Revision Specifiers
Prev | Chapter 3. Advanced Topics | Next

## Revision Specifiers

As we described in the section called “Revisions”, revision numbers in Subversion are pretty straightforward—integers that keep getting larger as you commit more changes to your versioned data. Still, it doesn't take long before you can no longer remember exactly what happened in each and every revision. Fortunately, the typical Subversion workflow doesn't often demand that you supply arbitrary revisions to the Subversion operations you perform. For operations that do require a revision specifier, you generally supply a revision number that you saw in a commit email, in the output of some other Subversion operation, or in some other context that would give meaning to that particular number.

| Note
Referring to revision numbers with an “`r`” prefix (`r314`, for example) is an established practice in Subversion communities, and is both supported and encouraged by many Subversion-related tools. In most places where you would specify a bare revision number on the command line, you may also use the `r` `NNN` syntax.

But occasionally, you need to pinpoint a moment in time for which you don't already have a revision number memorized or handy. So besides the integer revision numbers, svn allows as input some additional forms of revision specifiers: revision keywords and revision dates.

| Note
The various forms of Subversion revision specifiers can be mixed and matched when used to specify revision ranges. For example, you can use `-r `REV1`:`REV2`` where `REV1` is a revision keyword and `REV2` is a revision number, or where `REV1` is a date and `REV2` is a revision keyword, and so on. The individual revision specifiers are independently evaluated, so you can put whatever you want on the opposite sides of that colon.

### Revision Keywords

The Subversion client understands a number of revision keywords. These keywords can be used instead of integer arguments to the `--revision` (`-r`) option, and are resolved into specific revision numbers by Subversion:

`HEAD`

  The latest (or “youngest”) revision in the repository.

`BASE`

  The revision number of an item in a working copy. If the item has been locally modified, this refers to the way the item appears without those local modifications.

`COMMITTED`

  The most recent revision prior to, or equal to, `BASE`, in which an item changed.

`PREV`

  The revision immediately before the last revision in which an item changed. Technically, this boils down to `COMMITTED`-1.

As can be derived from their descriptions, the `PREV`, `BASE`, and `COMMITTED` revision keywords are used only when referring to a working copy path—they don't apply to repository URLs. `HEAD`, on the other hand, can be used in conjunction with both of these path types.

Here are some examples of revision keywords in action:

    $ svn diff -r PREV:COMMITTED foo.c
    # shows the last change committed to foo.c

    $ svn log -r HEAD
    # shows log message for the latest repository commit

    $ svn diff -r HEAD
    # compares your working copy (with all of its local changes) to the
    # latest version of that tree in the repository

    $ svn diff -r BASE:HEAD foo.c
    # compares the unmodified version of foo.c with the latest version of
    # foo.c in the repository

    $ svn log -r BASE:HEAD
    # shows all commit logs for the current versioned directory since you
    # last updated

    $ svn update -r PREV foo.c
    # rewinds the last change on foo.c, decreasing foo.c's working revision

    $ svn diff -r BASE:14 foo.c
    # compares the unmodified version of foo.c with the way foo.c looked
    # in revision 14

### Revision Dates

Revision numbers reveal nothing about the world outside the version control system, but sometimes you need to correlate a moment in real time with a moment in version history. To facilitate this, the `--revision` (`-r`) option can also accept as input date specifiers wrapped in curly braces (`{` and `}`). Subversion accepts the standard ISO-8601 date and time formats, plus a few others. Here are some examples.

    $ svn checkout -r {2006-02-17}
    $ svn checkout -r {15:30}
    $ svn checkout -r {15:30:00.200000}
    $ svn checkout -r {"2006-02-17 15:30"}
    $ svn checkout -r {"2006-02-17 15:30 +0230"}
    $ svn checkout -r {2006-02-17T15:30}
    $ svn checkout -r {2006-02-17T15:30Z}
    $ svn checkout -r {2006-02-17T15:30-04:00}
    $ svn checkout -r {20060217T1530}
    $ svn checkout -r {20060217T1530Z}
    $ svn checkout -r {20060217T1530-0500}
    …

| Note
Keep in mind that most shells will require you to, at a minimum, quote or otherwise escape any spaces that are included as part of revision date specifiers. Certain shells may also take issue with the unescaped use of curly braces, too. Consult your shell's documentation for the requirements specific to your environment.

When you specify a date, Subversion resolves that date to the most recent revision of the repository as of that date, and then continues to operate against that resolved revision number:

    $ svn log -r {2006-11-28}
    ------------------------------------------------------------------------
    r12 | ira | 2006-11-27 12:31:51 -0600 (Mon, 27 Nov 2006) | 6 lines
    …

You can also use a range of dates. Subversion will find all revisions between both dates, inclusive:

    $ svn log -r {2006-11-20}:{2006-11-29}
    …

| Warning
Since the timestamp of a revision is stored as an unversioned, modifiable property of the revision (see the section called “Properties”), revision timestamps can be changed to represent complete falsifications of true chronology, or even removed altogether. Subversion's ability to correctly convert revision dates into real revision numbers depends on revision datestamps maintaining a sequential ordering—the younger the revision, the younger its timestamp. If this ordering isn't maintained, you will likely find that trying to use dates to specify revision ranges in your repository doesn't always return the data you might have expected.

Prev | Up | Next
Chapter 3. Advanced Topics | Home | Peg and Operative Revisions