0800

my version 1.4
commit 15027957951b64cf874c3557a0f3547bd83b3ff6
Merge: 4a447f7... a6b4c97...
Author: Hoang Thanh Son <[email protected]>
Date: Sun Feb 8 19:02:46 2009 -0800

    Merge branch 'experiment'

That shows the tagger information, the date the commit was tagged, and the annotation message before showing the commit information.

SignedTags

You can also sign your tags with GPG, assuming you have a private key. All you have to do is use -s instead of -a:

$ git tag -s v1.5 -m 'my signed 1.5 tag'
You need a passphrase to unlock the secret key for user: "Hoang Thanh Son <[email protected]>"
1024-bit DSA key, ID F721C45A, created 2009-02-09

If you run git show on that tag, you can see your GPG signature attached to it:

$ git show v1.5 
tag v1.5 
Tagger: Hoang Thanh Son <[email protected]>
Date: Mon Feb 9 15:22:20 2009 -0800

my signed 1.5 tag
-----BEGIN PGP SIGNATURE----
Version: GnuPG v1.4.8 (Darwin)

iEYEABECAAYFAkmQurIACgkQON3DxfchxFr5cACeIMN+ZxLKggJQf0QYiQBwgySN Ki0An2JeAVUCAiJ7Ox6ZEtK+NvZAj82/=WryJ 
-----END PGP SIGNATURE----
commit 15027957951b64cf874c3557a0f3547bd83b3ff6
Merge: 4a447f7... a6b4c97...
Author: Hoang Thanh Son <[email protected]>
Date: Sun Feb 8 19:02:46 2009 -0800

    Merge branch 'experiment'

A bit later, you’ll learn how to verify signed tags.

Light weight Tags

Another way to tag commits is with a lightweight tag. This is basically the commit checksum stored in a file — no other information is kept. To create a lightweight tag, don’t supply the -a, -s, or -m option:

$ git tag v1.4-lw
$ git tag
v0.1
v1.3
v1.4
v1.4-lw
v1.5

This time, if you run git show on the tag, you don’t see the extra tag information. The command just shows the commit:

$ git show v1.4-lw 
commit 15027957951b64cf874c3557a0f3547bd83b3ff6
Merge: 4a447f7... a6b4c97...
Author: Hoang Thanh Son <[email protected]>
Date: Sun Feb 8 19:02:46 2009 -0800

    Merge branch 'experiment'

Verifying Tags

To verify a signed tag, you use git tag -v [tag-name]. This command uses GPG to verify the signature. You need the signer’s public key in your keyring for this to work properly:

$ git tag -v v1.4.2.1
object 883653babd8ee7ea23e6a5c392bb739348b1eb61
type commit
tag v1.4.2.1 
tagger Junio C Hamano <[email protected]> 1158138501 -0700

GIT 1.4.2.1

Minor fixes since 1.4.2, including git-mv and git-http with alternates.
gpg: Signature made Wed Sep 13 02:08:25 2006 PDT using DSA key ID F3119B9A
gpg: Good signature from "Junio C Hamano <[email protected]>"
gpg: aka "[jpeg image of size 1513]"
Primary key fingerprint: 3565 2A26 2040 E066 C9A7 4A7D C0C6 D9A4 F311 9B9A

If you don’t have the signer’s public key, you get something like this instead:

gpg: Signature made Wed Sep 13 02:08:25 2006 PDT using DSA key ID F3119B9A
gpg: Can't check signature: public key not found
error: could not verify the tag 'v1.4.2.1'

Tagging Later

You can also tag commits after you’ve moved past them. Suppose your commit history looks like this:

$ git log --pretty=oneline
15027957951b64cf874c3557a0f3547bd83b3ff6 Merge branch 'experiment'
a6b4c97498bd301d84096da251c98a07c7723e65 beginning write support
0d52aaab4479697da7686c15f77a3d64d9165190 one more thing
6d52a271eda8725415634dd79daabbc4d9b6008e Merge branch 'experiment'
0b7434d86859cc7b8c3d5e1dddfed66ff742fcbc added a commit function
4682c3261057305bdd616e23b64b0857d832627b added a todo file
166ae0c4d3f420721acbb115cc33848dfcc2121a started write support
9fceb02d0ae598e95dc970b74767f19372d61af8 updated rakefile
964f16d36dfccde844893cac5b347e7b3d44abbc commit the todo
8a5cbc430f1a9c3d00faaeffd07798508422908a updated readme

Now, suppose you forgot to tag the project at v1.2, which was at the "updated rakefile" commit. You can add it after the fact. To tag that commit, you specify the commit checksum (or part of it) at the end of the command:

$ git tag -a v1.2 9fceb02

You can see that you’ve tagged the commit:

$ git tag
v0.1
v1.2
v1.3
v1.4
v1.4-lw
v1.5

$ git show v1.2
tag v1.2
Tagger: Hoang Thanh Son <[email protected]>
Date: Mon Feb 9 15:32:16 2009 -0800

version 1.2
commit 9fceb02d0ae598e95dc970b74767f19372d61af8
Author: Magnus Chacon <[email protected]>
Date: Sun Apr 27 20:43:35 2008 -0700

    updated rakefile

...

