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I'm a bit of a newbie when it comes to QEMU, and I wondered how I tell it to put the serial ports output into a file?

Thanks

How about sending to it from outside the VM?

Pick a different character device instead of "file".

If you still need to write the output to a log, use chardev to specify the device. Here's an example of using chardev in place of "-serial stdio" to write the output to a log.

The "-chardev" option is explained in more detail in the QEMU manual.