chat(1)
NAME
chat - Automated conversational script with a modem
SYNOPSIS
chat [ options ] script
DESCRIPTION
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.
OPTIONS
-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.
-l <lock file>
Perform the UUCP style locking using the indicated lock file.
If the file could not be created then the chat program will fail.
The lock file will be deleted only if the chat program fails to
perform the script for any reason. If the script is successful the
lock file will be left on the disk. It is expected that the lock
file will be deleted when the pppd process no longer wishes to use
the serial device.
The use of a lock file with chat and pppd's lock option should not
be used at the same time. They are mutually exclusive options and
will cause one or the other program to fail to achieve the required
lock if you use both.
-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.
-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.
CHAT SCRIPT
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.
ABORT STRINGS
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.
TIMEOUT
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.
SENDING EOT
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.
GENERATING BREAK
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.
ESCAPE SEQUENCES
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.)
SEE ALSO
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(8), uucp(1)
COPYRIGHT
The chat program is in public domain. This is not the GNU public license.
If it breaks then you get to keep both pieces.