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This appendix presents the same information available in the Expect reference-manual entry (“man page”); we include it in the DejaGnu manual for convenient reference. The author of Expect, and of this reference, is Don Libes, of the National Institute of Standards and Technology (NIST).
A.1 Introduction | ||
A.2 Usage | ||
A.3 Commands | ||
A.4 Pretty-Printing | ||
A.5 Examples | ||
A.6 Caveats | ||
A.7 Bugs | ||
A.8 Expect Hints | ||
A.9 Expect Bibliography | ||
A.10 Acknowledgements |
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Expect is a program that “talks” to other interactive programs according to a script. Following the script, Expect knows what can be expected from a program and what the correct response should be. An interpreted language provides branching and high-level control structures to direct the dialogue. In addition, the user can take control and interact directly when desired, afterward returning control to the script.
expectk
is a mixture of Expect and Tk
. It behaves just
like Expect and Tk
’s wish
. Expect can also be used
directly in C or C++ (that is, without Tcl). See libexpect(3).
The name “Expect” comes from the idea of “send/expect” sequences
popularized by uucp
, kermit
, and other modem control
programs. However, unlike uucp
, Expect is generalized so
that it can be run as a user-level command with any program and task in
mind. (Expect can actually talk to several programs at the same
time.)
For example, here are some things Expect can do:
fsck
, and in response to its questions, answer “yes” or
“no”, or give control back to you, based on predetermined criteria.
rlogin
, telnet
, tip
, su
,
chgrp
, and so on.
There are a variety of reasons why the shell cannot perform these tasks. (Try, you’ll see.) All are possible with Expect.
In general, Expect is useful for running any program which requires interaction between the program and the user. All that is necessary is that the interaction can be characterized programmatically. Expect can also give the user back control (without halting the program being controlled) if desired. Similarly, the user can return control to the script at any time.
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Expect reads a file for a list of commands to execute. Expect may also be invoked implicitly on systems which support the ‘#!’ notation by marking the script executable, and making the first line in your script:
#!/usr/local/bin/expect -f
Of course, the path must accurately describe where Expect lives. ‘/usr/local/bin’ is just an example.
Here is a summary of the command line options you can use with Expect:
expect [ -dinN ] [ -c cmds ] [[ -f ] cmdfile ] [ args ]
‘--’ may be used to delimit the end of the options. This is useful if you want to pass an option-like argument to your script without it being interpreted by Expect. This can usefully be placed in the ‘#!’ line to prevent any option-like interpretation by Expect. For example, the following will leave the original arguments (including the script name) in the variable argv.
#!/usr/local/bin/expect --
Note that the usual getopt
and execve
conventions must be
observed when adding arguments to the ‘#!’ line.
Expect understands these command line options:
-c cmds
Execute the command cmds before any in the script. The command should be quoted to prevent being broken up by the shell. This option may be used multiple times. Multiple commands may be executed with a single ‘-c’ by separating them with semicolons. Commands are executed in the order they appear.
-d
Enables some debugging output, which primarily reports internal activity
of commands such as expect
and interact
. This option has
the same effect as ‘debug 1’ at the beginning of an Expect script,
plus the version of Expect is printed. (The strace
command is
useful for tracing statements, and the trace
command is useful
for tracing variable assignments.)
-f cmdfile
Read commands from cmdfile. The option itself is optional as it is only useful with using the ‘#!’ notation in a script(see above), so that other arguments may be supplied on the command line.
If the string ‘-’ is supplied as a filename, standard input is read instead. (Use ‘./-’ to read from a file actually named ‘-’.)
-i
Causes Expect to interactively prompt for commands instead of reading
them from a file. Prompting is terminated via the exit
command
or upon EOF. See interpreter
(below) for more information.
‘-i’ is assumed if neither a command file nor ‘-c’ is used.
-N
-n
The file ‘$expect_library/expect.rc’ is sourced automatically if present, unless the ‘-N’ option is used. Immediately after this, the file ‘~/.expect.rc’ is sourced automatically, unless the ‘-n’ option is used. Both of these are sourced after executing any ‘-c’ options.
args
Optional args are constructed into a list and stored in the
variable named argv
. argv
may be accessed with the usual
list access commands. For example, the length of argv
is
calculated by ‘llength $argv’.
argv[0]
is defined to be the name of the script. For example,
the following prints out the name of the script and the first two
arguments:
send_user [lrange $argv 0 2]
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Expect uses Tcl (Tool Command Language). Tcl provides
control flow (e.g., if, for, break), expression evaluation and several
other features such as recursion, procedure definition, etc. Commands
used here but not defined (e.g., set
, if
, exec
) are
Tcl commands (@pxref{Tcl}). Expect supports additional commands,
described below. Unless otherwise specified, commands return the empty
string.
Commands are listed alphabetically so that they can be quickly located.
However, new users may find it easier to start by reading the
descriptions of spawn
, send
, expect
, and
interact
, in that order. Then read the examples at the rear of
this man page. (In the text of this man page, Expect with an
uppercase "E" refers to the Expect program while expect
with a lower-case "e" refers to the expect
command within the
Expect program.)
close [ -i spawn_id ]
Closes the connection to the current process. Most interactive programs
will detect EOF on their stdin and exit; thus close
usually
suffices to kill the process as well. The ‘-i’ option declares the
process to close corresponding to the named spawn_id.
Both expect
and interact
will detect when the current
process exits and implicitly do a close
. But if you kill the
process by, say, ‘exec kill $pid’, you will need to explicitly call
close
.
No matter whether the connection is closed implicitly or explicitly, you
should call wait
to clear up the corresponding kernel process
slot. close
does not call wait
since there is no
guarantee that closing a process connection will cause it to exit. See
wait
below for more info.
debug [ -f file ] value
Causes further commands to send debugging information internal to Expect to stderr if value is non-zero. This output is disabled if value is 0. The debugging information includes every character received, and every attempt made to match the current output against the patterns.
If the optional file is supplied, all normal and debugging output is written to that file (regardless of the value of value). Any previous debugging output file is closed.
disconnect
Disconnects a forked process from the terminal. It continues running in the background. The process is given its own process group (if possible). Standard I/O is redirected to /dev/null.
