> if I am giving a wrong command I am getting error.
> But, from then onwards I am not getting output of
> even correct command. what could be wrong ??

    my $cmd = <STDIN>;
    print Writer $cmd;
    my $got = <Reader>;
    print $got;
[download]

This is exactly the problem that is explained in the manual:

This whole affair is quite dangerous, as you may block forever.

One quick way to fix this: Instead of bash, try bash 2>&1. Then $got will get the error message from bash.

To fix this properly, you would need to use the select function. select will tell you whether Reader has anything to read. Then read from Reader only when there is data available. This is difficult to do right, so I recommend that you try something else first.

Probably the best thing is as repson suggested and forget about IPC::Open2. Use $got = qx{$cmd} instead.

Note also that IPC::Open3 will do the same thing as open2, but catches stderr also, so that's another option.

Update: As Dominus kindly pointed out, this does not solve the immediate problem, but if you decide that differentiating between STDERR/STDOUT is something you will end up needing to do with this specific issue or in a future one, IPC::Open3 is something you may wish to look into. Obviously you can't use it as a drop-in replacement for open2 in your code and have things magically work. It's an alternative at more of a design level.

Says fastolfe:

> Note also that IPC::Open3 will do the same thing as open2,
>but catches stderr also, so that's another option.


It may be another option for someone, but not for galande. Did you read the original question? The problem was that galande is reading from the child's stdout when there is nothing there, and that causes the parent to block. Switching from Open2 to Open3 is not going to make a difference.

First off what you are actually doing here can be done with a simple qx// or backticks. print qw/$cmd/ could replace your loop. This code would be much more sensible than what you are doing if what you intend is simply to get the output of running $cmd.

Note: You should be running with taint checks on when running user input in the shell if there is more to this program then simply providing a direct wrapper around bash from the shell.

However if you require a persistent shell (though I don't see why you would in normal situations) then you are on the right track. The first problem is that for a persistent shell you would want to move open2 and the autoflush statements outside the loop. You should also print more than one line of output from the shell in the loop.
Here is my code to accomplish this task:

#same initialization
my $pid = open2( \*Reader, \*Writer, "/bin/bash" ) or warn $!;
Writer->autoflush();
Reader->autoflush();
STDIN->autoflush();
$SIG{ALRM} = sub {die}; # required for alarm call below
do {
    print ": "; # simple prompt
    my $cmd = <STDIN>;
    print Writer $cmd;
    my $got;
    while (1) {
        eval { alarm 3; $_=<Reader> }; # get line in 3 seconds or stop
+ reading
        alarm 0;
        last if ($@ or !(defined($_))); # exit loop if there was a tim
+eout or no data
        $got .= $_;
    }
    print $got;
} while(1);
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If you decide to use Expect, and you need to differentiate the STDOUT from the STDERR, (jcwren) RE: How to split output using Expect.pm has an idea that may possibly be of use to you.

--Chris

e-mail jcwren