How do I receive command output immediately?

Question

I'm using perl back-ticks syntax to run some commands. I would like the output of the command to written to a file and also printed out to stdout. I can accomplish the first by adding a > at the end of my back-ticked string, but I do not know hot to make the output be printed as soon as it is generated. If I do something like

print `command`; 

the output is printed only after command finished executing.

Thanks, Dave

Answer 1

You cannot do it with the backticks, as they return to the Perl program only when the execution has finished.

So,

print  `command1; command2; command3`;

will wait until command3 finishes to output anything.

You should use a pipe instead of backticks to be able to get output immediately:

open (my $cmds, "-|", "command1; command2; command3");
while (<$cmds>) {
        print;
}
close $cmds;

After you've done this, you then have to see if you want or not buffering (depending on how immediate you want the output to be): Suffering from buffering?

To print and store the output, you can open() a file to write the output to:

open (my $cmds, "-|", "command1; command2; command3");
open (my $outfile, ">", "file.txt");
while (<$cmds>) {
        print;
        print $outfile $_;
}
close $cmds;
close $outfile;