For direct low-level printer control from a terminal, without specific software, I found the following solution with full credit thanks to user: http://stackexchange.com/users/6463673/meuh

Sharing here as may be of use to other users in the 3d Printing community, and I was unable to source a "complete" solution to this elsewhere.


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Step 1)
Create a custom python script that allows you to set arbitrary baud rates (make executable with `chmod u+x`).

    #!/usr/bin/python
    # set nonstandard baudrate. Original Question: http://unix.stackexchange.com/a/327366/119298
    import sys,array,fcntl
    
    # from /usr/lib/python2.7/site-packages/serial/serialposix.py
    # /usr/include/asm-generic/termbits.h for struct termios2
    #  [2]c_cflag [9]c_ispeed [10]c_ospeed
    def set_special_baudrate(fd, baudrate):
        TCGETS2 = 0x802C542A
        TCSETS2 = 0x402C542B
        BOTHER = 0o010000
        CBAUD = 0o010017
        buf = array.array('i', [0] * 64) # is 44 really
        fcntl.ioctl(fd, TCGETS2, buf)
        buf[2] &= ~CBAUD
        buf[2] |= BOTHER
        buf[9] = buf[10] = baudrate
        assert(fcntl.ioctl(fd, TCSETS2, buf)==0)
        fcntl.ioctl(fd, TCGETS2, buf)
        if buf[9]!=baudrate or buf[10]!=baudrate:
            print("failed. speed is %d %d" % (buf[9],buf[10]))
            sys.exit(1)
    
    set_special_baudrate(0, int(sys.argv[1]))


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Step 2) Run the script to set your baud rate.  
`./set_custom_baud_rate.py <> /dev/ttyUSB0 250000`


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Step 3) You can now monitor your printer output in a terminal window simply with:  
 `tail -f /dev/ttyUSB0`


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Step 4) And finally, open up a new terminal window, and you can directly send M or Gcode to your printer; example:  
`echo "M115" >> /dev/ttyUSB0`