chat - Automated conversational script with a modem
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
- -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
- -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. You can also use the
TIMEOUT string in order to specify the timeout.
- -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.
- Start with the echo option turned on. Echoing may also be
turned on or off at specific points in the chat script by using the
ECHO keyword. When echoing is enabled, all output from the modem is
echoed to stderr.
- Enables environment variable substitution within chat
scripts using the standard $xxx syntax.
- Request that the chat script be executed in a
verbose mode. The chat program will then log the execution state of
the chat script as well as all text received from the modem and the output
strings sent to the modem. The default is to log through the SYSLOG; the
logging method may be altered with the -S and -s flags.
- Request that the chat script be executed in a stderr
verbose mode. The chat program will then log all text received from
the modem and the output strings sent to the modem to the stderr device.
This device is usually the local console at the station running the chat
or pppd program.
- Use stderr. All log messages from '-v' and all error
messages will be sent to stderr.
- Do not use the SYSLOG. By default, error messages are sent
to the SYSLOG. The use of -S will prevent both log messages from '-v' and
error messages from being sent to the SYSLOG.
- -T <phone number>
- Pass in an arbitrary string, usually a phone number, that
will be substituted for the \T substitution metacharacter in a send
- -U <phone number 2>
- Pass in a second string, usually a phone number, that will
be substituted for the \U substitution metacharacter in a send string.
This is useful when dialing an ISDN terminal adapter that requires two
- If the script is not specified in a file with the -f
option then the script is included as parameters to 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
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 ssword: 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.
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
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
or NO CARRIER
. 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
sequence. It is written in the script as in the following
- ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ATDT5551212
This sequence will expect nothing; and then send the string ATZ. The expected
response to this is the string OK
. When it receives OK
string ATDT5551212 to dial the telephone. The expected string is
. 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
This sequence allows for clearing previously set 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.
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.
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:
- ABORT BUSY
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 ...
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
sequence. It is written in the script as in the following
- REPORT CONNECT ABORT BUSY '' ATDT5551212 CONNECT '' ogin:
This sequence will expect nothing; and then send the string ATDT5551212 to dial
the telephone. The expected string is CONNECT
. If the string
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
This sequence allows for clearing previously set 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
. 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 'BUSY'
ABORT 'NO CARRIER'
all output resulting from modem configuration and dialing is not visible, but
starting with the CONNECT
) message, everything will be
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
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)
- ABORT 'BUSY'
'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:
- ATZ OK ATDT5551212 CONNECT TIMEOUT 10 ogin:--ogin: TIMEOUT
5 assword: 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
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. 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
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.
- represents a backspace character.
- 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
- Delay for one second. The program uses sleep(1) which will
delay to a maximum of one second. (not valid in expect.)
- Insert a BREAK (not valid in expect.)
- Send a newline or linefeed character.
- Send a null character. The same sequence may be represented
by \0. (not valid in expect.)
- Pause for a fraction of a second. The delay is 1/10th of a
second. (not valid in expect.)
- Suppress writing the string to the SYSLOG file. The string
?????? is written to the log in its place. (not valid in
- Send or expect a carriage return.
- 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.
- Send or expect a tab character.
- Send the phone number string as specified with the
-T option (not valid in expect.)
- Send the phone number 2 string as specified with the
-U option (not valid in expect.)
- Send or expect a backslash character.
- Collapse the octal digits (ddd) into a single ASCII
character and send that character. (some characters are not valid in
- 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.)
Environment variables are available within chat scripts, if the -E
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,
is replaced for the variable.
program will terminate with the following completion codes.
- The normal termination of the program. This indicates that
the script was executed without error to the normal conclusion.
- 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.
- 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.
- 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
- The first string marked as an ABORT condition
- The second string marked as an ABORT condition
- The third string marked as an ABORT condition
- The fourth string marked as an ABORT condition
- The other termination codes are also strings marked as an
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 is in public domain. This is not the GNU public license.
If it breaks then you get to keep both pieces.