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manual page [] for chat 1.8
$Id: chat.8,v 1.1.1.1 1994/11/12 05:25:32 lars Exp $
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chat - Automated conversational script with a modem chat [ options ] script

The chat program defines a conversational exchange between the computer and the modem. Its primary purpose is to establish the connection between the Point-to-Point Protocol Daemon (pppd) and the remote's pppd process.

-f <chat file> Read the chat script from the chat file. The use of this option is mutually exclusive with the chat script parameters. The user must have read access to the file. Multiple lines are permitted in the file. Space or horizontal tab characters should be used to separate the strings.

-t <timeout> Set the timeout for the expected string to be received. If the string is not received within the time limit then the reply string is not sent. An alternate reply may be sent or the script will fail if there is no alternate reply string. A failed script will cause the chat program to terminate with a non-zero error code.

-r <report file> Set the file for output of the report strings. If you use the keyword REPORT, the resulting strings are written to this file. If this option is not used and you still use REPORT keywords, the stderr file is used for the report strings.

-v Request that the chat script be executed in a verbose mode. The chat program will then log all text received from the modem and the output strings which it sends to the SYSLOG.

script If the script is not specified in a file with the -f option then the script is included as parameters to the chat program.

The chat script defines the 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

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

Once it received the login prompt the chat program will send the string ppp and then expect the prompt "ssword:". When it receives the prompt for the password, it will send 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 string. Since it is normally stored on a disk file, it should not contain variable information. It is generally not acceptable to look for time strings, network identification strings, or other variable pieces of data as an expect string.

To help correct for characters which may be corrupted during the initial sequence, look for the string "ogin:" rather than "login:". It is possible that the leading "l" character may be received in error and you may never find the string even though it was sent by the system. For this reason, scripts look for "ogin:" rather than "login:" and "ssword:" rather than "password:".

A very simple script might look like this:

ogin: ppp ssword: hello2u2

In other words, expect ....ogin:, send ppp, expect ...ssword:, send hello2u2.

In actual practice, simple scripts are rare. At the vary least, you should include sub-expect sequences should the original string not be received. For example, consider the following script:

ogin:--ogin: ppp ssowrd: hello2u2

This would be a better script than the simple one used earlier. This would look for the same login: prompt, however, if one was not received, a single return sequence is sent and then it will look for login: again. Should line noise obscure the first login prompt then sending the empty line will usually generate a login prompt again.

Many modems will report the status of the call as a string. These strings may be CONNECTED or NO CARRIER or BUSY. It is often desirable to terminate the script should the modem fail to connect to the remote. The difficulty is that a script would not know exactly which modem string it may receive. On one attempt, it may receive BUSY while the next time it may receive NO CARRIER.

These "abort" strings may be specified in the script using the ABORT sequence. It is written in the script as in the following example:

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

This sequence will expect nothing; and then send the string ATZ. The expected response to this is the string OK. When it receives OK, the string ATDT5551212 to dial the telephone. The expected string is CONNECT. If the string CONNECT is received the remainder of the script is executed. However, should the modem find a busy telephone, it will send the string BUSY. This will cause the string to match the abort character sequence. The script will then fail because it found a match to the abort string. If it received the string NO CARRIER, it will abort for the same reason. Either string may be received. Either string will terminate the chat script.

A report string is similar to the ABORT string. The difference is that the strings, and all characters to the next control character such as a carriage return, are written to the report file.

The report strings may be used to isolate the transmission rate of the modem's connect string and return the value to the chat user. The analysis of the report string logic occurs in conjunction with the other string processing such as looking for the expect string. The use of the same string for a report and abort sequence is probably not very useful, however, it is possible.

The report strings to no change the completion code of the program.

These "report" strings may be specified in the script using the REPORT sequence. It is written in the script as in the following example:

REPORT CONNECT ABORT BUSY '' ATDT5551212 CONNECT '' ogin: account

This sequence will expect nothing; and then send the string ATDT5551212 to dial the telephone. The expected string is CONNECT. If the string CONNECT is received the remainder of the script is executed. In addition the program will write to the expect-file the string "CONNECT" plus any characters which follow it such as the connection rate.

The initial timeout value is 45 seconds. This may be changed using the -t parameter.

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

ATZ OK ATDT5551212 CONNECT TIMEOUT 10 ogin:--ogin: TIMEOUT 5 assowrd: hello2u2

This will change the timeout to 10 seconds when it expects the login: prompt. The timeout is then changed to 5 seconds when it looks for the password prompt.

The timeout, once changed, remains in effect until it is changed again.

The special reply string of EOT indicates that the chat program should send an EOT character to the remote. This is normally the End-of-file character sequence. A return character is not sent following the EOT.

R The EOT sequence may be embedded into the send string using the sequence ^D.

The special reply string of BREAK will cause a break condition to be sent. The break is a special signal on the transmitter. The normal processing on the receiver is to change the transmission rate. It may be used to cycle through the available transmission rates on the remote until you are able to receive a valid login prompt.

R The break sequence may be embedded into the send string using the \\K sequence.

The expect and reply strings may contain escape sequences. All of the sequences are legal in the reply string. Many are legal in the expect. Those which are not valid in the expect sequence are so indicated.

'' Expects or sends a null string. If you send a null string then it will still send the return character. This sequence may either be a pair of apostrophe or quote 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. It 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 will delay 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 a fraction of a second. The delay is 1/10th of a second. (not valid in expect.)

\\\\q Suppress writing the string to the SYSLOG file. 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. This may 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.

\\\\\\\\ Send or expect a backslash character.

\\\\ddd Collapse the octal digits (ddd) into a single ASCII character and send that character. (some characters are not valid in expect.)

\^^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.)

The chat program will terminate with the following completion codes.

0 The normal termination of the program. This indicates that the script was executed without error to the normal conclusion.

1 One or more of the parameters are invalid or an expect string was too large for the internal buffers. This indicates that the program as not properly executed.

2 An error occurred during the execution of the program. This may be due to a read or write operation failing for some reason or chat receiving a signal such as SIGINT.

3 A timeout event occurred when there was an expect string without having a "-subsend" string. This may mean that you did not program the script correctly for the condition or that some unexpected event has occurred and the expected string could not be found.

4 The first string marked as an ABORT condition occurred.

5 The second string marked as an ABORT condition occurred.

6 The third string marked as an ABORT condition occurred.

7 The fourth string marked as an ABORT condition occurred.

... The other termination codes are also strings marked as an ABORT condition.

Using the termination code, it is possible to determine which event terminated the script. It is possible to decide if the string "BUSY" was received from the modem as opposed to "NO DIAL TONE". While the first event may be retried, the second will probably have little chance of succeeding during a retry.

Additional information about chat scripts may be found with UUCP documentation. The chat script was taken from the ideas proposed by the scripts used by the uucico program.

uucico(1), uucp(1)

The chat program is in public domain. This is not the GNU public license. If it breaks then you get to keep both pieces.