Using maven to output the version number to a text file

Sure. Create a text file somewhere in src/main/resources, call it version.txt (or whatever)

File content:

${project.version}

now in your pom.xml, inside the build element, put this block:

<build>
  <resources>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>true</filtering>
      <includes>
        <include>**/version.txt</include>
      </includes>
    </resource>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>false</filtering>
      <excludes>
        <exclude>**/version.txt</exclude>
      </excludes>
    </resource>
    ...
  </resources>
</build>

after every build, the file (which you can find in target/classes) will contain the current version.

Now if you want to move the file somewhere else automatically, you are probably going to need to execute an ant task via the maven-antrun-plugin.

Something like this:

  <build>
    ...
    <plugins>
      <plugin>
        <artifactId>maven-antrun-plugin</artifactId>
         <version>1.4</version>
         <executions>
          <execution>
            <phase>process-resources</phase>
            <configuration>
               <tasks>
                 <copy file="${project.build.outputDirectory}/version.txt"
                   toFile="..." overwrite="true" />
              </tasks>
            </configuration>
            <goals>
              <goal>run</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
   </plugins>
 </build>