dip provides a number of commands that configure your serial line and modem. Some of these are obvious, such as port, which selects a serial port, and speed, databits, stopbits, and parity, which set the common line parameters. The modem command selects a modem type. Currently, the only type supported is HAYES (capitalization required). You have to provide dip with a modem type, or else it will refuse to execute the dial and reset commands. The reset command sends a reset string to the modem; the string used depends on the modem type selected. For Hayes-compatible modems, this string is ATZ.

The flush code can be used to flush out all responses the modem has sent so far. Otherwise, a chat script following reset might be confused because it reads the OK responses from earlier commands.

The init command selects an initialization string to be passed to the modem before dialing. The default for Hayes modems is "ATE0 Q0 V1 X1“, which turns on echoing of commands and long result codes, and selects blind dialing (no checking of dial tone). Modern modems have a good factory default configuration, so this is a little unnecessary, though it does no harm.

The dial command sends the initialization string to the modem and dials up the remote system. The default dial command for Hayes modems is ATD.

The echo command serves as a debugging aid. Calling echo on makes dip echo to the console everything it sends to the serial device. This can be turned off again by calling echo off.

dip also allows you to leave script mode temporarily and enter terminal mode. In this mode, you can use dip just like any ordinary terminal program, writing the characters you type to the serial line, reading data from the serial line, and displaying the characters. To leave this mode, enter Ctrl-].

The get command is dip’s way of setting a variable. The simplest form is to set a variable to a constant, as we did in cowslip.dip. You may, however, also prompt the user for input by specifying the keyword ask instead of a value:

DIP> get $local ask
Enter the value for $local: _

A third method is to obtain the value from the remote host. Bizarre as it seems at first, this is very useful in some cases. Some SLIP servers will not allow you to use your own IP address on the SLIP link, but will rather assign you one from a pool of addresses whenever you dial in, printing some message that informs you about the address you have been assigned. If the message looks something like "Your address: 192.168.5.74“, the following piece of dip code would let you pick up the address:

# finish login
wait address: 10
get $locip remote

This is the command used to echo text to the console from which dip was started. Any of dip’s variables may be used in print commands. Here’s an example:

DIP> print Using port $port at speed $speed
Using port ttyS3 at speed 38400

dip understands only a predefined set of variables. A variable name always begins with a dollar symbol and must be written in lowercase letters.

The $local and $locip variables contain the local host’s name and IP address. When you store the canonical hostname in $local, dip will automatically attempt to resolve the hostname to an IP address and to store it in the $locip variable. A similar but backward process occurs when you assign an IP address to the $locip variable; dip will attempt to perform a reverse lookup to identify the name of the host and store it in the $local variable.

The $remote and $rmtip variables operate in the same way for the remote host’s name and address. $mtu contains the MTU value for the connection.

These five variables are the only ones that may be assigned values directly using the get command. A number of other variables are set as a result of the configuration commands bearing the same name, but may be used in print statements; these variables are $modem, $port, and $speed.

$errlvl is the variable through which you can access the result of the last command executed. An error level of 0 indicates success, while a nonzero value denotes an error.

The if command is a conditional branch, rather than a full-featured programming if statement. Its syntax is:

if var 
                     op 
                     number goto label

The expression must be a simple comparison between one of the variables $errlvl, $locip, and $rmtip. var must be an integer number; the operator op may be one of ==, !=, <, >, <=, and >=.

The goto command makes the execution of the script continue at the line following that bearing the label. A label must be the first word on the line and must be followed immediately by a colon.

These commands help implement simple chat scripts in dip. The send command outputs its arguments to the serial line. It does not support variables, but understands all C-style backslash character sequences, such as \n for newline and \b for backspace. The tilde character (~) can be used as an abbreviation for carriage return/newline.

The wait command takes a word as an argument and will read all input on the serial line until it detects a sequence of characters that match this word. The word itself may not contain any blanks. Optionally, you may give wait a timeout value as a second argument; if the expected word is not received within that many seconds, the command will return with an $errlvl value of 1. This command is used to detect login and other prompts.

The sleep command may be used to wait for a certain amount of time; for instance, to patiently wait for any login sequence to complete. Again, the interval is specified in seconds.

These commands are used to flip the serial line to SLIP mode and configure the interface.

The mode command is the last command executed by dip before going into daemon mode. Unless an error occurs, the command does not return.

mode takes a protocol name as argument. dip currently recognizes SLIP, CSLIP, SLIP6, CSLIP6, PPP, and TERM as valid names. The current version of dip does not understand adaptive SLIP, however.

After enabling SLIP mode on the serial line, dip executes ifconfig to configure the interface as a point-to-point link, and invokes route to set the route to the remote host.

If, in addition, the script executes the default command before mode, dip creates a default route that points to the SLIP link.