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What wins: the command line

Command-line options always beat saved settings. For any given setting:

  1. If you passed it on the command line, that value is used.
  2. Otherwise the saved preference is used.
  3. Otherwise the built-in default is used.

So if your preferences say "Next" but you run jnext --machine 48k, you get a 48K — for that run only, with nothing overwritten. This makes it safe to keep comfortable defaults for everyday use and still override them for a one-off, and it is why scripts should pass explicit options rather than depend on whatever a particular machine happens to have saved.

Any setting, from the command line

Every setting of the configuration file can be given for one run with --set section.key=value, using the file's own section and key names and values. Repeat it for more:

jnext --set audio.gain_db=-6 --set startup.silent=false --set paths.last_load_dir=/home/me/games

Three settings also have flags of their own, because they have menus of their own: --cpu-speed (3.5, 7, 14 or 28 MHz), --scale (1 to 3) and --crt (off, scanlines, soft, tv or pc). The keys, and the flag beside each key that has one, are listed by jnext --help and in jnext(1) under Settings for --set.

  • The later one wins. A flag and --set are the same setting, so --cpu-speed 28 --set startup.cpu_speed=0 starts at 3.5 MHz, in either order.
  • Nothing is written back. A value from the command line governs that run only; jnext.conf keeps what it had, even when something else makes JNEXT save the file (loading a program adds it to File > Recent Files, say).
  • The window takes over. Once the run is going, a change you make in it (Preferences OK or Apply, a menu, a file dialog that moves the last directory) replaces the command line's value from then on. Preferences shows the saved values, not the command line's.
  • A bad value stops the start-up, with a message naming the key and the value and exit status 1. A key the running frontend has no use for (a window setting with --headless, say) only produces a warning, so one command line can serve the window, the SDL-only build and headless runs.
  • The CPU speed is a starting value. On the Next machine NextZXOS sets its own speed while it boots, as the real firmware does.
  • A few things only --set can say: startup.silent=false plays sound for one run although the file says muted; startup.tape_fast_load=true forces fast loading; esp.allowed_hosts= (empty) allows any host over a saved list. An empty controller (--joy1-device "" or --set input.joy1_device=) picks Automatic over a saved controller.

Headless runs ignore the configuration file completely, so an automated test produces the same result on any machine.

The full list of options is in the jnext(1) man page, also available as USAGE.md.