ch09_26.htm

the unix power tools

><PRE
CLASS="screen"
># <CODE
CLASS="userinput"
><B
>passwd libes</B
></CODE
>
Changing password for libes on thunder.
New password:
Retype new password:</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>This is fine for a single password.
But suppose you have accounts of your own on a number of unrelated
computers and you would like them all to have the same password.
Or suppose you are a system administrator establishing 1000 accounts at
the beginning of each semester.
All of a sudden, an automated <EM
CLASS="emphasis"
>passwd</EM
> makes a lot of sense.
Here is an Expect script to do just that:
automate <EM
CLASS="emphasis"
>passwd</EM
> so that it can be called from a shell script.</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>spawn passwd [lindex $argv 0]
set password [lindex $argv 1]
expect &quot;password:&quot;
send &quot;$password\r&quot;
expect &quot;password:&quot;
send &quot;$password\r&quot;
expect eof</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The first line starts the <EM
CLASS="emphasis"
>passwd</EM
> program with the username passed as
an argument.
The next line saves the password in a variable for convenience.
As in shell scripts, variables do not have to be declared in advance.</P
><P
CLASS="para"
>In the third line, the <EM
CLASS="emphasis"
>expect</EM
> command looks for the pattern
<CODE
CLASS="literal"
>password:</CODE
>.
<EM
CLASS="emphasis"
>expect</EM
> waits until the pattern is found before continuing.</P
><P
CLASS="para"
>After receiving the prompt, the next line sends a password to the
current process.
The <EM
CLASS="emphasis"
>\r</EM
> indicates a carriage-return.
(Most of the usual C string conventions are supported.)
There are two <EM
CLASS="emphasis"
>expect</EM
>-<EM
CLASS="emphasis"
>send</EM
> sequences because
<EM
CLASS="emphasis"
>passwd</EM
> asks the password to
be typed twice as a spelling verification.
There is no point to this in a non-interactive <EM
CLASS="emphasis"
>passwd</EM
>,
but the script has to do it because <EM
CLASS="emphasis"
>passwd</EM
> assumes it is
interacting with a human who does not type consistently.</P
><P
CLASS="para"
>The final command <CODE
CLASS="literal"
>expect&nbsp;eof</CODE
> causes the script to wait for the
end-of-file in the output of <EM
CLASS="emphasis"
>passwd</EM
>.
Similar to <EM
CLASS="emphasis"
>timeout</EM
>, <EM
CLASS="emphasis"
>eof</EM
> is another keyword pattern.
This final <EM
CLASS="emphasis"
>expect</EM
> effectively waits for <EM
CLASS="emphasis"
>passwd</EM
> to
complete execution before returning control to the script.</P
><P
CLASS="para"
>Take a step back for a moment.
Consider that this problem could be solved in a different way.
You could edit the source to <EM
CLASS="emphasis"
>passwd</EM
> (should you be so lucky as to
have it) and modify it so that given an optional flag, it reads its
arguments from the command line just the way that the Expect
script does.
If you lack the source and have to write <EM
CLASS="emphasis"
>passwd</EM
> from scratch, of
course, then you will have to worry about how to encrypt passwords,
lock and write the password database, etc.
In fact, even if you only modify the existing code, you may find it
surprisingly complicated code to look at.
The <EM
CLASS="emphasis"
>passwd</EM
> program does some very tricky things.
If you do get it to work, pray that nothing
changes when your system is upgraded.
If the vendor adds NIS, Kerberos, shadow passwords, a different
encryption function, or some other new feature, you will have to
revisit the code.</P
><P
CLASS="para"
>Expect comes with many example scripts that demonstrate how you can
do many things that are impossible with traditional shells.
For example, the <EM
CLASS="emphasis"
>passmass</EM
> script lets you update your password on
many unrelated machines simultaneously.
The <EM
CLASS="emphasis"
>rftp</EM
> script provides your regular <EM
CLASS="emphasis"
>ftp</EM
> client with
additional commands to do recursive FTP in either direction.
The <EM
CLASS="emphasis"
>cryptdir</EM
> script encrypts all the files in a directory.
And an amusing script is provided that lets two <EM
CLASS="emphasis"
>chess</EM
>
processes play each other.
Expect has no limit to the number of interactive programs it can drive
at the same time.
[The UNIX system may limit Expect, though, by controlling the
maximum number of processes or other system resources available. -<EM
CLASS="emphasis"
>JP</EM
>&nbsp;]<A
CLASS="indexterm"
NAME="AUTOID-11583"
></A
><A
CLASS="indexterm"
NAME="AUTOID-11584"
></A
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-9912-SECT-1.3"
>9.26.3 Testing: A Story </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-11588"
></A
>Many people use Expect for testing.
You can test interactive programs as easily as you can automate them.
And hardware lends itself to testing with Expect, too.
For example, we solved a thorny problem when we had to deal with an
unreliable bank of modems.
We were receiving dozens of calls each week reporting &quot;the modem is hung.&quot;
No indication of which modem, of course.
And it was always too late for us to ask the user to try something
to investigate the problem.
The connection was gone by then.
Our solution was an Expect script that hourly connected to each modem
and exercised it.
Any problems were recorded so that we had a clear and full history of
each modem's behavior.
As soon as a defective or hung modem was encountered, the
Expect script would send email to the system administrator.
With this script in place, reports of modem problems from our users
dropped to zero.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-9912-SECT-1.4"
>9.26.4 Other Problems </A
></H3
><P
CLASS="para"
>These are just a few of the problems that can be solved with Expect.
And as with all Expect solutions, recompilation of the original
programs is unnecessary.
You don't even need the source code!
Expect handles many other problems as well.
For example, Expect can wrap existing interactive tools with GUI wrappers.
This means you can wrap interactive programs with Motif-like frontends
to control applications by buttons, scrollbars, and other graphic
elements.
And Expect scripts work great as CGI scripts or from
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>cron</EM
> (<A
CLASS="linkend"
HREF="ch40_12.htm"
TITLE="Periodic Program Execution: The cron Facility "
>40.12</A
>)</SPAN
>
or <EM
CLASS="emphasis"
>inetd</EM
>
[the daemon that controls Internet services provided by a
system-<EM
CLASS="emphasis"
>JP</EM
>&nbsp;].
Finally, learning Expect may be easier than you think.
Expect can watch you interact and then produce an Expect script for you.
Interaction automation can't get much easier than this!</P
><P
CLASS="para"
>More information on Expect is available in <EM
CLASS="emphasis"
>Exploring Expect</EM
>, by
Don Libes, from O'Reilly &amp; Associates.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>DL</SPAN
></P
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