The following fragment uses disconnect
to continue running the
script in the background.
if [fork]!=0 exit disconnect …
The following script reads a password, and then runs a program every
hour that demands a password each time it is run. The script supplies
the password so that you only have to type it once. (See the
system
command which demonstrates how to turn off password
echoing.)
send_user "password?\ " expect_user -re "(.*)\n" for {} 1 {} { if [fork]!=0 {exec sleep 3600;continue} disconnect spawn priv_prog expect Password: send "$expect_out(1,string)\r" … exit } |
An advantage to using disconnect
over the shell asynchronous
process feature (&
) is that Expect can save the terminal
parameters prior to disconnection, and then later apply them to new
ptys. With &
, Expect does not have a chance to read the
terminal’s parameters since the terminal is already disconnected by the
time Expect receives control.
exit [ status ]
Kills Expect. All connections to spawned processes are closed. Closure will be detected as an EOF by spawned processes.
Exit generates a signal 0 (see trap
), but otherwise takes no
other actions beyond what the normal _exit
procedure does. Thus,
spawned processes that do not check for EOF may continue to run. (A
variety of conditions are important to determining, for example, what
signals a spawned process will be sent, but these are system-dependent.)
Spawned processes that continue to run will be inherited by init.
status (or 0 if not specified) is returned as the exit status of
Expect. exit
is implicitly executed if the end of the
script is reached.
expect [[ -opts ] pat1 body1 ] … [ -opts ] patn [ bodyn ]
waits until one of the patterns matches the output of a spawned process, a specified time period has passed, or an end-of-file is seen. If the final body is null, it may be omitted.
Patterns from the most recent expect_before
command are
implicitly used before any other patterns. Patterns from the most
recent expect_after
command are implicitly used after any other
patterns.
If the arguments to the entire expect
statement require more than
one line, all the arguments may be “braced” into one so as to avoid
terminating each line with a backslash. In this one case, the usual Tcl
substitutions will occur despite the braces.
If a pattern is the keyword eof
, the corresponding body is
executed upon end-of-file. If a pattern is the keyword timeout
,
the corresponding body is executed upon timeout. The default timeout
period is 10 seconds but may be set, for example to 30, by the command
‘set timeout 30’. An infinite timeout may be designated by the
value -1. If a pattern is the keyword default
, the corresponding
body is executed upon either timeout or end-of-file.
If a pattern matches, then the corresponding body is executed.
expect
returns the result of the body (or null if no pattern matched).
In the event that multiple patterns match, the one appearing first is
used to select a body.
Each time new output arrives, it is compared to each pattern in the order
they are listed. Thus, you may test for absence of a match by making
the last pattern something guaranteed to appear, such as a prompt.
In situations where there is no prompt, you must use
timeout
(just like you would if you were interacting manually).
Patterns are specified in two ways. By default, patterns are specified
as with Tcl’s string match
command. (Such patterns are also
similar to C-shell filename expansion, usually referred to as “glob”
patterns).
For example, the following fragment looks for a successful login. (Note
that abort
is presumed to be a procedure defined elsewhere in the
script.)
expect { connected break busy { print busy\n continue } failed abort "invalid password" abort timeout abort } |
Quotes are necessary on the fourth pattern since it contains a space, which would otherwise separate the pattern from the action. Patterns with the same action (such as the 3rd and 4th) require listing the actions again. This can be avoid by using regexp-style patterns (see below). More information on forming glob-style patterns can be found in the Tcl manual.
Alternatively, regexp-style patterns follow the syntax defined by Tcl’s
regexp
(short for “regular expression”) command. regexp
patterns are introduced with the option ‘-re’. The previous
example can be rewritten using a regexp as:
expect { connected break busy { print busy\n continue } -re "failed|invalid password" abort timeout abort } |
Both types of patterns are “unanchored”. This means that patterns do not have to match the entire string, but can begin and end the match anywhere in the string (as long as everything else matches). Use ‘^’ to match the beginning of a string, and ‘$’ to match the end. Note that if you do not wait for the end of a string, your responses can easily end up in the middle of the string as they are echoed from the spawned process. While still producing correct results, the output can look unnatural. Thus, use of ‘$’ is encouraged if you can exactly describe the characters at the end of a string.
The ‘-nocase’ option causes uppercase characters of the output to compare as if they were lowercase characters. The pattern is not affected.
While reading output, more than 2000 bytes can force earlier bytes to be
“forgotten”. This may be changed with the function match_max
.
(Note that excessively large values can slow down the pattern matcher.)
If patlist is full_buffer
, the corresponding body is
executed if match_max bytes have been received and no other
patterns have matched.
Upon matching a pattern (or eof
or buffer_full
), any
matching and previously unmatched output is saved in the array element
‘expect_out(buffer)’. Up to 9 regexp substring matches are saved
in the array elements ‘expect_out(1,string)’ through
‘expect_out(9,string)’. For each substring,
‘expect_out(x,start)’ holds the starting index; the ending
index is stored in ‘expect_out(x,end)’. These indices are in
a form suitable for lrange
; x corresponds to the substring
position in the pattern. 0 refers to the entire pattern itself. For
example, if a process has produced output of ‘abcdefgh\n’, the
result of:
expect "cd"
is as if the following statements had executed:
set expect_out(0,start) 2 set expect_out(0,end) 3 set expect_out(0,string) cd set expect_out(buffer) abcd
and ‘efgh\n’ is left in the output buffer. For the output ‘abbbcabkkkka\n’, the result of:
expect -re "b(b*).*(k+)"
is as if the following statements had executed:
set expect_out(0,start) 1 set expect_out(0,end) 10 set expect_out(0,string) bbbcabkkkk set expect_out(1,start) 2 set expect_out(1,end) 3 set expect_out(1,string) bb set expect_out(2,start) 10 set expect_out(2,end) 10 set expect_out(2,string) k set expect_out(buffer) abbbcabkkkk
and ‘a\n’ is left in the output buffer. The pattern ‘*’ will flush the output buffer without reading any more output from the process.
