Section: Maintenance Commands (8)
Updated: 22 May 1999
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.
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:
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:
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:
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.
Comments can be embedded in the chat script. A comment is a line which starts with the # (hash) character in column 1. Such comment lines are just ignored by the chat program. If a '#' character is to be expected as the first character of the expect sequence, you should quote the expect string. If you want to wait for a prompt that starts with a # (hash) character, you would have to write something like this:
:# Now wait for the prompt and send logout string
'# ' logout:
If the string to send starts with an at sign (@), the rest of the string is taken to be the name of a file to read to get the string to send. 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 provides a way for chat to communicate with another program, for example, a program to prompt the user and receive a password typed in.
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:
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.
This sequence allows for clearing previously set ABORT strings. ABORT strings are kept in an array of a pre-determined size (at compilation time); CLR_ABORT will reclaim the space for cleared entries so that new strings can use that space.
The SAY directive allows the script to send strings to the user at the 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 will normally be redirected to the file /etc/ppp/connect-errors.
SAY strings must be enclosed in single or double quotes. If carriage return and line feed are needed in the string to be output, you must explicitly add them to your string.
The SAY strings could be used to give progress messages in sections of the script where you want to have 'ECHO OFF' but still let the user know what is happening. An example is:
SAY "Dialling your ISP...\n"
SAY "Waiting up to 2 minutes for connection ... "
SAY "Connected, now logging in ...\n"
SAY "Logged in OK ...\n" etc ...:
This sequence will only present the SAY strings to the user and all the details of the script will remain hidden. For example, if the above script works, the user will see:
:Dialling your ISP...
Waiting up to 2 minutes for connection ... Connected, now logging in ...
Logged in OK ...:
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:
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.
This sequence allows for clearing previously set REPORT strings. REPORT strings are kept in an array of a pre-determined size (at compilation time); CLR_REPORT will reclaim the space for cleared entries so that new strings can use that space.
The echo options controls whether the output from the modem is echoed to stderr. This option may be set with the -e option, but it can also be controlled by the ECHO keyword. The "expect-send" pair ECHO ON enables echoing, and ECHO OFF disables it. With this keyword you can select which parts of the conversation should be visible. For instance, with the following script:
ABORT 'NO CARRIER'
The HANGUP options control whether a modem hangup should be considered as an error or not. This option is useful in scripts for dialling systems which will hang up and call your system back. The HANGUP options can be ON or OFF.
When HANGUP is set OFF and the modem hangs up (e.g., after the first stage of logging in to a callback system), chat will continue running the script (e.g., waiting for the incoming call and second stage login prompt). As soon as the incoming call is connected, you should use the HANGUP ON directive to reinstall normal hang up signal behavior. Here is an (simple) example script:
'Callback login:' call_back_ID
ABORT "Bad Login"
'Callback Password:' Call_back_password
ABORT "NO CARRIER"
The initial timeout value is 45 seconds. This may be changed using the -t parameter. You can also specify "TIMEOUT 0".
To change the timeout value for the next expect string, the following example may be used:
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 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. 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. 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.
Environment variables are available within chat scripts, if the -E option was specified in the command line. The metacharacter $ is used to introduce the name of the environment variable to substitute. If the substitution fails, because the requested environment variable is not set, nothing is replaced for the variable.
The chat program will terminate with the following completion codes.
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.
The chat program is in public domain. This is not the GNU public license. If it breaks then you get to keep both pieces.
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