Programming the Pico
There's a couple of ways to get the necessary binary files for programming the Pico. You can build them yourself from this repo, or you can download pre-compiled images here. The files are provide for both UART and USB console modes. UF2 files can be used the standard bootsel button USB way, or the ELF format files for GDB users.
The pshell console
pshell provides a Unix-like command line interface via a serial interface. This can be via UART or emulated over USB to a separately hosted terminal program in VT100 mode. In Linux for example you could connect via UART wih
minicom -c on -b 115200 -o -D /dev/serial0
or via USB with
minicom -c on -b 115200 -o -D /dev/ttyACM0
The screen command can also be used, but does not supoort xmodem transfers.
Getting files to and from the Pico
Along the line you may need to transfer files back and forth between your host and the Pico. You'd do this using the xmodem protocol, supported by both the Pico and minicom.
The xput command intructs pshell to receive a file from the host. The xget command does the opposite, it tells the Pico to transmit a file to the host. xmodem transfers single files. A tar command allows you create Linux compatible tarballs with multiple files, which can then be transferred as a single file.