Normally, the matched output is discarded from Expect’s internal buffers. This may be prevented by prefixing a pattern with the ‘-n’ option. The name, placement, and existence of this option is subject to change in a future release. Therefore, it should not be used in permanent scripts. However, it is especially useful in experimenting (which is why it has a one-character name).
By default, patterns are matched against output from the current process; however, the ‘-i’ option declares the output from the named spawn_id be matched against any following patterns (up to the next ‘-i’). For example, the following example waits for ‘connected’ from the current process, or ‘busy’, ‘failed’ or ‘invalid password’ from the spawn_id named by ‘$proc2’.
expect { connected break -i $proc2 busy { print busy\n continue } -re "failed|invalid password" abort timeout abort } |
The variable any_spawn_id
may be used to match patterns to any
spawn_id that is named with another ‘-i’ option associated with a
pattern. Upon matching a pattern (or eof
or buffer_full
),
the variable expect_out(spawn_id)
is set to the spawn_id
which produced the matching output.
Actions such as break
and continue
cause control
structures (i.e., for
, proc
) to behave in the usual way.
The special argument ‘-expect’ to continue
allows
expect
itself to continue executing rather than returning as it
normally would.
This is useful for avoiding explicit loops or repeated expect
statements. The following example is part of a fragment to automate
rlogin
. The continue
avoids having to write a second
expect
statement (to look for the prompt again) if the
rlogin
prompts for a password.
expect { Password: { system stty -echo send_user "password (for $user) on $host: " expect_user -re "(.*)\n" send_user "\n" send "$expect_out(1,string)\r" system stty echo continue -expect } incorrect { send_user "invalid password or account\n" exit } timeout { send_user "connection to $host timed out\n" exit } eof { send_user "connection to $host failed: \ $expect_out(buffer)" exit } -re $prompt } |
For example, the following fragment might help a user guide an
interaction that is already totally automated. In this case, the
terminal is put into raw mode. If the user presses ‘+’, a variable
is incremented. If ‘p’ is pressed, several returns are sent to the
process, perhaps to poke it in some way, and ‘i’ lets the user
interact with the process, effectively stealing away control from the
script. In each case, the ‘continue -expect’ allows the current
expect
to continue pattern matching after executing the current
action.
system stty raw -echo expect_after { -i $user_spawn_id "p" {send "\r\r\r"; continue -expect} "+" {incr foo; continue -expect} "i" {interact; continue -expect} "quit" exit } |
‘continue -expect’ resets the timeout timer.
expect_after [ expect args ]
Takes the same arguments as expect
; however, it returns
immediately. Pattern-action pairs from the most recent
expect_after
are implicitly added to any following expect
commands. If a pattern matches, it is treated as if it had been
specified in the expect
command itself, and the associated body
is executed in the context of the expect
command. If patterns
from both expect
and expect_after
can match, the
expect
pattern is used.
Unless overridden by a ‘-i’ option, expect_after
patterns
match against the spawn_id
defined at the time that the
expect_after
command was executed (not when its pattern is
matched).
expect_before [ expect args ]
takes the same arguments as expect
; however, it returns
immediately. Pattern-action pairs from the most recent
expect_before
are implicitly added to any following expect
commands. If a pattern matches, it is treated as if it had been
specified in the expect
command itself, and the associated body
is executed in the context of the expect
command. If patterns
from both expect_before
and expect
can match, the
expect_before
pattern is used.
Unless overridden by a ‘-i’ option, expect_before
patterns
match against the spawn_id
defined at the time that the
expect_before
command was executed (not when its pattern is
matched).
expect_user [ expect args ]
Like expect
, but reads characters from stdin (i.e.
keystrokes from the user). By default, reading is performed in cooked
mode. Thus, lines must end with a return in order for expect
to
see them. This may be changed via stty
(see the system
command below).
expect_version [[ -exit ] version ]
is useful for assuring that the script is compatible with the current version of Expect.
With no arguments, the current version of Expect is returned. This version may then be encoded in your script. If you actually know that you are not using features of recent versions, you can specify an earlier version.
Versions consist of up to three numbers separated by dots. First is the
major number. Scripts written for versions of Expect with a
different major number will almost certainly not work.
expect_version
returns an error if the major numbers do not
match.
Second is the minor number. Scripts written for a version with a
greater minor number than the current version may depend upon some new
feature and might not run. expect_version
returns an error if
the major numbers match, but the script minor number is greater than
that of the running Expect.
Third is a number that plays no part in the version comparison. However, it is incremented when the Expect software distribution is changed in any way, such as by additional documentation or optimization. It is reset to 0 upon each new minor version.
With the ‘-exit’ option, Expect prints an error and exits if the version is out of date.
There have been three major versions of Expect. The first was never officially released and only existed for two months, as I experimented and designed the basic style of Expect. The second version lasted a year and a half until the time when Tcl 6 and Expect 3 were issued. Version 6 of Tcl was incompatible with earlier versions, but John Ousterhout (Tcl’s author) suggested that enough experience had been gained that such changes were appropriate, and this might be the last time it could be done because further delay would be that much more painful due to the ever-growing number of people using it. I feel the same way. I hope that the current version of Expect will last many years without the introduction of incompatibilities that might render scripts obsolete.
During its one and a half year public lifetime, the second version of Expect was requested (and perhaps even used) by over 3000 sites. I received numerous suggestions for improvements or future directions. Many of these either appear in the current version or are addressed in the Expect FAQ file.
fork
Creates a new process. The new process is an exact copy of the current
Expect process. On success, fork
returns 0 to the new (child)
process and return the process ID of the child process to the parent
process. On failure (invariably due to lack of resources, e.g., swap
space, memory), fork
returns -1 to the parent process, and no
child process is created.
Forked processes exit via the exit
command, just like the
original process. Forked processes are allowed to write to the log
files. If you do not disable debugging or logging in most of the
processes, the result can be confusing.
Some pty implementations may be confused by multiple readers and
writers, even momentarily. Thus, it is safest to fork
before
spawning processes.
getpid
Returns the process id of the current process.
interact [ string1 body1 ] … [ stringn [ bodyn ]]
Gives control of the current process to the user, so that keystrokes are sent to the current process, and the stdout and stderr of the current process are returned.
