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.

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>