How to list active sub-modules in a Maven project?

I have a complex project where there are many directories that have POM files, but only some of which are sub-modules (possibly transitively) of a particular parent project.

Obviously, Maven knows the list of relevant files because it parses all the <module> tags to find them. But, I only see a list of the <name>s in the [INFO] comments, not the paths to those modules.

Is there a way to have Maven output a list of all the POM files that provided references to projects that are part of the reactor build for a given project?

This is quite simple but it only gets the artifactId, from the root (or parent) module:

mvn --also-make dependency:tree | grep maven-dependency-plugin | awk '{ print $(NF-1) }'

If you want the directories

mvn -q --also-make exec:exec -Dexec.executable="pwd"

mvn help:evaluate -Dexpression=project.modules

mvn help:evaluate -Dexpression=project.modules[0]
mvn help:evaluate -Dexpression=project.modules[1]

IFS=$'\n'
modules=($(mvn help:evaluate -Dexpression=project.modules | grep -v "^\[" | grep -v "<\/*strings>" | sed 's/<\/*string>//g' | sed 's/[[:space:]]//'))
for module in "${modules[@]}"
do
    echo "$module"
done

The following script prints artifactId's of all sub-modules:

mvn -Dexec.executable='echo' -Dexec.args='${project.artifactId}' exec:exec -q

Example output:

build-tools
aws-sdk-java-pom
core
annotations
utils
http-client-spi
http-client-tests
http-clients
apache-client
test-utils
sdk-core
...

Here's a way to do this on Linux outside of Maven, by using strace.

$ strace -o opens.txt -f -e open mvn dependency:tree > /dev/null
$ perl -lne 'print $1 if /"(.*pom\.xml)"/' opens.txt 

The first line runs mvn dependency:tree under strace, asking strace to output to the file opens.txt all the calls to the open(2) system call, following any forks (because Java is threaded). This file looks something like:

9690  open("/etc/ld.so.cache", O_RDONLY) = 3
9690  open("/lib/libncurses.so.5", O_RDONLY) = 3
9690  open("/lib/libdl.so.2", O_RDONLY) = 3

The second line asks Perl to print any text inside quotes that happens to end in pom.xml. (The -l flag handles printing newlines, the -n wraps the code single quotes in a loop that simply reads any files on the command line, and the -e handles the script itself which uses a regex to find interesting calls to open.)

It'd be nice to have a maven-native way of doing this :-)