String-body pairs may be specified as arguments, in which case the body
is executed when the corresponding string is entered. (By default, the
string is not sent to the current process.) The interpreter
command is assumed, if the final body is missing.
If the arguments to the entire interact
statement require more
than one line, all the arguments may be “braced” into one so as to avoid
terminating each line with a backslash. In this one case, the usual Tcl
substitutions will occur despite the braces.
For example, the following command runs interact with the following string-body pairs defined: When <C-Z> is pressed, Expect is suspended. When <C-A> is pressed, the user sees ‘you typed a control-A’ and the process is sent a ‘^A’. When $ is pressed, the user sees the date. When <C-C> is pressed, Expect exits. If foo is entered, the user sees ‘bar’. When ~~ is pressed, the Expect interpreter runs interactively.
set CTRLZ \032 interact { $CTRLZ {exec kill -STOP 0} \001 {send_user "you typed a control-A\n"; send "\001" } $ {send_user "The date is [exec date]."} \003 exit foo {send_user "bar"} ~~ } |
In string-body pairs, strings are matched in the order they are listed as arguments. Strings that partially match are not sent to the current process in anticipation of the remainder coming. If characters are then entered such that there can no longer possibly be a match, only the part of the string will be sent to the process that cannot possibly begin another match. Thus, strings that are substrings of partial matches can match later, if the original strings that was attempting to be match ultimately fails.
By default, string matching is exact with no wild cards. (In contrast,
the expect
command uses glob-style patterns by default.) The
‘-re’ option forces the string to be interpreted as a regexp-style
pattern. In this case, matching substrings are stored in the variable
interact_out
similarly to the way expect
stores its output
in the variable expect_out
.
The option ‘-eof’ introduces an action that is executed upon
end-of-file. The ‘-eof’ option applies to the most recently
specified process (such as via ‘-input’ or -output
). If the
‘-eof’ option precedes all spawned processes, then it applies to
all spawned processes that do not have an ‘-eof’ option. The
default ‘-eof’ action is return
, so that interact
simply returns upon any EOF.
The option ‘-timeout’ introduces a timeout (in seconds) and action
that is executed after no characters have been read for a given time.
The ‘-timeout’ option applies to the most recently specified
process. If the ‘-timeout’ option precedes all spawned processes,
then it applies to all spawned processes that do not have a
‘-timeout’ option. There is no default -timeout
. The
special variable timeout
(used by the expect
command) has
no affect on this timeout.
For example, the following statement could be used to autologout users who have not typed anything for an hour but who still get frequent system messages:
interact -input $user_spawn_id \ -output $spawn_id -f \ -timeout 3600 return
Actions such as break
and continue
cause control
structures (i.e., for
, proc
) to behave in the usual way.
However return
causes interact to return to its caller, while
‘return -tcl’ causes interact
to cause a return in its
caller. For example, if ‘proc foo’ called interact
which
then executed the action ‘return -tcl’, ‘proc foo’ would
return. (This means that if interact
calls interpreter
interactively typing return
will cause the interact to continue,
while ‘return -tcl’ will cause the interact to return to its
caller.)
During interact
, raw mode is used so that all characters may be
passed to the current process. If the current process does not catch
job control signals, it will stop if sent a stop signal (by default
<C-Z>). To restart it, send a continue signal (such as by
‘kill -CONT pid’). If you really want to send a
SIGSTOP
to such a process (by <C-Z>), consider spawning
csh
first and then running your program. On the other hand, if
you want to send a SIGSTOP
to Expect itself, first press
<ESC> (the escape character), and then press <C-Z>.
String-body pairs can be used as a shorthand for avoiding having to enter the interpreter and execute commands interactively. The previous terminal mode is used while the body of a string-body pair is being executed.
The ‘-f’ option (for fast) skips the possibility of a temporary mode switch during pair processing. This consequently prevents characters from being lost when the terminal is returned to raw mode (an unfortunate feature of the terminal driver) at the end of a key-body pair execution. The only reason not to use ‘-f’ is if your action depends on running in cooked mode.
The ‘-F’ option indicates that all following options behave as if they each were declared with ‘-f’.
The previous example is restated below in a more efficient form, using
‘-F’. The first line cannot use ‘-f’ because it would leave
the user back in the shell in raw mode. The last line would remain in
raw mode but it is overridden by the interpreter
command itself,
which forces it into cooked mode temporarily. The second line requires
no extra ‘\r’ because send
automatically adds one. The
other lines need no change and run fine with ‘-f’.
set CTRLZ \032 interact { $CTRLZ {kill -STOP 0} -F \001 {send_user "you typed a control-A\n"; send "\001" } $ {send_user "The date is [exec date]."} \003 exit foo {send_user "bar"} ~~ } |
By default, actions that change the value of spawn_id will not affect
the behavior of interact even if input or output sources were originally
associated with spawn_id
.
The ‘-update’ option forces spawn_id
to be reexamined after
evaluation of an action. This could be used, for example, so that
pressing a particular function key would switch to interacting with a
different process.
The ‘-echo’ option sends characters that match the following pattern back to the process that generated them as each character is read. This may be useful when the user needs to see feedback from partially typed patterns.
If a pattern is being echoed but eventually fails to match, the
characters are sent to the spawned process. If the spawned process then
echoes them, the user will see the characters twice. ‘-echo’ is
probably only appropriate in situations where the user is unlikely to
not complete the pattern. For example, the following excerpt is from
rftp
, the recursive-ftp script, where the user is prompted to
enter ~g, ~p, or ~l, to get, put, or list the current
directory recursively. These are so far away from the normal ftp
commands, that the user is unlikely to type ~ followed by anything
else, except mistakenly, in which case, they’ll probably just ignore the
result anyway.
interact { -echo ~g {getcurdirectory 1} -echo ~l {getcurdirectory 0} -echo ~p {putcurdirectory} }
The ‘-flush’ option sends characters that match the following pattern on to the output process as characters are read.
This is useful when you wish to let a program echo back the pattern. For example, the following might be used to monitor where a person is dialing (a Hayes-style modem). Each time atd is seen the script logs the rest of the line.
proc lognumber {} { interact -flush -f -re "(.*)\r" return puts $log "[exec date]: dialed $interact_out(1,string)" } interact -flush -f "atd" lognumber |
During interact
, previous use of log_user
is ignored. In
particular, interact
will force its output to be logged (sent to
the standard output) since it is presumed the user doesn’t wish to
interact blindly.
