git log

One of the more helpful options is -p, which shows the difference introduced in each commit. You can also use -2, which limits the output to only the last two entries:

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:

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:

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:

Table 2-1. Useful options for `git log --pretty=format`
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 email
`%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

You may be wondering what the difference is between author and committer. The author is the person who originally wrote the work, whereas 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 Distributed Git.

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:

This type of output will become more interesting as we go through branching and merging in the next chapter.

Those are only some simple output-formatting options to git log – there are many more. Table 2-2 lists the options we’ve covered so far, as well as some other common formatting options that may be useful, along with how they change the output of the log command.

Table 2-2. Common options to `git log`
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 (for example, “2 weeks 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 oneline, short, full, fuller, and format (where you specify your own format).

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 like "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.)

Another really helpful filter is the -S option which takes a string and only shows the commits that introduced a change to the code that added or removed that string. For instance, if you wanted to find the last commit that added or removed a reference to a specific function, you could call:

$ git log -Sfunction_name

The last really useful option to pass to git log as a filter is a path. If you specify a directory or file name, you can limit the log output to commits that introduced a change to those files. This is always the last option and is generally 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.

Table 2-3. Options to limit the output of `git log`
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.
`--grep`	Only show commits with a commit message containing the string
`-S`	Only show commits adding or removing code matching the string

For example, if you want to see which commits modifying test files in the Git source code history are merged and were committed by Junio Hamano 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 attributes are in use
acd3b9e - Enhance hold_lock_file_for_{update,append}() API
f563754 - demonstrate breakage of detached checkout with symbolic link HEAD
d1a43f2 - reset --hard/read-tree --reset -u: remove unmerged new paths
51a94af - Fix "checkout --track -b newbranch" on detached HEAD
b0ad11e - pull: allow "git pull origin $something:$current_branch" into an unborn branch

Of the nearly 40,000 commits in the Git source code history, this command shows the 6 that match those criteria.

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