Setting environment variable 'TURTLEBOT_STAGE_MAP_FILE'

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I'm trying to get the turtlebot stage simulator running from these instructions: http://wiki.ros.org/turtlebot_stage/T.. .

But, putting the first line in the terminal:

roslaunch turtlebot_stage turtlebot_in_stage.launch

gives this output:

... logging to /home/yaseen-14/.ros/log/0b120c28-319b-11e5-ac3b-685d4316eccf/roslaunch-yaseen14-HP-Pavilion-m6-Notebook-PC-11910.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

Invalid <arg> tag: environment variable 'TURTLEBOT_STAGE_MAP_FILE' is not set. 

Arg xml is <arg default=" $(env TURTLEBOT_STAGE_MAP_FILE)" name="map_file"/>
The traceback for the exception was written to the log file

Not sure why this is happening but would appreciate if anyone could help me out. Cheers!