Bamboo version 5.15.5 build 51518 - 28 Mar 17
Hello, I have a shell script that does this:
echo "custom=hello" > 'assembly.properties'
Pretty dead simple. When I use the "Inject Bamboo variables" task to read in that file my log shows this:

Injected variable bamboo.inject.version=2.3.6466.26912 in RESULT scope