How can I capture the stdin and stdout of system command from a Perl script?

Question

In the middle of a Perl script, there is a system command I want to execute. I have a string that contains the data that needs to be fed into stdin (the command only accepts input from stdin), and I need to capture the output written to stdout. I've looked at the various methods of executing system commands in Perl, and the open function seems to be what I need, except that it looks like I can only capture stdin or stdout, not both.

At the moment, it seems like my best solution is to use open, redirect stdout into a temporary file, and read from the file after the command finishes. Is there a better solution?

Answer 1

I think you want to take a look at IPC::Open2

Answer 2

IPC::Open2/3 are fine, but I've found that usually all I really need is IPC::Run3, which handles the simple cases really well with minimal complexity:

use IPC::Run3;    # Exports run3() by default

run3( \@cmd, \$in, \$out, \$err );

The documentation compares IPC::Run3 to other alternatives. It's worth a read even if you don't decide to use it.

Answer 3

The perlipc documentation covers many ways that you can do this, including IPC::Open2 and IPC::Open3.

Answer 4

Somewhere at the top of your script, include the line

use IPC::Open2;

That will include the necessary module, usually installed with most Perl distributions by default. (If you don't have it, you could install it using CPAN.) Then, instead of open, call:

$pid = open2($cmd_out, $cmd_in, 'some cmd and args');

You can send data to your command by sending it to $cmd_in and then read your command's output by reading from $cmd_out.

If you also want to be able to read the command's stderr stream, you can use the IPC::Open3 module instead.

Answer 5

IPC::Open3 would probably do what you want. It can capture STDERR and STDOUT.

http://metacpan.org/pod/IPC::Open3

Answer 6

A very easy way to do this that I recently found is the IPC::Filter module. It lets you do the job extremely intuitively:

$output = filter $input, 'somecmd', '--with', 'various=args', '--etc';

Note how it invokes your command without going through the shell if you pass it a list. It also does a reasonable job of handling errors for common utilities. (On failure, it dies, using the text from STDERR as its error message; on success, STDERR is just discarded.)

Of course, it’s not suitable for huge amounts of data since it provides no way of doing any streaming processing; also, the error handling might not be granular enough for your needs. But it makes the many simple cases really really simple.

Answer 7

There is a special perl command for it

open2()

More info can be found on: http://sunsite.ualberta.ca/Documentation/Misc/perl-5.6.1/lib/IPC/Open2.html

Answer 8

If you do not want to include extra packages, you can just do

open(TMP,">tmpfile");
print TMP  $tmpdata ;
open(RES,"$yourcommand|");
$res = "" ;
while(<RES>){
$res .= $_ ;
}

which is the contrary of what you suggested, but should work also.

Answer 9

I always do it this way if I'm only expecting a single line of output or want to split the result on something other than a newline:

my $result = qx( command args 2>&1 );  
my $rc=$?;  
# $rc >> 8 is the exit code of the called program.

if ($rc != 0 ) {  
    error();  
}  

If you want to deal with a multi-line response, get the result as an array:

my @lines = qx( command args 2>&1 );  

foreach ( my $line ) (@lines) {  
    if ( $line =~ /some pattern/ ) {  
        do_something();  
    }  
}