SVN是以版本号（revision number）来记录版本库的每一次改变，一般的SVN操作不需要用到版本号，但是有些SVN操作需要指定版本号。我们可以指定一个明确的整数版本号，但是也可以使用SVN关键字来指代某个特殊的版本号，SVN会真正计算出它所指代的实际整数版本号：
HEAD：版本库中最新的版本；
BASE：某个工作副本项的版本，注意这个是你上次update该项时的版本号，可能晚于当前最新的版本号；
COMMITTED：某个工作副本项最近修改的版本，与BASE相同或更早；
PREV：COMMITTED - 1。
HEAD针对于版本库，另外3个针对于某个工作副本目录或文件。
示例：
$ 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