How does one find out the command line corresponding to GUI app (eg, LibreOffice Writer)?

Run libreoffice --writer to start LibreOffice Writer.

In this case, I guessed that it might be libreoffice or similar, so I entered libre in a terminal and pressed tab twice. libreoffice was one of the options, so I ran that. It started something from which one could choose to go to Writer, Calc, Impress, etc. Since you asked specifically about Writer, I looked at man libreoffice, which mentions -writer as one of the command line options. Running libreoffice -writer worked but resulted in

$ libreoffice -writer
Warning: -writer is deprecated.  Use --writer instead.

And so I got to libreoffice --writer.

In general, one could try things like the following:

• Guess and try names, based on the name of the application. Use Tab-completion for help. Read man pages for options.
• Run the program (using a GUI menu or however else you run it), guess the process name, and check using ps aux | grep guessed_name.
• If you know that the program belongs to installed package X, run dpkg -L X. It will list installed files from package X, look for /usr/bin, /bin, /sbin, etc in the output.

• One can find out the process corresponding to a window as follows :

  Run xprop _NET_WM_PID, navigate to the target window (without clicking!), and click on it. This will print the PID corresponding to the window, for example

  _NET_WM_PID(CARDINAL) = 7394
  

  Now run ps -p 7394 (with 7394 replaced by the PID you got) to find out the process name for the given PID.

To find a command, try using command auto-complete:

type 'libre', then press ['Tab'], giving you:

libreoffice

then add ' --help', finally giving you:

libreoffice --help

This will give you ALL the possible options you might need ..

(snip)
LibreOffice 3.4  340m1(Build:402)

Usage: soffice [options] [documents...]

Options:
--minimized    keep startup bitmap minimized.
--invisible    no startup screen, no default document and no UI.
--norestore    suppress restart/restore after fatal errors.
--quickstart   starts the quickstart service
--nologo       don't show startup screen.
 :
--writer       create new text document.
--calc         create new spreadsheet document.
--draw         create new drawing.
--impress      create new presentation.
--base         create new database.
:

One very easy way which does not involve any guessing at all:

• Run alacarte (that's 'Main Menu' in the Gnome menu).
• Follow these steps to find out an application's command name:

1. Select the application.
2. Click Properties.
3. Find the command name for the application in the field Command, in this example it is file-roller.