GRASS Troubleshooting: how to face the first difficulties of the beginning...

When you are using a software for the first time always expect problems! For any problem related to a specific GRASS command (you ran r.whatever and it caused error n°78371647....) see the GRASS official site or the users mailing list (learn to use this precious resource!). In this part of the tutorial you will only find the main problems first time users have using GRASS, expecially if they are not so confident with Unix-like Operative Systems. This part has been written taking into account a long GRASS teaching experience (since 1996...) This is an advice for Windows users who start using GRASS: remember to close terminals, menus or forms only when the processes you ran have completely finished! If you don't check what's written inside the windows, you could shut a terminal (or a form) with an active process and this can cause the program not to respond anymore. Don't worry, with a little practice everything becomes much easier!

Running GRASS you receive this message: "concurrent user not allowed" and GRASS doesn't start...

Two cases are possible:

Somebody else is running GRASS with the same user, from another terminal, in the same mapset. A user can only run one GRASS instance within the same mapset.
The last time you used GRASS, you didn't close the session properly. Go to the mapset directory, look for the hidden file .gislock and erase it. Remember that to quit GRASS you must select exit from the menu File and the graphic interface quits, but on the terminal, GRASS is still open! So, to close GRASS properly it is necessary to write also exit on the terminal and press enter. A common first time user error is to exit only from GRASS graphical interface and close the terminal with the X without prompting exit.

Running a command on a monitor you have this error:"monitor x... is in use by user_name" (user_name it's YOUR user name)

The monitor is locked by a previous un-finished operation. Try the following operations: a) close this operation (press mouse right button on the monitor or on its form); b) kill the process in this way: first digit ps -a |grep x on the terminal to know the PID (process id), then prompt kill -9 PID ; c) kill the process as seen before then manually delete the locking file by typing: rm /GRASS_install_directory/locks/computer_name/*

After running some commands to create new maps you can't see any output

Many commands can create maps but they don't display them automatically: you will have to display them manually using the Graphical interface or d.rast, d.vect. Some operations that takes a long while to be concluded run in background, and you will receive an e-mail when they are finished.

You are trying to display a file or to obtain an elaboration but you can't see anything in the monitor and apparently the result of the elaboration contains no data

Make sure that the right region has been set with g.region and adapt the region to the map that must be displayed/elaborated.

You can't find some data anymore (and you do know they exists!)

Check if you have set the right location and mapset, and if you have the right permissions to read/write these files/directories.

You wrote r.mapcalc formulas but they don't work

Maps name cannot contain algebric symbols (like +/-) or r.mapcalc will see them as operators. Remember also to check syntax, quotes, spaces, filenames... and try again.

You are running a command from the console, entering goofy filename, and GRASS can't find it. But you're sure that the name is goofy!

Map and file names are case sensitive, so Goofy or GOOFY or goofy or goofY are four different maps or files! Check name's capital letters (you can display a list of available maps using g.list or listing the file on the terminal or using a file manager) and try again.

You want to use maps thet have been created by someone else (or you created them a long time ago and now you can't remember anything about them!) and you need to know: file contents, how they have been created, if they have been reclassed or not, if they have been generated by r.mapcalc and you would like to know which formulas have been used...

In the hist directory of the mapset there's a textfile for each map: there you can find file history, including information about reclass and any r.mapcalc operation, or if the original file has been imported from an interchange format. To read these files use a text editor or run r.info.

For further informations on GRASS installation and many more usage problems see GRASS FAQ on the official site..