Real-time commands are single characters that Grbl executes immediately, even in the middle of a program and regardless of the queue. The app sends them through the Hold, Resume and Stop buttons and the override sliders; you almost never type them by hand. Characters from 0x80 upwards cannot be typed, they are sent as bytes.
? · Status report
Grbl answers with state and position, such as <Idle|MPos:0.000,0.000,0.000|FS:0,0>. The app asks several times per second.
! · Feed hold
Decelerates under control and stops; the position is kept. The spindle keeps running. In the app: Hold button.
~ · Cycle start / resume
Continues after hold or M0. In the app: Resume button.
Strg+X (0x18) · Soft reset
Aborts everything immediately and restarts Grbl. If a move is running, the position is uncertain afterwards (alarm 3). A hold (!) before the reset keeps the position.
0x84 · Safety door
Acts like an opened safety door: hold, spindle off, optionally parking.
0x85 · Jog cancel
Stops a $J= move immediately and clears its queue. The app sends it when you release a continuous-jog arrow.
0x90 bis 0x94 · Feed override
Sets the feed to 100 % (0x90), raises or lowers it by 10 % (0x91/0x92) or by 1 % (0x93/0x94). Grbl allows 10 to 200 %. The app's speed goes up to 500 %; above 200 % it rescales the F values itself.
0x95 bis 0x97 · Rapid override
Rapids at 100 %, 50 % or 25 %.
0x99 bis 0x9D · Spindle override
Spindle speed to 100 % (0x99), ±10 % (0x9A/0x9B) or ±1 % (0x9C/0x9D); 10 to 200 % are allowed.
0x9E · Spindle stop in hold
Switches the spindle off during a hold and back on when resuming.
0xA0 / 0xA1 · Coolant toggle
Toggles flood (0xA0) or mist (0xA1) coolant while running.