CHAT(8) BSD System Manager's Manual CHAT(8)NAMEchat — Automated conversational script with a modem
SYNOPSISchat [-evVsS] [-f chat_file] [-r report_file] [-t timeout]
[-T phone_number] [-U phone_number]
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(8)) and the remote's
pppd(8) process.
OPTIONS
The following options are provided:
-f chat_file Read the chat script from 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 ter‐
minate 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.
-v Request that the script be executed in a verbose
mode. The chat program will then log the execu‐
tion 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
syslog(3); the logging method may be altered with
the -S and -s flags. Logging is done to the
“local2” facility at level “info” for verbose
tracing and level “err” for some errors.
-V Request that script be executed in a stderr ver‐
bose 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 syslog(3). By default, error messages
are sent to syslog(3). The use of -S will prevent
both log messages from -v and error messages from
being sent to syslog(3).
-T phone_number Pass in an arbitrary string, usually a phone num‐
ber, that will be substituted for the \T substitu‐
tion metacharacter in a send string.
-U phone_number 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 parame‐
ters to the chat program.
CHAT SCRIPT
The chat script defines the communications.
A script consists of one or more “expect-send” pairs of strings, sepa‐
rated by spaces, with an optional “subexpect-subsend” string pair, sepa‐
rated 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 con‐
tain 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 possi‐
ble 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
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
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 desir‐
able 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.
CLR_ABORT STRINGS
This sequence allows for clearing previously set “ABORT” strings.
“ABORT” strings are kept in an array of a pre-determined size (at compi‐
lation time); “CLR_ABORT will reclaim the space for cleared” entries so
that new strings can use that space.
SAY STRINGS
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 min. for connection... "
CONNECT ''
SAY "Connected, now logging in...\n"
ogin: account
ssword: pass
$ SAY "Logged in OK ...\n"
...
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 min. for connection... Connected, now logging in...
Logged in OK ...
REPORT STRINGS
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 analy‐
sis 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 addi‐
tion the program will write to the expect-file the string “CONNECT” plus
any characters which follow it such as the connection rate.
CLR_REPORT STRINGS
This sequence allows for clearing previously set “REPORT” strings.
“REPORT” strings are kept in an array of a pre-determined size (at compi‐
lation time); “CLR_REPORT” will reclaim the space for cleared entries so
that new strings can use that space.
ECHO
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.
HANGUP
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 a (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
...
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
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.
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 pro‐
cessing 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 charac‐
ter. 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 sec‐
ond. (not valid in expect.)
\q Suppress writing the string to syslogd(8). 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.)
TERMINATION CODES
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 ter‐
minated 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 suc‐
ceeding during a retry.
SEE ALSOuucico(1), uucp(1), syslog(3), syslogd(8)
Additional information about chat scripts may be found with UUCP documen‐
tation. The chat script was taken from the ideas proposed by the scripts
used by the uucico(1) program.
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.
BSD September 27, 1996 BSD
