Expect question

RaduH has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks,
Here's the piece of code I have trouble with:

    $sftp = Expect->spawn("sftp $toyName\@$targetServer") || die("Coul
+dn't spawn sftp, $!");
    $sftp->expect(5, "Password:  ") || die("Never got password prompt 
+on $targetServer, ".$sftp->exp_error()."\n");
    print($sftp "$toyPassword\r");
    $sftp->expect(5, "sftp\> ") || die("Never got sftp prompt on $targ
+etServer, ".$sftp->exp_error()."\n");
    print ($sftp "put $backupFileName\r");
    $sftp->expect(120, "sftp> ") || die("Never got password prompt on 
+$targetServer after upload, ".$stfp->exp_error()."\n");
[download]

I read the documentation in the Expect module and I saw it can be (well, IS) very strict when it comes to matching the expected strings and the "Password: " string is one particular example being discussed. They warn about the exact number of blank spaces to expect after the colon. I was careful with all of these but I still cannot match the "Password: " prompt (two blanks after : on purpose..)
Here's the resulting output:

Spawned 'sftp user@<host_ip>'
      spawn id(3)
      Pid: 23271
      Tty: /dev/pts/2
 at /usr/lib/perl5/site_perl/5.8.8/Expect.pm line 181
      Expect::spawn('Expect', 'sftp user@<host_ip>') called at lifch.p
+l line 126
      main::upgrade_procedure() called at lifch.pl line 267
expect(Expect=GLOB(0xb96b40) 5 Password:  ) called...
expect(): handling param 'Password:  '...
expect(): exact match 'Password:  '...
Starting EXPECT pattern matching...
 at /usr/lib/perl5/site_perl/5.8.8/Expect.pm line 561
      Expect::expect('Expect=GLOB(0xb96b40)', 5, 'Password:  ') called
+ at lifch.pl line 127
      main::upgrade_procedure() called at lifch.pl line 267
spawn id(3): beginning expect.
      Timeout: 5 seconds.
      Current time: Fri Oct 26 17:04:42 2007
spawn id(3): list of patterns:
  #1: -ex `Password:  '


spawn id(3): Does `'
match:
  pattern #1: -ex `Password:  '? No.

Waiting for new data (5 seconds)...
spawn id(3): new data.
spawn id(3): read 33 byte(s).

spawn id(3): Does `Connecting to <host_ip>...\r\n'
match:
  pattern #1: -ex `Password:  '? No.

Waiting for new data (5 seconds)...
spawn id(3): new data.
spawn id(3): read 10 byte(s).

spawn id(3): Does `Connecting to <host_ip>...\r\nPassword: '
match:
  pattern #1: -ex `Password:  '? No.

Waiting for new data (4 seconds)...
TIMEOUT
Returning from expect with TIMEOUT or EOF
Never got password prompt on <host_ip>, 1:TIMEOUT
Closing spawn id(3).
 at /usr/lib/perl5/site_perl/5.8.8/Expect.pm line 1431
      Expect::hard_close('Expect=GLOB(0xb96b40)') called at /usr/lib/p
+erl5/site_perl/5.8.8/Expect.pm line 1621
      Expect::DESTROY('Expect=GLOB(0xb96b40)') called at lifch.pl line
+ 0
      eval {...} called at lifch.pl line 0
spawn id(3) closed.
Pid 23271 of spawn id(3) terminated, Status: 0x100
[download]

What am I still doing wrong? Thanks!

Comment on Expect question Select or Download Code

Replies are listed 'Best First'.

Re: Expect question
by jasonk (Parson) on Oct 26, 2007 at 23:33 UTC

You indicate that you put two blanks after the colon on purpose, but then the script times out waiting for the word 'Password' followed by a colon, followed by a single space. So what was the purpose of the second space?

We're not surrounded, we're in a target-rich environment!

