CHAT(8) Maintenance Procedures CHAT(8)
chat - automated conversational exchange tool
program implements a conversational text-based exchange between
the computer and any serial device, including (but not limited to) a
modem, an ISDN TA, and the remote peer itself, establishing a connection
between the Point-To-Point Protocol daemon (pppd
) and the remote pppd
command supports the following options: -f <chat file>
Read the chat
script from the chat
option is mutually exclusive with the chat
parameters. You must have read
access to use the
file. Multiple lines are permitted in the file.
Use the space or horizontal tab characters 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, the reply string is not sent. If
specified, a 'subexpect' (alternate reply) string
can be sent. Otherwise, if no alternate reply
strings remain, the chat
script fails. 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 the -r
option is not
used and you use the REPORT
keyword, the stderr
file is used for the report strings. -e
Start with the echo option turned on. You turn
echo on or off at specific points in the chat
script 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
Request that the chat
script execute in a verbose
mode. The chat
program logs the execution state
of the chat
script as well as all text received
from the modem and output strings sent to the
modem. The default is to log through syslog(3C)
with facility local
2; the logging method is
alterable using the -S
Request that the chat
script be executed in a stderr
verbose mode. The chat
program logs all
text received from the modem and output strings
sent to the modem to stderr
is usually the
local console at the station running the chat
. Log messages from -v
messages are sent to stderr
Do not use syslog. By default, error messages are
set to syslog. This option prevents log messages
and error messages from being sent to
syslog. -T <phone number>
Pass in an arbitrary string (usually a telephone
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 telephone
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, the script is included as parameters to
script defines 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
ogin:-BREAK-ogin: ppp ssword: hello2u2
The example indicates that the chat
program should expect the string
"ogin:". If it fails to receive a login prompt within the time interval
allotted, it sends a break sequence to the remote and then expects the
string "ogin:". If the first "ogin:" is received, the break sequence is
Upon receiving the login prompt, the chat
program sends the string "ppp"
and then expects the prompt "ssword:". When the password prompt is
received, it sends 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
The expect sequence should contain only what is needed to identify the
received data. Because it's stored on a disk file, it should not contain
variable information. Generally it is not acceptable to look for time
strings, network identification strings, or other variable pieces of data
as an expect string.
To correct for characters that are corrupted during the initial sequence,
look for the string "ogin:" rather than "login:". The leading "l"
character may be received in error, creating problems in finding the
string. For this reason, scripts look for "ogin:" rather than "login:"
and "ssword:" rather than "password:".
An example of a simple script follows:
ogin: ppp ssword: hello2u2
The example can be interpreted as: expect ogin:, send ppp, expect
...ssword:, send hello2u2.
When login to a remote peer is necessary, simple scripts are rare. At
minimum, you should include sub-expect sequences in case the original
string is not received. For example, consider the following script:
ogin:--ogin: ppp ssword: hello2u2
This script is more effective than the simple one used earlier. The
string looks for the same login prompt; however, if one is not received,
a single return sequence is sent and then the script looks for login:
again. If line noise obscures the first login prompt, send the empty line
to generate a login prompt again.
Comments can be embedded in the chat
script. Comment lines are ignored by
the chat program. A comment starts with the hash ("#") character in
column one. If a #
character is expected as the first character of the
expect sequence, quote the expect string. If you want to wait for a
prompt that starts with a #
character, write something like this:
# Now wait for the prompt and send logout string
'# ' logout
Sending Data From A File
If the string to send begins with an at sign ("@"), the remainder of the
string is interpreted as the name of the file that contains the string.
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
to communicate with another program, for example, a program
to prompt the user and receive a password typed in.
Many modems report the status of a call as a string. These status strings
are often "CONNECTED" or "NO CARRIER" or "BUSY." If the modem fails to
connect to the remote, you can terminate the script. Abort strings may be
specified in the script using the ABORT sequence. For example:
ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK ATDT5551212 CONNECT
This sequence expects nothing and sends the string ATZ. The expected
response is the string OK. When OK is received, the string ATDT5551212
dials the telephone. The expected string is CONNECT. If CONNECT is
received, the remainder of the script is executed. When the modem finds a
busy telephone, it sends the string BUSY, causing the string to match the
abort character sequence. The script fails because it found a match to
the abort string. If the NO CARRIER string is received, it aborts for the
same reason. Clr_Abort
The CLR_ABORT sequence clears previously set ABORT strings. ABORT strings
are kept in an array of a pre-determined size; CLR_ABORT reclaims the
space for cleared entries, enabling new strings to use that space.
The SAY string enables the script to send strings to a user at a terminal
via standard error. If chat
is being run by pppd
is running as a
daemon (detached from its controlling terminal), standard error is
normally redirected to the /etc/ppp/connect-errors
SAY strings must be enclosed in single or double quotes. If carriage
return and line feed are required for the output, you must explicitly add
them to your string.