The ‘-o’ option causes following key-body pairs to be applied to
the output of the current process. This can be useful, for example,
when dealing with hosts that send unwanted characters during a
telnet
session.
By default, interact
expects the user to be writing stdin and
reading stdout of the Expect process itself. The ‘-u’ option (for
“user”) makes interact
look for the user as the process named
by its argument (which must be a spawned id).
This allows two unrelated processes to be joined together without using
an explicit loop. To aid in debugging, Expect diagnostics always go to
stderr (or stdout for certain logging and debugging information). For
the same reason, the interpreter
command will read interactively
from stdin.
For example, the following fragment creates a login process. Then it dials the user (not shown), and finally connects the two together. Of course, any process may be substituted for login. A shell, for example, would allow the user to work without supplying an account and password.
spawn login set login $spawn_id spawn tip modem … ;# dial back out to user … ;# connect user to login interact -u $login |
To send output to multiple processes, list each spawn id prefaced by a
‘-output’ option. Input for a group of output spawn ids may be
determined by a spawn id prefaced by a ‘-input’ option. All
following options and strings (or patterns) apply to this input until
another ‘-input’ option appears. If no ‘-input’ appears,
‘-output’ implies ‘-input $user_spawn_id -output’.
(Similarly, with patterns that do not have ‘-input’.) If one
‘-input’ is specified, it overrides $user_spawn_id
. If a
second ‘-input’ is specified, it overrides $spawn_id
.
Additional ‘-input’ options may be specified.
The two implied input processes default to having their outputs
specified as $spawn_id
and $user_spawn_id
(in reverse).
If a ‘-input’ option appears with no ‘-output’ option,
characters from that process are discarded.
The ‘-i’ option introduces a replacement for the current
spawn_id
when no other ‘-input’ or ‘-output’ options
are used.
interpreter
causes the user to be interactively prompted for Expect and Tcl commands. The result of each command is printed.
Actions such as break
and continue
cause control
structures (i.e., for
, proc
) to behave in the usual way.
However return
causes interpreter to return to its caller, while
‘return -tcl’ causes interpreter
to cause a return in its
caller. For example, if ‘proc foo’ called interpreter
which
then executed the action .BR ‘return -tcl’ , ‘proc foo’ would
return. Any other command causes interpreter
to continue
prompting for new commands.
By default, the prompt contains two integers. The first integer
describes the depth of the evaluation stack (i.e., how many procedures
have yet to return). The second integer is the Tcl history identifier.
The prompt can be set by defining a procedure called prompt1
whose return value becomes the next prompt. If a statement has open
quotes, parens, braces, or brackets, a secondary prompt (by default
‘+> ’) is issued upon newline. The secondary prompt may be set by
defining a procedure called prompt2
.
During interpreter
, cooked mode is used, even if the its caller
was using raw mode.
log_file [[-a] file ]
If a filename is provided, log_file
will record a transcript of
the session (beginning at that point) in the file. log_file
will
stop recording if no argument is given. Any previous log file is
closed.
The ‘-a’ option forces output to be logged that was suppressed by
the log_user
command.
The log_file
command appends to old files rather than
truncating them, for the convenience of being able to turn logging off
and on multiple times in one session. A simple way to always start with
a fresh log file is to delete the log file before using the
log_file
command for the first time in a script. For example:
exec rm transcript log_file transcript
log_user expression
By default, the send/expect dialogue is logged to standard output (and a logfile if open). This logging is disabled by the command ‘log_user 0’ and reenabled by ‘log_user 1’.
match_max [-d] [-i spawn_id ] [size]
defines the size of the buffer (in bytes) used internally by
expect
. With no size argument, the current size is
returned.
With the ‘-d’ option, the default size is set. (The initial default is 2000.) With the ‘-i’ option, the size is set for the named spawn id, otherwise it is set for the current process.
overlay [-n spawn_id…] program [args]
Executes ‘program args’ in place of the current Expect program, which terminates. A bare hyphen argument forces a hyphen in front of the command name as if it was a login shell. All spawn ids are closed except for those named as arguments. These are mapped onto the named file descriptors n.
Spawn ids are mapped to file descriptors for the new program to inherit.
For example, the following line runs chess
and allows it to be
controlled by the current process—say, a chess master.
overlay -0 $spawn_id -1 $spawn_id -2 $spawn_id chess
This is more efficient than ‘interact -u’. However, it sacrifices the ability to do programmed interaction since the Expect process is no longer in control.
Note that no controlling terminal is provided. Thus, if you disconnect or remap standard input, programs that do job control (shells, login, etc) will not function properly.
parity [-d] [-i spawn_id] [value]
Defines whether parity should be retained or stripped from the output of spawned processes. If value is zero, parity is stripped, otherwise it is not stripped. With no value argument, the current value is returned.
With the ‘-d’ option, the default parity value is set. (The initial default is 1, i.e., parity is not stripped.) With the ‘-i’ option, the parity value is set for the named spawn id, otherwise it is set for the current process.
send [-s] [-h] [-i spawn_id] [-raw] args
Sends args to the current process. Strings are interpreted following Tcl rules. For example, the command
send "hello world\r"
sends the characters ‘h’ ‘e’ ‘l’ ‘l’ ‘o’
‘ ’ ‘w’ ‘o’ ‘r’ ‘l’ ‘d’
‘<RET>’ to the current process. (Tcl includes a command
similar to printf
(called format
) which can build
arbitrarily complex strings.)
Characters are sent immediately although programs with line-buffered input will not read the characters until a return character is sent. A return character is denoted ‘\r’.
The ‘-i’ option declares that the string be sent to the named spawn
id. If the spawn id is user_spawn_id
, and the terminal is in raw
mode, newlines in the string are translated to return-newline sequences
so that they appear as it the terminal was in cooked mode. The
‘-raw’ option disables this translation.
