chat(8) - Linux man page
DISCLAIMER
This page is derived from the Linux chat manual page. Some sections
have been omitted or modified to make them more applicable to the
implementation of pppd dialout used in the Trimble NetRS product. For
the complete page, consult any Linux system or look on the web.
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
No command line options to chat are directly available on the
Trimble NetRS. The user supplied chatscript is fed to chat.
If the debug option is set for pppd, then the -v option
is enabled for chat.
This requests 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.
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 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
SENDING DATA FROM A FILE
This section is not applicable on the Trimble NetRS.
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.
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 compilation 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 explicitely 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 ...
ogin: account
ssword: pass
$ SAY "Logged in OK ... 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 ...
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 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.
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 compilation 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'
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 an (simple)
example script:
- ABORT 'BUSY'
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 ...
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 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.
- \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
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 substition fails,
because the requested environment variable is not set,
nothing is replaced for the variable.
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 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.
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(1), 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.