The following variables are available to shell scripts

BUILD_NUMBER
  The current build number, such as "153"
BUILD_ID
  The current build id, such as "2005-08-22_23-59-59" (YYYY-MM-DD_hh-mm-ss)
JOB_NAME
  Name of the project of this build, such as "foo"
BUILD_TAG
  String of "hudson-${JOBNAME}-${BUILD_NUMBER}". Convenient to put into a resource file, a jar file, etc for easier identification.
EXECUTOR_NUMBER
  The unique number that identifies the current executor (among executors of the same machine) that's carrying out this build. This is the number you see in the "build executor status", except that the number starts from 0, not 1.
JAVA_HOME
  If your job is configured to use a specific JDK, this variable is set to the JAVA_HOME of the specified JDK. When this variable is set, `PATH` is also updated to have `$JAVA_HOME/bin`.
WORKSPACE
  The absolute path of the workspace.
HUDSON_URL
  Full URL of Hudson, like `http://server:port/hudson/`
SVN_REVISION
  For Subversion-based projects, this variable contains the revision number of the module.
CVS_BRANCH
  For CVS-based projects, this variable contains the branch of the module. If CVS is configured to check out the trunk, this environment variable will not be set.

To understand how environmental variables provided by Hudson can be utilized by Ant, study the following target:

    <target name="printinfo">
      <property environment="env" />
      <echo message="${env.BUILD_TAG}"/>
    </target>