The chat script defines communications. A script consists of one or more "expect-send" pairs of strings separated by spaces, with an optional "subexpect-subsend" string pair, separated by a dash (as in the following example:)

ogin:-BREAK-ogin: ppp ssword: hello2u2

The example indicates that the chat program should expect the string "ogin:". If it fails to receive a login prompt within the time interval allotted, it sends a break sequence to the remote and then expects the string "ogin:". If the first "ogin:" is received, the break sequence is not generated.

Upon receiving the login prompt, the chat program sends the string "ppp" and then expects the prompt "ssword:". When the password prompt is received, it sends the password hello2u2.

A carriage return is normally sent following the reply string. It is not expected in the "expect" string unless it is specifically requested by using the \r character sequence.

The expect sequence should contain only what is needed to identify the received data. Because it's stored on a disk file, it should not contain variable information. Generally it is not acceptable to look for time strings, network identification strings, or other variable pieces of data as an expect string.

To correct for characters that are corrupted during the initial sequence, look for the string "ogin:" rather than "login:". The leading "l" character may be received in error, creating problems in finding the string. For this reason, scripts look for "ogin:" rather than "login:" and "ssword:" rather than "password:".

An example of a simple script follows:

ogin: ppp ssword: hello2u2

The example can be intrepreted as: expect ogin:, send ppp, expect ...ssword:, send hello2u2.

When login to a remote peer is necessary, simple scripts are rare. At minimum, you should include sub-expect sequences in case the original string is not received. For example, consider the following script:

ogin:--ogin: ppp ssword: hello2u2

This script is more effective than the simple one used earlier. The string looks for the same login prompt; however, if one is not received, a single return sequence is sent and then the script looks for login: again. If line noise obscures the first login prompt, send the empty line to generate a login prompt again.

Comments can be embedded in the chat script. Comment lines are ignored by the chat program. A comment starts with the hash ("#") character in column one. If a # character is expected as the first character of the expect sequence, quote the expect string. If you want to wait for a prompt that starts with a # character, write something like this:

# Now wait for the prompt and send logout string 
'# ' logout

If the string to send begins with an at sign ("@"), the remainder of the string is interpreted as the name of the file that contains the string. If the last character of the data read is a newline, it is removed. The file can be a named pipe (or fifo) instead of a regular file. This enables chat to communicate with another program, for example, a program to prompt the user and receive a password typed in.

Many modems report the status of a call as a string. These status strings are often "CONNECTED" or "NO CARRIER" or "BUSY." If the modem fails to connect to the remote, you can terminate the script. Abort strings may be specified in the script using the ABORT sequence. For example:

ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ATDT5551212 CONNECT

This sequence expects nothing and sends the string ATZ. The expected response is the string OK. When OK is received, the string ATDT5551212 dials the telephone. The expected string is CONNECT. If CONNECT is received, the remainder of the script is executed. When the modem finds a busy telephone, it sends the string BUSY, causing the string to match the abort character sequence. The script fails because it found a match to the abort string. If the NO CARRIER string is received, it aborts for the same reason.

The CLR_ABORT sequence clears previously set ABORT strings. ABORT strings are kept in an array of a pre-determined size; CLR_ABORT reclaims the space for cleared entries, enabling new strings to use that space.

The SAY string enables the script to send strings to a user at a terminal via standard error. If chat is being run by pppd and pppd is running as a daemon (detached from its controlling terminal), standard error is normally redirected to the /etc/ppp/connect-errors file.

SAY strings must be enclosed in single or double quotes. If carriage return and line feed are required for the output, you must explicitly add them to your string.

The SAY string can provide progress messages to users even with "ECHO OFF." For example, add a line similar to the following to the script:

ABORT BUSY  
ECHO OFF  
SAY "Dialing your ISP...\n"  
'' ATDT5551212  
TIMEOUT 120 
SAY "Waiting up to 2 minutes for connection ..." 
CONNECT ''  
SAY "Connected, now logging in ...\n" 
ogin: account 
ssword: pass 
$ \c
SAY "Logged in OK ... \n"

This sequence hides script detail while presenting the SAY string to the user. In this case, you will see:

Dialing your ISP... Waiting up to 2 minutes for connection...Connected, now logging in... Logged in OK ...

REPORT is similar to the ABORT string. With REPORT, however, strings and all characters to the next control character (such as a carriage return), are written to the report file.

REPORT strings can be used to isolate a modem's transmission rate from its CONNECT string and return the value to the chat user. Analysis of the REPORT string logic occurs in conjunction with other string processing, such as looking for the expect string. It's possible to use the same string for a REPORT and ABORT sequence, but probably not useful.