Sharing Tags

By default, the git push command doesn’t transfer tags to remote servers. You will have to explicitly push tags to a shared server after you have created them. This process is just like sharing remote branches you can run git push origin [tagname].

$ git push origin v1.5
Counting objects: 50, done.
Compressing objects: 100% (38/38), done.
Writing objects: 100% (44/44), 4.56 KiB, done.
Total 44 (delta 18), reused 8 (delta 1)
To [email protected]:schacon/simplegit.git
* [new tag] v1.5 -> v1.5

If you have a lot of tags that you want to push up at once, you can also use the --tags option to the git push command. This will transfer all of your tags to the remote server that are not already there.

$ git push origin --tags
Counting objects: 50, done.
Compressing objects: 100% (38/38), done.
Writing objects: 100% (44/44), 4.56 KiB, done.
Total 44 (delta 18), reused 8 (delta 1)
To [email protected]:wingadium1/simplegit.git 
  * [new tag] v0.1 -> v0.1
  * [new tag] v1.2 -> v1.2
  * [new tag] v1.4 -> v1.4
  * [new tag] v1.4-lw -> v1.4-lw
  * [new tag] v1.5 -> v1.5

Now, when someone else clones or pulls from your repository, they will get all your tags as well.

Tips and Tricks

Before we finish this chapter on basic Git, a few little tips and tricks may make your Git experience a bit simpler, easier, or more familiar. Many people use Git without using any of these tips, and we won’t refer to them or assume you’ve used them later in the book; but you should probably know how to do them.

Auto-Completion

If you use the Bash shell, Git comes with a nice auto-completion script you can enable. Download the Git source code, and look in the contrib/completion directory; there should be a file called git-completion.bash. Copy this file to your home directory, and add this to your .bashrc file:

source ˜/.git-completion.bash

If you want to set up Git to automatically have Bash shell completion for all users, copy this script to the /opt/local/etc/bash_completion.d directory on Mac systems or to the /etc/bash_completion.d/ directory on Linux systems. This is a directory of scripts that Bash will automatically load to provide shell completions.

If you’re using Windows with Git Bash, which is the default when installing Git on Windows with msysGit, auto-completion should be preconfigured.

Press the Tab key when you’re writing a Git command, and it should return a set of suggestions for you to pick from:

$ git co<tab><tab>
commit config

In this case, typing git co and then pressing the Tab key twice suggests commit and config. Adding m<tab> completes git commit automatically. This also works with options, which is probably more useful. For instance, if you’re running a git log command and can’t remember one of the options, you can start typing it and press Tab to see what matches:

$ git log --s<tab> 
--shortstat --since= --src-prefix= --stat --summary

That’s a pretty nice trick and may save you some time and documentation reading.

Git Aliases

Git doesn’t infer your command if you type it in partially. If you don’t want to type the entire text of each of the Git commands, you can easily set up an alias for each command using git config. Here are a couple of examples you may want to set up:

$ git config --global alias.co checkout
$ git config --global alias.br branch
$ git config --global alias.ci commit
$ git config --global alias.st status

This means that, for example, instead of typing git commit, you just need to type git ci. As you go on using Git, you’ll probably use other commands frequently as well; in this case, don’t hesitate to create new aliases.

This technique can also be very useful in creating commands that you think should exist. For example, to correct the usability problem you encountered with unstaging a file, you can add your own unstage alias to Git:

$ git config --global alias.unstage 'reset HEAD --'

This makes the following two commands equivalent:

$ git unstage fileA $ git reset HEAD fileA

This seems a bit clearer. It’s also common to add a last command, like this:

$ git config --global alias.last 'log -1 HEAD'

This way, you can see the last commit easily:

$ git
last commit 66938dae3329c7aebe598c2246a8e6af90d04646
Author: Josh Goebel <[email protected]>
Date: Tue Aug 26 19:48:51 2008 +0800

    test for current head

    Signed-off-by: Hoang Thanh Son <[email protected]>

As you can tell, Git simply replaces the new command with whatever you alias it for. However, maybe you want to run an external command, rather than a Git subcommand. In that case, you start the command with a ! character. This is useful if you write your own tools that work with a Git repository. We can demonstrate by aliasing git visual to run gitk:

$ git config --global alias.visual "!gitk"

Summary

At this point, you can do all the basic local Git operations — creating or cloning a repository, making changes, staging and committing those changes, and viewing the history of all the changes the repository has been through. Next, we’ll cover Git’s killer feature: its branching model.

Series Navigation<< Pro Git Chapter 1: Getting StartedPro Git Chapter 3: Git Branching >>

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