Bamboo variables don't get substituted when using a Script file (not an inline script)

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simple	18-Mar-2013 16:13:54	Starting task 'Script' of type 'com.atlassian.bamboo.plugins.scripttask:task.builder.script'
command	18-Mar-2013 16:13:54	Beginning to execute external process for build 'Project-1 - Plan-2 - Default Job'\n ... running command line: \n/bin/sh /tmp/a.sh\n ... in: /Users/akhachatryan/Atlassian/BambooHome4.4.4/xml-data/build-dir/PROJ1-PLAN2-JOB1\n
error	18-Mar-2013 16:13:55	/tmp/a.sh: line 1: ${bamboo.buildKey}: bad substitution
simple	18-Mar-2013 16:13:55	Failing task since return code was 1 while expected 0
simple	18-Mar-2013 16:13:55	Finished task 'Script'
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