The ‘-s’ option forces output to be sent “slowly”, thus avoid the
common situation where a computer outtypes an input buffer that was
designed for a human who would never outtype the same buffer. This
output is controlled by the value of the variable send_slow
which
takes a two element list. The first element is an integer that
describes the number of bytes to send atomically. The second element is
a real number that describes the number of seconds by which the atomic
sends must be separated. For example, ‘set send_slow 10 .001’
would force ‘send -s’ to send strings with 1 millisecond in between
each 10 characters sent.
The ‘-h’ option forces output to be sent (somewhat) like a human
actually typing. Human-like delays appear between the characters. (The
algorithm is based upon a Weibull distribution, with modifications to
suit this particular application.) This output is controlled by the
value of the variable send_human
which takes a five element list.
The first two elements are average interarrival time of characters in
seconds. The first is used by default. The second is used at word
endings, to simulate the subtle pauses that occasionally occur at such
transitions. The third parameter is a measure of variability, where .1
is quite variable, 1 is reasonably variable, and 10 is quite invariable.
The extremes are 0 to infinity. The last two parameters are,
respectively, a minimum and maximum interarrival time. As an example,
the following command types a lot like the author (a fast and consistent
typist):
set send_human {.1 .3 1 .05 2} send -h "I'm hungry. Let's do lunch."
while the following might be more suitable after a hangover:
set send_human {.4 .4 .2 .5 100} send -h "Goodd party lash night!"
Note that errors are not simulated, although you can set up error correction situations yourself by embedding mistakes and corrections in a send argument.
It is a good idea to precede the first send
to a process by an
expect
. expect
will wait for the process to start, while
send
cannot. In particular, if the first send
completes
before the process starts running, you run the risk of having your data
ignored. In situations where interactive programs offer no initial
prompt, you can precede send
by a delay as in:
# To avoid hints on how to break in, # there is no prompt for an external password. # Wait for 5 seconds for exec to complete spawn telnet very.secure.gov exec sleep 5 send password\r |
send_error args
Like send
, except that the arguments are sent to standard
error rather than the current process.
send_log args
Like send
, except that the arguments are only sent to the log
file (see log_file
). The arguments are ignored if no log file is
open.
send_spawn args
An alias for send
. If you are use expectk
or some other
variant of Expect in the Tk environment, send
is defined by Tk
for an entirely different purpose. send_spawn
is provided for
compatibility between environments.
send_user args
is like send
, except that the arguments are sent to stdout rather
than the current process.
spawn [args] program [args]
Creates a new process running ‘program args’. Its
stdin
, stdout
, and stderr
are connected to Expect,
so that they may be read and written by other Expect commands. The
connection is broken by close
or if the process itself closes any
of the file descriptors.
When a process is started by spawn
, the variable spawn_id
is set to a descriptor referring to that process. The process described
by spawn_id
is considered the current process.
spawn_id
may be read or written, in effect providing job control.
user_spawn_id
is a predefined variable containing a descriptor
which refers to the user. For example, when spawn_id
is set to
this value, expect
behaves like expect_user
. Do not
assume the value of user_spawn_id
will remain the same from one
version of Expect to another.
tty_spawn_id
is a predefined variable containing a descriptor
for ‘/dev/tty’. This may be the same as
user_spawn_id
but may be different if user_spawn_id
has
been redirected. If ‘/dev/tty’ does not exist (such as in a
cron
, at
, or batch
script), then
tty_spawn_id
is not defined. This may be tested as:
if [info vars tty_spawn_id] { # /dev/tty exists … } else { # /dev/tty doesn't exist # probably in cron, batch, or at script … }
spawn
returns the UNIX process id. Note that the UNIX process id
is not equivalent to the descriptor in spawn_id
.
By default, spawn
echoes the command name and arguments. The
‘-noecho’ argument stops spawn
from doing this.
The option ‘-console’ causes console output to be redirected to the spawned process. This is not supported on all systems.
Internally, spawn
uses a pty, initialized the same way as the
user’s tty. This is further initialized so that all settings are
sane
(according to stty(1)). If the variable
stty_init
is defined, it is interpreted in the style of
stty
arguments as further configuration. For example, ‘set
stty_init raw’ will cause further spawned processes’s terminals to start
in raw mode. ‘-nottycopy’ skips the initialization based on the
user’s tty. ‘-nottyinit’ skips the sane
initialization.
Normally, spawn
takes little time to execute. If you notice
spawn taking a significant amount of time, it is probably encountering
ptys that are wedged. A number of tests are run on ptys to avoid
entanglements with errant processes. (These take 10 seconds per wedged
pty.) Running Expect with the ‘-d’ option will show if Expect is
encountering many ptys in odd states. If you cannot kill the processes
to which these ptys are attached, your only recourse may be to reboot.
If program cannot be spawned successfully because exec
fails (e.g. when program does not exist), an error message will be
returned by the next interact
or expect
command as if
program had run and produced the error message as output. This
behavior is a natural consequence of the implementation of spawn
.
Internally, spawn forks, after which the spawned process has no way to
communicate with the original Expect process except by communication via
the spawn id.
strace level
Causes following statements to be printed before being executed. (The
Tcl trace
command traces variables.) level indicates how
far down in the call stack to trace. For example, the following command
runs Expect while tracing the first 4 levels of calls, but none below
that.
expect -c "strace 4" script.exp
system args
Gives args to sh
as input, just if it had been typed as a
command from a terminal. Expect waits until the shell terminates. The
return status from sh
is handled the same way that exec
handles its return status.
In contrast to exec
which redirects stdin and stdout to the
script, system
performs no redirection (other than that indicated
by the string itself). Thus, it is possible to use programs which must
talk directly to ‘/dev/tty’. For the same reason, the results of
system
are not recorded in the log.
system
understands and evaluates certain cases of stty
directly, in order to efficiently handle mode switching during
interpeter
and interact
. In particular, the arguments
‘raw’ or ‘-cooked’ put the terminal into raw mode. The
arguments ‘-raw’ or ‘cooked’ put the terminal into cooked
mode. The arguments ‘echo’ and ‘-echo’ put the terminal into
echo and noecho mode respectively.