[reply]

Re^2: Expect question

by RaduH (Scribe) on Oct 26, 2007 at 23:38 UTC

expect(): handling param 'Password:  '...
expect(): exact match 'Password:  '...
[download]

[reply]
[d/l]

Re^3: Expect question

by Crackers2 (Parson) on Oct 27, 2007 at 00:37 UTC

I might be wrong of course, but it looks to me like that's just the parsing of the parameter you gave to the function, not what was received from the server.

Could the problem have something to do with the "exact match" part? You can see it's trying to match against 'Connecting to <host_ip>...\r\nPassword: '

So perhaps you need something like '.*Password: ' as a pattern? (assuming it takes regexes).

[reply]
[d/l]
[select]

Re^4: Expect question

by RaduH (Scribe) on Oct 29, 2007 at 15:30 UTC

Re^3: Expect question

by jasonk (Parson) on Oct 29, 2007 at 19:21 UTC

These two lines of debugging output both display your expect pattern as it is being built by Expect, meaning the text in those two lines is from your code, not from the server.

We're not surrounded, we're in a target-rich environment!

[reply]

Re: Expect question
by Argel (Prior) on Oct 27, 2007 at 01:46 UTC

print $expect_instance "$username}\r";
( $which, $why, $match, $before, $after) = $expect_instance->expect( $
+timeout, 'Password:');
if( ! $which ) { myerror( 'send the password:'.$why );
[download]

I think you are getting hung up on the spaces when either they do not matter or you could just deal with them later on in your perl code. Though it's been a few years since I touched Expect.pm so maybe it is more rigid than before.

[reply]
[d/l]

Re: Expect question
by andyford (Curate) on Oct 27, 2007 at 11:21 UTC

Ignore me if it won't work for you, but something to consider: If you used pre-positioned ssh keys to authenticate instead of passwords, all your problems described here would just go away. Additionally you won't have to retool your Expect machinations when you hit a different server that acts a bit differently. There are many discussions and tutorials on the web, just search for something like 'using ssh key' and you'll retrieve a wealth of information.

non-Perl: Andy Ford

[reply]

Re^2: Expect question

by RaduH (Scribe) on Oct 29, 2007 at 15:01 UTC

Yup, I did think about that and I will use it. However, some scenarios need to go through the more painful experience, cannot always afford the ssh key trick. Thanks for the advice though, good point!

[reply]

Re: Expect question
by RaduH (Scribe) on Oct 29, 2007 at 15:48 UTC

I was about to try the few lines proposed by Argel when I noticed that example was using single quotes '...'rather than doubles like I was using "..." Before trying those lines I just replaced my doubles with single quotes and changed nothing else. it worked just fine. I even killed the spaces trailing the colon after password and it still matched just fine.

The string I was trying to match was the "Password:" part of the entire return from the server, which looked like "Connecting to <ip>... \n\rPassword: ". I know using 'Password:' works but I have no idea why. (just went back and read some more about single and double quotes, but I'm still in the fog)

Since this is a slightly different problem, I just wanted to post here my findings and thank you for your replies. I'll ask the next question in a separate thread.

[reply]

Re^2: Expect question

by Argel (Prior) on Oct 29, 2007 at 19:54 UTC

As a side note, I suspect I was normally using single quotes because that's good practice if the string does not contain anything that needs to be interpolated. Though I suppose actually making them constants would be even better practice but I hate 'use constant'. What I really want is a constant that I can treat like a variable, so if ReadOnly was ever made Core I would likely use that.

Anyway, thanks for passing the solution on as I'm sure others can benefit and it's always great to hear how things turned out when someone asks a question. Err, well, except for paco, where the mystery is surely better than the answer (unless he has a great hot dog recipe).

[reply]

Re^3: Expect question

by RaduH (Scribe) on Oct 31, 2007 at 14:01 UTC

I was running Perl 5.8.8 on a Suse 10.2 machine. ... About that hot dog recipe, if you or anybody else get your hands on it, please post :)

[reply]

(OT) Re^4: Expect question

by Argel (Prior) on Oct 31, 2007 at 17:01 UTC


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