There is an example script here:
linuxcnc.org/docs/2.7/html/gcode/m-code.html#mcode:m100-m199
The first line is important, "#!/bin/bash" tells linux that the script should be passed to "bash" (which is basically the Linux command line) to be interpreted.
The file needs to be saved as plain text in the same folder as the HAL and INI files (typically, but see the link there on file search paths) and it needs to be called "m101" or similar. It also needs to be set as executable (ie, it can run as a program). You can do this by right-clicking the file icon in the the fle manager and selecting "allow execution" or similar.
