Bash/[SHELF][1], 243
========

<!-- language-all: lang-bsh -->

"<b>SHE</b>ll go<b>LF</b>" is a golfing library for Bash which provides a few useful aliases. This is a valid answer as the library existed and was on GitHub before the challenge was posted.

How to run
----

This takes the initial position (separated by commas as specified; this adds a lot of characters to the code) as its first argument, and the instructions on standard input.

    source shelf.sh #you must load SHELF first
    source rover.sh 1,2,N<<<MRMLM #now run the script via source so it has access to SHELF

Sample output
-----

    2,4,N
Code
---


    o=$1
    for x in `Y . '& '`;do
    d $x R&&o=`y NESW ESWN<<<$o`
    d $x L&&o=`y NESW WNES<<<$o`
    d $x M&&{
    z=(`y , \ <<<$o`)
    case ${z[2]} in
    N) z[1]=$[z[1]+1];;S) z[1]=$[z[1]-1];;W) z[0]=$[z[0]-1];;E) z[0]=$[z[0]+1];;esac
    o=`P ${z[@]}|y \  ,`
    }
    done
    p $o

Explanation
------

`d` is for comparison; it returns 0 if its two arguments are equal and 1 otherwise, it can then have other commands chained onto it with `&&` and `||`.

`y` is like `tr` (but done through `sed`).

`Y` is like `sed 's/.../.../g'` for its two arguments.

`P` is `echo -e -n`; `p` is just `echo -e`.

    o=$1                         #save first argument to a variable
    for x in `Y . '& '`;do       #add a space after every char on stdin, then loop for each one
    d $x R&&o=`y NESW ESWN<<<$o` #if it is R, change $o to turn right
    d $x L&&o=`y NESW WNES<<<$o` #similarly for L
    d $x M&&{                    #if it is M,
    z=(`y , \ <<<$o`)            #replace commas with spaces and convert to array
    case ${z[2]} in              #switch...case statement
    #adjust the coordinates in the array, depending on where the rover is facing 
    N) z[1]=$[z[1]+1];;S) z[1]=$[z[1]-1];;W) z[0]=$[z[0]-1];;E) z[0]=$[z[0]+1];;esac
    #turn the data back into a comma-separated string
    o=`P ${z[@]}|y \  ,`
    }
    done #end loop
    p $o #print output

  [1]: https://github.com/professorfish/bash-shelf