Report strings may be specified in the script using the REPORT sequence. For example:

REPORT CONNECT
ABORT BUSY
ATDT5551212 CONNECT
ogin: account

The above sequence expects nothing, then sends the string ATDT5551212 to dial the telephone. The expected string is CONNECT. If CONNECT is received, the remainder of the script is executed. In addition, the program writes the string CONNECT to the report file (specified by -r) in addition to any characters that follow.

CLR_REPORT clears previously set REPORT strings. REPORT strings are kept in an array of a pre-determined size; CLR_REPORT reclaims the space for cleared entries so that new strings can use that space.

ECHO determines if modem output is echoed to stderr. This option may be set with the -e option, but can also be controlled by the ECHO keyword. The "expect-send" pair ECHO ON enables echoing, and ECHO OFF disables it. With ECHO, you can select which parts of the conversation should be visible. In the following script:

ABORT   'BUSY' 
ABORT   'NO CARRIER'
""    AT&F
OK\r\n  ATD1234567
\r\n    \c
ECHO    ON
CONNECT \c
ogin:   account

All output resulting from modem configuration and dialing is not visible, but output is echoed beginning with the CONNECT (or BUSY) message.

The HANGUP option determines if a modem hangup is considered as an error. HANGUP is useful for dialing systems that hang up and call your system back. HANGUP can be ON or OFF. When HANGUP is set to OFF and the modem hangs up (for example, following the first stage of logging in to a callback system), chat continues running the script (for example, waiting for the incoming call and second stage login prompt). When the incoming call is connected, use the HANGUP ON string to reinstall normal hang up signal behavior. An example of a simple script follows:

ABORT   'BUSY'
 ""  AT&F
 OK\r\n  ATD1234567
 \r\n    \c
 CONNECT \c
 'Callback login:' call_back_ID
 HANGUP OFF
 ABORT "Bad Login"
 'Callback Password:' Call_back_password
 TIMEOUT 120
 CONNECT \c
 HANGUP ON
 ABORT "NO CARRIER"
 ogin:--BREAK--ogin: real_account

The initial timeout value is 45 seconds. Use the -t parameter to change the intial timeout value.

To change the timeout value for the next expect string, the following example can be used:

''"AT&F
 OK ATDT5551212
 CONNECT \c
 TIMEOUT 10
 ogin:--ogin: username
 TIMEOUT 5
 assword: hello2u2

The example changes the timeout to ten seconds when it expects the login: prompt. The timeout is changed to five seconds when it looks for the password prompt.

Once changed, the timeout value remains in effect until it is changed again.

The EOT special reply string instructs the chat program to send an EOT character to the remote. This is equivalent to using ^D\c as the reply string. The EOT string normally indicates the end-of-file character sequence. A return character is not sent following the EOT. The EOT sequence can embedded into the send string using the sequence ^D.

The BREAK special reply string sends a break condition. The break is a special transmitter signal. Many UNIX systems handle break by cycling through available bit rates, and sending break is often needed when the remote system does not support autobaud. BREAK is equivalent to using \K\c as the reply string. You embed the break sequence into the send string using the \K sequence.

Expect and reply strings can contain escape sequences. Reply strings accept all escape sequences, while expect strings accept most sequences. A list of escape sequences is presented below. Sequences that are not accepted by expect strings are indicated.

''

Expects or sends a null string. If you send a null string, chat sends the return character. If you expect a null string, chat proceeds to the reply string without waiting. This sequence can be a pair of apostrophes or quote mark characters.

\b

Represents a backspace character.

\c

Suppresses the newline at the end of the reply string. This is the only method to send a string without a trailing return character. This sequence must be at the end of the send string. For example, the sequence hello\c will simply send the characters h, e, l, l, o. (Not valid in expect.)

\d

Delay for one second. The program uses sleep(1) which delays to a maximum of one second. (Not valid in expect.)

\K

Insert a BREAK. (Not valid in expect.)

\n

Send a newline or linefeed character.

\N

Send a null character. The same sequence may be represented by \0. (Not valid in expect.)

\p

Pause for 1/10th of a second. (Not valid in expect.)

\q

Suppress writing the string to syslog. The string ?????? is written to the log in its place. (Not valid in expect.)

\r

Send or expect a carriage return.

\s

Represents a space character in the string. Can be used when it is not desirable to quote the strings which contains spaces. The sequence 'HI TIM' and HI\sTIM are the same.

\t

Send or expect a tab character.

\T

Send the phone number string as specified with the -T option. (Not valid in expect.)

\U

Send the phone number 2 string as specified with the -U option. (Not valid in expect.)

\\

Send or expect a backslash character.

\ddd

Collapse the octal digits (ddd) into a single ASCII character and send that character. (\000 is not valid in an expect string.)

^C

Substitute the sequence with the control character represented by C. For example, the character DC1 (17) is shown as ^Q. (Some characters are not valid in expect.)