The SAY string can provide progress messages to users even with "ECHO
OFF." For example, add a line similar to the following to the script:
SAY "Dialing your ISP...\n"
SAY "Waiting up to 2 minutes for connection ..."
SAY "Connected, now logging in ...\n"
SAY "Logged in OK ... \n"
This sequence hides script detail while presenting the SAY string to the
user. In this case, you will see:
Dialing your ISP...
Waiting up to 2 minutes for connection...Connected, now logging in...
Logged in OK ...
REPORT is similar to the ABORT string. With REPORT, however, strings and
all characters to the next control character (such as a carriage return),
are written to the report file.
REPORT strings can be used to isolate a modem's transmission rate from
its CONNECT string and return the value to the chat
user. Analysis of the
REPORT string logic occurs in conjunction with other string processing,
such as looking for the expect string. It's possible to use the same
string for a REPORT and ABORT sequence, but probably not useful.
Report strings may be specified in the script using the REPORT sequence.
The above sequence expects nothing, then sends the string ATDT5551212 to
dial the telephone. The expected string is CONNECT. If CONNECT is
received, the remainder of the script is executed. In addition, the
program writes the string CONNECT to the report file (specified by -r
addition to any characters that follow. Clr_Report
CLR_REPORT clears previously set REPORT strings. REPORT strings are kept
in an array of a pre-determined size; CLR_REPORT reclaims the space for
cleared entries so that new strings can use that space.
ECHO determines if modem output is echoed to stderr
. This option may be
set with the -e
option, but can also be controlled by the ECHO keyword.
The "expect-send" pair ECHO ON enables echoing, and ECHO OFF disables
it. With ECHO, you can select which parts of the conversation should be
visible. In the following script:
ABORT 'NO CARRIER'
All output resulting from modem configuration and dialing is not visible,
but output is echoed beginning with the CONNECT (or BUSY) message.
The HANGUP option determines if a modem hangup is considered as an error.
HANGUP is useful for dialing systems that hang up and call your system
back. HANGUP can be ON or OFF. When HANGUP is set to OFF and the modem
hangs up (for example, following the first stage of logging in to a
callback system), chat
continues running the script (for example, waiting
for the incoming call and second stage login prompt). When the incoming
call is connected, use the HANGUP ON string to reinstall normal hang up
signal behavior. An example of a simple script follows:
'Callback login:' call_back_ID
ABORT "Bad Login"
'Callback Password:' Call_back_password
ABORT "NO CARRIER"
The initial timeout value is 45 seconds. Use the -t
parameter to change
the initial timeout value.
To change the timeout value for the next expect string, the following
example can be used:
The example changes the timeout to ten seconds when it expects the login:
prompt. The timeout is changed to five seconds when it looks for the
Once changed, the timeout value remains in effect until it is changed
The EOT special reply string instructs the chat
program to send an EOT
character to the remote. This is equivalent to using ^D\c as the reply
string. The EOT string normally indicates the end-of-file character
sequence. A return character is not sent following the EOT. The EOT
sequence can embedded into the send string using the sequence ^D. BREAK
The BREAK special reply string sends a break condition. The break is a
special transmitter signal. Many UNIX systems handle break by cycling
through available bit rates, and sending break is often needed when the
remote system does not support autobaud. BREAK is equivalent to using
\K\c as the reply string. You embed the break sequence into the send
string using the \K sequence.
Expect and reply strings can contain escape sequences. Reply strings
accept all escape sequences, while expect strings accept most sequences.
A list of escape sequences is presented below. Sequences that are not
accepted by expect strings are indicated. ''
Expects or sends a null string. If you send a null string, chat
sends the return character. If you expect a null string, chat
proceeds to the reply string without waiting. This sequence
can be a pair of apostrophes or quote mark 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. This sequence 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 delays 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 1/10th of a second. (Not valid in expect.) \q
Suppress writing the string to syslog. 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. Can 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
(Not valid in expect.) \U
Send the phone number 2 string as specified with the -U
(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. (\000 is not valid in an expect
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
is specified on the command line. The metacharacter $
introduces 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.
program terminates with the following completion codes: 0
Normal program termination. Indicates that the script was
executed without error to normal conclusion. 1
One or more of the parameters are invalid or an expect string
was too large for the internal buffers. Indicates that the
program was not properly executed. 2
An error occurred during the execution of the program. This may
be due to a read or write operation failing or chat
signal such as SIGINT. 3
A timeout event occurred when there was an expect string
without having a "-subsend" string. This indicates that you may
not have programmed the script correctly for the condition or
that an unexpected event 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
To determine which event terminated the script, use the termination code.
It is possible to decide if the string "BUSY" was received from the modem
versus "NO DIALTONE." While the first event may be retried, the second
probably will not succeed during a retry.
for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|Interface Stability | Evolving |
SEE ALSO sleep(1)
Additional information on chat
scripts are available with UUCP
documentation. The chat
script format was taken from scripts used by the uucico
August 29, 2021 CHAT(8)