When UUCP decodes this address, it reads the first system name (arthur) and sends it to that system. The UUCP processes on arthur then examine the rest of the address and realize the mail is to be sent on to warlock. If you have permission to forward through arthur, UUCP on arthur sends the mail through to warlock for you. You can have many hops in an address, as long as each system you are connecting to allows the pass-through and can connect to the next machine on the list. For example, the address would send data through arthur, warlock, chatton, and vader in order, and then to the user alex:

arthur!warlock!chatton!vader!alex

The addresses must be specified in the proper hop order or the address will fail. This multi-hop addressing can be very useful if a number of friends have local connections to other machines, allowing you to easily set up a complex network. The hard part is usually tracking the names of the systems involved. (The exclamation mark in the address is called a bang, so the address above is spoken as “arthur-bang-warlock-bang-chatton-bang-vader-bang-alex.”)

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Tip:  
Some shells don’t like the bang character because it is interpreted as a special shell command. Shells, such as the C Shell, use the exclamation mark to recall previous commands so you must replace the bang with a \ to prevent the shell’s interpretation. Addresses then become arthur\!chatton\!yvonne. This looks funny, but you get used to it.

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Depending on how you have your UUCP system set, it may call out to the other systems in an address whenever something is submitted to it, or if callout times are limited, the data may be spooled until a call is allowed. You have already seen how to set callout times in the /usr/lib/uucp/sys and /usr/lib/uucp/Systems files.

A quick caution about relying on UUCP for delivery of information: If the systems that are being used are not set to call immediately when there is something queued, your data can take a long time to get to its destination. For example, if one of the hops in your address only calls the next machine in the address once a day, you may have a 24-hour delay in delivery. This can be exacerbated by each machine in the network.

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Tip:  
Don’t rely on the contents of your data sent through UUCP to be kept confidential. Once on a remote system, any user with access privileges to the queue could snoop into your data. Ideally, the file permissions prevent anyone but the superuser accessing the data, but not all systems keep tight security. If you must send sensitive data, encrypt it and let the recipient know the decryption key through another format (not in a mail message!).

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UUCP deals with all transfers as jobs, a term you’ll encounter often when working with UUCP and its documentation. A job is a command that is to be executed on the remote system, a file that is to be transferred to or from the remote, or any other task that you want performed between the two systems.

Sending Email with UUCP

Most utilities like mail packages understand the UUCP addresses, so you don’t have to worry about email not reaching the proper destination. You don’t usually have to make any changes at all to applications running under Linux to get them to understand this UUCP address format. In the last section you saw how the mail package can be used with UUCP addresses.

You can use any of the usual mail command options to modify the behavior of the package. For example, to send the contents of the file data_1 to yvonne on system chatton through the system arthur, and tag the mail with a subject heading, issue the following command:

mail -s “Data file” arthur\!chatton\!yvonne < data_1

Most mail packages available for Linux, including X-based mailers, work perfectly well with UUCP addresses, as well as the more common Internet addresses, but you may want to check before adopting a new mail package.