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CHAT(8) System Manager's Manual CHAT(8)
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.
-e 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.
-E Enables environment variable substitution within chat
scripts using the standard $xxx syntax.
-v 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.
-V 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.
-s Use stderr. All log messages from '-v' and all error
messages will be sent to stderr.
-S 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 string.
-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 numbers.
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 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 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:
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.
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:
ABORT BUSY
ECHO OFF
SAY "Dialling 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" 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:
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.
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 'BUSY'
ABORT 'NO CARRIER'
'' ATZ
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 starting with the CONNECT (or BUSY) message,
everything will be echoed.
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:
ABORT 'BUSY'
'' ATZ
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
etc ...
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 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 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. 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.
'' 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.
\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. (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.)
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.
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.
This page is part of the ppp (An implementation of the Point-to-
Point Protocol (PPP)) project. Information about the project can
be found at ⟨https://ppp.samba.org/⟩. If you have a bug report
for this manual page, see
⟨https://github.com/paulusmack/ppp/issues⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/paulusmack/ppp⟩ on 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-07-19.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Chat Version 1.22 22 May 1999 CHAT(8)
Pages that refer to this page: pon(1), pppd(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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