The SGI serial port pinout is described at: http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?coll=hdwr&db=bks&fname=/SGI_EndUser/Indigo2_OG/apa.html.
Chances are that your linux server has a male DB9 connector. If all you have is USB, find a cheap RS232 / USB adaptor. The pinout for the DB9 is all over the web. Here is one example: http://www.aggsoft.com/rs232-pinout-cable/serial-port-db9.htm.
You need to find a circular male 8 pin mini-DIN connector to plug into the SGI, such as Digikey part number CP-2080-ND. Apple Macintosh computers also used the same connector, so you might just be able to cut the cord off an old Apple serial cable.
You only need to connect three wires: TX, RX, GND. To make a cable straight from the SGI to the linux server, you need to connect the TX from one machine to the RX of the other machine and vice versa. According to the pinouts on the links above, make the following connections:
Minicom on the linux server works fine for this. Install
minicom -s to configure it. Here is an
example of settings I had to change:
minicom with the serial port all plugged
in, and when the SGI boots you should start to see stuff at which
point quickly press <esc> as prompted.