The following example illustrates how to use system
to
temporarily disable echoing. This could be used in otherwise-automatic
scripts to avoid embedding passwords in them. (For more discussion of
this, see section Expect Hints.)
system stty -echo send_user "Password: " expect_user -re "(.*)\n" set password $expect_out(1,string) system stty echo |
trap [[command] signals]
Causes the given command to be executed upon future receipt of any
of the given signals. If command is absent, the signal actions
are reset to their defaults. If command is the string
SIG_IGN
, the signals are ignored. signals is either a
single signal or a list of signals. Signals may be specified
numerically or symbolically as per signal(3). The ‘SIG’
prefix may be omitted. ONEXIT
(signal 0) is raised upon exit
from Expect.
With no arguments, trap
prints the commands associated with each
signal number.
For example, ‘trap {send_user "Ouch!"} SIGINT’ prints ‘Ouch!’ each time the user presses <C-c>. The default behavior is restored by ‘trap SIGINT’.
Note that output may be lost if signals arrive during reads (although this is usually the desired behavior).
trap
will not let you override the action for SIGALRM
as
this is used internally to Expect. The disconnect
command sets
SIGALRM
to SIG_IGN
(ignore). You can reenable this as
long as you disable it during subsequent spawn commands.
Few checks on signals are made. For example, trap
does not
prevent you from registering signals that the kernel refuses to catch.
See signal(3) for more information.
wait [-i spawn_id]
Delays until a signal is received or the named spawned process (or the current process if none is named) terminates (or stops due to tracing). (See wait(2) for more information.)
wait
returns two integers. The first integer is the process id
of the process that was waited upon. In this case, the second integer
is WEXITSTATUS
(see wait(2)). If your system does not
support WEXITSTATUS
, the raw exit value is returned. If an error
occurs during execution of the wait, the integers returned are -1
followed by errno
.
The ‘-i’ option declares the process to wait corresponding to the named spawn id (NOT the process id).
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A vgrind
definition is available for pretty-printing Expect
scripts. Assuming the vgrind
definition supplied with the Expect
distribution is correctly installed, you can use it as:
vgrind -lexpect file
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It many not be apparent how to put everything together that is described here. I encourage you to read and try out the many examples in the ‘example’ directory of the Expect distribution. Some of them are real programs. Others are simply illustrative of certain techniques, and of course, a couple are just quick hacks. The ‘INSTALL’ file has a quick overview of these programs.
The Expect papers (see section Expect Bibliography) are also useful although invariably shorter. However, there is a significant amount of explanatory text accompanying those examples.
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Expect takes a rather liberal view of scoping. In particular, variables
read by commands specific to the Expect program will be sought first
from the local scope, and if not found, in the global scope. For
example, this obviates the need to place ‘global timeout’ in every
procedure you write that uses expect
. On the other hand,
variables written are always in the local scope (unless a global
command has been issued).
If you cannot enable the multispawning capability (i.e., your system
supports neither select
(BSD), poll
(SVr>
2), nor
something equivalent), Expect will only be able to control a single
process at a time. In this case, do not attempt to set spawn_id
,
nor should you execute processes via exec
while a spawned process
is running. Furthermore, you will not be able to expect
from
multiple processes (including the user as one) at the same time.
Terminal parameters can have a big effect on scripts. For example, if a script is written to look for echoing, it will misbehave if echoing is turned off. For this reason, Expect forces ‘sane’ terminal parameters by default. Unfortunately, this can make things unpleasant for other programs. As an example, the Emacs shell wants to change the “usual” mappings: newlines get mapped to newlines instead of carriage-return newlines, and echoing is disabled. This allows one to use emacs to edit the input line. Unfortunately, Expect cannot possibly guess this.
You can request that Expect not override its default setting of terminal parameters, but you must then be very careful when writing scripts for such environments. In the case of Emacs, avoid depending upon things like echoing and end-of-line mappings.
The commands that accepted arguments braced into a single list (the
expect
variants and interact
) use a heuristic to decide if
the list is actually one argument or many. The heuristic can fail only
in the case when the list actually does represent a single argument
which has multiple embedded ‘\n’ characters with non-whitespace
characters between them. This seems sufficiently improbable; however,
the argument ‘-brace’ can be used to force a single argument to be
handled as a single argument. This could conceivably be used with
machine-generated Expect code.
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It was really tempting to name the program sex
(for either
“Smart EXec” or “Send-EXpect”), but good sense (or perhaps just
Puritanism) prevailed.
Tcl 6.0 through 6.3 have a bug which may produce the error
‘Tcl_WaitPids got unknown process’ followed by a core dump. The
problem is that Tcl assumes it knows about all forked processes. When
it waits for one of its own (i.e., in exec
) and stumbles across
one that was spawned by Expect, it generates that error. Until this is
fixed, make sure you do a wait
on any spawned processes that have
exited before you call exec
. system
is safe from this
bug, so if you do not need the differences provided by exec
, you
can use system
meanwhile. Tcl 6.2 has a partial fix; core is not
dumped, but Expect loses the possibility of waiting on the process if
your system does not support waitpid
.
Since Tcl uses C-style null-terminated strings, there is no way to
represent strings with nulls in them. Expect will record such output to
the log and stdout, but it will strip them out before performing string
matching or storing in the variable expect_out
.
When a shell is spawned on an HP-UX system, it complains about not being able to access the tty. However, it runs anyway. You’ll have to discard that message in your scripts, though. If you figure out why this occurs please let me know.
Ultrix 4.1 (at least the latest versions around here) considers timeouts of above 1000000 to be equivalent to 0.
Telnet (verified only under SunOS 4.1.2) hangs if TERM
is not
set. This is a problem under cron
and at
, which do not
define TERM
. Thus, you must set it explicitly—to what type is
usually irrelevant. It just has to be set to something! The following
probably suffices for most cases.
set env(TERM) vt100
Some implementations of ptys are designed so that the kernel throws away any unread output after 10 to 15 seconds (actual number is implementation-dependent) after the process has closed the file descriptor. Thus, Expect programs such as
spawn date exec sleep 20 expect
will fail. To avoid this, invoke non-interactive programs with
exec
rather than spawn
. While such situations are
conceivable, in practice I have never encountered a situation in which
the final output of a truly interactive program would be lost due to
this behavior.
