I was struggling with the same thing, running a shell script that set variables, then wanting to use the variables in the shared-pom. The goal was to have environment variables replace strings in my project files using the com.google.code.maven-replacer-plugin.
Using ${env.foo} or ${env.FOO} didn't work for me. Maven just wasn't finding the variable. What worked was passing the variable in as a command-line parameter in Maven. Here's the setup:
Set the variable in the shell script. If you're launching Maven in a sub-script, make sure the variable is getting set, e.g. using source ./maven_script.sh to call it from the parent script.
In shared-pom, create a command-line param that grabs the environment variable:
<plugin>
...
<executions>
<executions>
...
<execution>
...
<configuration>
<param>${foo}</param> <!-- Note this is *not* ${env.foo} -->
</configuration>
In com.google.code.maven-replacer-plugin, make the replacement value ${foo}.
In my shell script that calls maven, add this to the command: -Dfoo=$foo