MAVEN_HOME, MVN_HOME or M2_HOME

I've personally never found it useful to set M2_HOME.

What counts is your $PATH environment. Hijacking part of the answer from Danix, all you need is:

export PATH=/Users/xxx/sdk/apache-maven-3.0.5/bin:$PATH

The mvn script computes M2_HOME for you anyway for what it's worth.

MAVEN_HOME is used for maven 1 and M2_HOME is used to locate maven 2. Having the two different _HOME variables means it is possible to run both on the same machine. And if you check old mvn.cmd scripts they have something like,

@REM ----------------------------------------------------------------------------
@REM Maven2 Start Up Batch script
@REM
@REM Required ENV vars:
@REM JAVA_HOME - location of a JDK home dir
@REM
@REM Optional ENV vars
@REM M2_HOME - location of maven2's installed home dir
@REM MAVEN_BATCH_ECHO - set to 'on' to enable the echoing of the batch commands
@REM MAVEN_BATCH_PAUSE - set to 'on' to wait for a key stroke before ending
@REM MAVEN_OPTS - parameters passed to the Java VM when running Maven
@REM     e.g. to debug Maven itself, use
@REM set MAVEN_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000
@REM MAVEN_SKIP_RC - flag to disable loading of mavenrc files
@REM ----------------------------------------------------------------------------

See that @REM M2_HOME - location of maven2's installed home dir

Anyway usage of this pattern is now deprecated with maven 3.5 as per the documentation.

Based on problems in using M2_HOME related to different Maven versions installed and to simplify things, the usage of M2_HOME has been removed and is not supported any more MNG-5823, MNG-5836, MNG-5607

So now the mvn.cmd look like,

@REM -----------------------------------------------------------------------------
@REM Apache Maven Startup Script
@REM
@REM Environment Variable Prerequisites
@REM
@REM   JAVA_HOME          Must point at your Java Development Kit installation.
@REM   MAVEN_BATCH_ECHO  (Optional) Set to 'on' to enable the echoing of the batch commands.
@REM   MAVEN_BATCH_PAUSE (Optional) set to 'on' to wait for a key stroke before ending.
@REM   MAVEN_OPTS        (Optional) Java runtime options used when Maven is executed.
@REM   MAVEN_SKIP_RC     (Optional) Flag to disable loading of mavenrc files.
@REM -----------------------------------------------------------------------------

So what you need is JAVA_HOME to be set correctly. As per the new installation guide (as of 12/29/2017), Just add the maven bin directory path to the PATH variable. It should do the trick.

ex : export PATH=/opt/apache-maven-3.5.2/bin:$PATH

Here is my Maven setup. You can use it as an example. You don't need anything else in order to use Maven.

M2_HOME is used for both Maven 2 and 3

export M2_HOME=/Users/xxx/sdk/apache-maven-3.0.5
export M2=$M2_HOME/bin
export MAVEN_OPTS="-Xmx1048m -Xms256m -XX:MaxPermSize=312M"
export PATH=$M2:$PATH

M2_HOME (and the like) is not to be used as of Maven 3.5.0. See MNG-5607 and Release Notes for details.