On the other hand, Cray UNICOS ptys throw away any unread output immediately after the process has closed the file descriptor. I have reported this to Cray and they are working on a fix.
Sometimes a delay is required between a prompt and a response, such as when a tty interface is changing UART settings or matching baud rates by looking for start/stop bits. Usually, all this requires is to sleep for a second or two. A more robust technique is to retry until the hardware is ready to receive input. The following example uses both strategies:
send "speed 9600\r"; exec sleep 1 expect { timeout {send "\r"; continue -expect} $prompt } |
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There are a couple of things about Expect that may be non-intuitive. This section attempts to address some of these things with a couple of suggestions.
A common expect problem is how to recognize shell prompts. Since these are customized differently by differently people and different shells, portably automating rlogin can be difficult without knowing the prompt. A reasonable convention is to have users store a regular expression describing their prompt (in particular, the end of it) in the environment variable EXPECT_PROMPT. Code like the following can be used. If EXPECT_PROMPT does not exist, this code still has a good chance of functioning correctly.
set prompt "(%|#|\$) $" ;# default prompt if [info exists env(EXPECT_PROMPT)] { set prompt $env(EXPECT_PROMPT) } expect -re $prompt |
I encourage you to write expect
patterns that include the end of
whatever you expect to see. This avoids the possibility of answering a
question before seeing the entire thing. In addition, while you may
well be able to answer questions before seeing them entirely, if you
answer early, your answer may appear echoed back in the middle of the
question. In other words, the resulting dialogue will be correct but
look scrambled.
Most prompts include a space character at the end. For example, the prompt from ftp is ‘f’, ‘t’, ‘p’, ‘>’ and ‘ ’. To match this prompt, you must account for each of these characters. It is a common mistake not to include the blank. Put the blank in explicitly.
If you use a pattern of the form ‘X*’, the ‘*’ will match all the output received from the end of X to the last thing received. This sounds intuitive but can be somewhat confusing because the phrase “last thing received” can vary depending upon the speed of the computer and the processing of I/O both by the kernel and the device driver.
In particular, humans tend to see program output arriving in huge chunks (atomically) when in reality most programs produce output one line at a time. Assuming this is the case, the ‘*’ in the pattern of the previous paragraph may only match the end of the current line, even though there seems to be more, because at the time of the match that was all the output that had been received.
expect
has no way of knowing that further output is coming unless
your pattern specifically accounts for it.
Even depending on line-oriented buffering is unwise. Not only do programs rarely make promises about the type of buffering they do, but system indigestion can break output lines up so that lines break at seemingly random places. Thus, if you can express the last few characters of a prompt when writing patterns, it is wise to do so.
If you are waiting for a pattern in the last output of a program and the
program emits something else instead, you will not be able to detect
that with the timeout
keyword. The reason is that expect
will not time out—instead it will get an eof
indication. Use
that instead. Even better, use both. That way if that line is ever
moved around, you will not have to edit the line itself.
Newlines are usually converted to carriage return, linefeed sequences when output by the terminal driver. Thus, if you want a pattern that explicitly matches the two lines, from, say, ‘printf("foo\nbar")’, you should use the pattern ‘foo\r\nbar’.
A similar translation occurs when reading from the user, via
expect_user
. In this case, when you press return, it will be
translated to a newline. If Expect then passes that to a program which
sets its terminal to raw mode (like telnet
), there is going to be
a problem, as the program expects a true return. (Some programs are
actually forgiving in that they will automatically translate newlines to
returns, but most do not.) Unfortunately, there is no way to find out
that a program put its terminal into raw mode.
Rather than manually replacing newlines with returns, the solution is to use the command ‘system stty raw’, which will stop the translation. Note, however, that this means that you will no longer get the cooked line-editing features.
interact
implicitly sets your terminal to raw mode so this
problem will not arise then.
It is often useful to store passwords (or other private information) in Expect scripts. This is not recommended since anything that is stored on a computer is susceptible to being accessed by anyone. Thus, interactively prompting for passwords from a script is a smarter idea than embedding them literally. Nonetheless, sometimes such embedding is the only possibility.
Unfortunately, the UNIX file system has no direct way of creating files
which are executable but unreadable. Systems which support
setgid
shell scripts may indirectly simulate this as follows:
Create the Expect script (that contains the secret data) as usual. Make
its permissions be 750 (‘-rwxr-x---’) and owned by a trusted group,
i.e., a group which is allowed to read it. If necessary, create a new
group for this purpose. Next, create a /bin/sh
script with
permissions 2751 (‘-rwxr-s--x’) owned by the same group as before.
The result is a script which may be executed (and read) by anyone. When invoked, it runs the Expect script.
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@xref{Tcl}, for discussion of the Tcl language that underlies Expect.
See libexpect(3), for discussion of a library of C functions
implementing expect
functionality.
A number of papers provide further reading:
Don Libes, Proceedings of the Summer 1990 USENIX Conference, Anaheim, California, June 11-15, 1990.
expect
to Automate System Administration TasksDon Libes, Proceedings of the 1990 USENIX Large Installation Systems Administration Conference, Colorado Springs, Colorado, October 17-19, 1990.
John Ousterhout, Proceedings of the Winter 1990 USENIX Conference, Washington, D.C., January 22-26, 1990.
Don Libes, Computing Systems, Vol. 4, No. 2, University of California Press Journals, November 1991.
Don Libes, Proceedings of the Summer 1992 USENIX Conference, pp. 135-144, San Antonio, TX, June 12-15, 1992.
Don Libes, Software—Practice & Experience, John Wiley & Sons, West Sussex, England, to appear.
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Don Libes, of the National Institute of Standards and Technology (NIST), implemented Expect.
Thanks to John Ousterhout for Tcl, and Scott Paisley for inspiration. Thanks to Rob Savoye for Expect’s autoconfiguration code.
The ‘HISTORY’ file documents much of the evolution of Expect. It makes interesting reading and might give you further insight to this software. Thanks to the people mentioned in it who sent me bug fixes or gave other assistance.
Design and implementation of Expect was paid for by the U.S. government and is therefore in the public domain. However the author and NIST would like credit if this program and documentation or portions of them are used.
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