Two computers

In this tutorial you join two computers with network cards and a cable, give each a name, and use rsh and rcp to run commands on one from the other and to copy files between them.

What you need

• Two working computers, each built as in Your first computer: case, CPU, RAM, EEPROM, a mcos disk, battery and screen. A second disk is made the same way as the first, so each computer has its own copy of mcos and its own files.
• Two Network Cards (a wired card: its recipe uses a cable; see its page). Wireless WLAN cards work the same way in this tutorial and need no cable, within their range.
• Some cable (a craft gives eight)

The whole thing

Once both computers have a network card and a cable between them, this is every command, in order (each step below explains its part):

# on the second computer
hostname beta
rc rshd start              # rc rshd enable: at every boot too

# on the first computer
hostname alpha
rsh beta hostname          # prints beta: it ran there
rsh beta 'ls /nothing'; echo $?    # beta's error, and its exit status, 1
echo "sent from alpha" > note.txt
rcp note.txt beta:/home/note.txt
rsh beta cat /home/note.txt        # sent from alpha
rcp beta:/home/note.txt ./back.txt
rsh beta                   # a shell on beta; exit (or ~.) to come back

Steps

1. Fit a card in each

With both computers off, open each case view and put a Network Card in an expansion (EXP) socket. A card in a slot is on its computer’s bus, and each card has an address of its own (an id, like a MAC address on a real network card).

2. Run a cable between them

Place the two cases a few blocks apart, then a line of cable from one to the other so a cable touches the side of each case (not its front). Cables connect to each other by themselves. You will see the cable blocks join into a line.

A cable is a shared wire, not a private one: the two cases are two hosts on the same cable, and each case’s other devices still belong to that case alone. A case ends a run of cable: a cable cannot pass through a case to reach another. See the bus and networking.

3. Boot both and name them

Turn both computers on. On the first, type:

hostname alpha

On the second:

hostname beta

You will see nothing printed. hostname with no name prints the current one. Without a name set, a computer’s name is its first network card’s address. The name is kept in /etc/hostname, so the computer has it again after a reboot. See hostname.

To see the card itself, run lsdev -i: the network card appears with its id: address.

4. Start the remote shell service on beta

A computer can be reached by rsh only while it runs rshd, the remote shell service (remote shell). On beta, type:

rc rshd start

Give it a moment to start. Run rc with no arguments to see the services: rshd should be listed as running. To start it at every boot, run rc rshd enable (services, rc).

rshd has no passwords and no encryption: anyone whose card reaches the computer can run commands on it. That is how the Berkeley r-commands it is named for worked, and why it is a service that is off until you turn it on.

5. Run a command on the other computer

On alpha:

rsh beta hostname

You will see beta: the command ran there and its output came back here. Try one that fails:

rsh beta 'ls /nothing'; echo $?

You will see beta’s error message, then 1: the command’s exit status comes back as rsh’s own.

For a shell on the other computer, give no command:

rsh beta

Your prompt is now beta’s. Anything you run runs there. To come back, type exit, or at the start of a line type ~. to cut the connection.

6. Copy files

Make a file on alpha:

echo "sent from alpha" > note.txt
rcp note.txt beta:/home/note.txt
rsh beta cat /home/note.txt

You will see sent from alpha. The other direction works too:

rcp beta:/home/note.txt ./back.txt

A directory needs -r: rcp -r mydir beta:/home/. Without it, a directory is refused.

When something goes wrong

Congratulations

Your computers now talk. A computer that wakes when another sends it a message, sockets programs can use, and more are on the networking page. Last, manage disks with the Disk Station.