custom m code

Can anyone recommend me a manual for creating custom m codes? I know there are a lot of examples out there but they are all very case specific and none explains how the card gets addressed and where to place bash files.

The documentation is here:
linuxcnc.org/docs/html/gcode/m-code.html#mcode:m100-m199

You save the file to a directory specified in the ini file for your config.
linuxcnc.org/docs/html/config/ini-config.html#_rs274ngc_section
It can also be in the directory specified in the "OPEN_FILE=" path specified in the Display section of your ini file.

For the most part custom M-codes work by directly setting the values of hal pins, such as the hal pin for a parallel port board "parport.0.pin-14-out"

Thanks for the quick reply. i will try that tomorrow.
bernd