Anyone knows how to write to /dev/ttyACM0 in bash? Any advice is appreciated?
By writing to the device you open it, the character gets sent and close it again. This shortly sets DTR which lets the Arduino reset (autoreset feature, that's necessary for a convenient upload functionality). Because that character is received by the Arduino during it's reset it doesn't have any consequences. On most Arduinos you can disable the autoreset feature by putting a 10µF capacitor between RST and GND.