<basename property="jar.filename" file="${lib.jarfile}"/>
will set jar.filename to myjar.jar, if lib.jarfile is defined as either a full-path filename (eg., /usr/local/lib/myjar.jar), a relative-path filename (eg., lib/myjar.jar), or a simple filename (eg., myjar.jar).
<basename property="cmdname" file="D:/usr/local/foo.exe"
          suffix=".exe"/>
will set cmdname to foo.
<property environment="env"/>
<basename property="temp.dirname" file="${env.TEMP}"/>