autoexpect - generate an Expect script from watching a session
] [ program args...
autoexpect watches you interacting with another program and creates an Expect
script that reproduces your interactions. For straightline scripts, autoexpect
saves substantial time over writing scripts by hand. Even if you are an Expect
expert, you will find it convenient to use autoexpect to automate the more
mindless parts of interactions. It is much easier to cut/paste hunks of
autoexpect scripts together than to write them from scratch. And if you are a
beginner, you may be able to get away with learning nothing more about Expect
than how to call autoexpect.
The simplest way to use autoexpect is to call it from the command line with no
arguments. For example:
By default, autoexpect spawns a shell for you. Given a program name and
arguments, autoexpect spawns that program. For example:
% autoexpect ftp ftp.cme.nist.gov
Once your spawned program is running, interact normally. When you have exited
the shell (or program that you specified), autoexpect will create a new script
for you. By default, autoexpect writes the new script to
"script.exp". You can override this with the -f flag followed by a
new script name.
The following example runs "ftp ftp.cme.nist.gov" and stores the
resulting Expect script in the file "nist".
% autoexpect -f nist ftp ftp.cme.nist.gov
It is important to understand that autoexpect does not guarantee a working
script because it necessarily has to guess about certain things - and
occasionally it guesses wrong. However, it is usually very easy to identify
and fix these problems. The typical problems are:
- Timing. A surprisingly large number of programs (rn, ksh,
zsh, telnet, etc.) and devices (e.g., modems) ignore keystrokes that
arrive "too quickly" after prompts. If you find your new script
hanging up at one spot, try adding a short sleep just before the previous
You can force this behavior throughout by overriding the variable
"force_conservative" near the beginning of the generated script.
This "conservative" mode makes autoexpect automatically pause
briefly (one tenth of a second) before sending each character. This
pacifies every program I know of.
This conservative mode is useful if you just want to quickly reassure
yourself that the problem is a timing one (or if you really don't care
about how fast the script runs). This same mode can be forced before
script generation by using the -c flag.
Fortunately, these timing spots are rare. For example, telnet ignores
characters only after entering its escape sequence. Modems only ignore
characters immediately after connecting to them for the first time. A few
programs exhibit this behavior all the time but typically have a switch to
disable it. For example, rn's -T flag disables this behavior.
The following example starts autoexpect in conservative mode.
The -C flag defines a key to toggle conservative mode. The following example
starts autoexpect (in non-conservative mode) with ^L as the toggle. (Note
that the ^L is entered literally - i.e., enter a real control-L).
autoexpect -C ^L
The following example starts autoexpect in conservative mode with ^L as the
autoexpect -c -C ^L
- Echoing. Many program echo characters. For example, if you
type "more" to a shell, what autoexpect actually sees is:
you typed 'm',
computer typed 'm',
you typed 'o',
computer typed 'o',
you typed 'r',
computer typed 'r',
Without specific knowledge of the program, it is impossible to know if you
are waiting to see each character echoed before typing the next. If
autoexpect sees characters being echoed, it assumes that it can send them
all as a group rather than interleaving them the way they originally
appeared. This makes the script more pleasant to read. However, it could
conceivably be incorrect if you really had to wait to see each character
- Change. Autoexpect records every character from the
interaction in the script. This is desirable because it gives you the
ability to make judgements about what is important and what can be
replaced with a pattern match.
On the other hand, if you use commands whose output differs from run to run,
the generated scripts are not going to be correct. For example, the
"date" command always produces different output. So using the
date command while running autoexpect is a sure way to produce a script
that will require editing in order for it to work.
The -p flag puts autoexpect into "prompt mode". In this mode,
autoexpect will only look for the the last line of program output - which
is usually the prompt. This handles the date problem (see above) and most
The following example starts autoexpect in prompt mode.
The -P flag defines a key to toggle prompt mode. The following example
starts autoexpect (in non-prompt mode) with ^P as the toggle. Note that
the ^P is entered literally - i.e., enter a real control-P.
autoexpect -P ^P
The following example starts autoexpect in prompt mode with ^P as the
autoexpect -p -P ^P
flag disables informational messages produced by autoexpect.
flag names a quote character which can be used to enter characters
that autoexpect would otherwise consume because they are used as toggles.
The following example shows a number of flags with quote used to provide a way
of entering the toggles literally.
autoexpect -P ^P -C ^L -Q ^Q
I don't know if there is a "style" for Expect programs but autoexpect
should definitely not be held up as any model of style. For example,
autoexpect uses features of Expect that are intended specifically for
computer-generated scripting. So don't try to faithfully write scripts that
appear as if they were generated by autoexpect. This is not useful.
On the other hand, autoexpect scripts do show some worthwhile things. For
example, you can see how any string must be quoted in order to use it in a Tcl
script simply by running the strings through autoexpect.
"Exploring Expect: A Tcl-Based Toolkit for Automating Interactive
by Don Libes, O'Reilly and Associates, January 1995.
Don Libes, National Institute of Standards and Technology
are in the public domain. NIST and I would
appreciate credit if these programs or parts of them are used.