Question

I have the following command line that outputs vmstat every second with a time stamp on each line via the perl script:

vmstat 15 | /home/Beer/addtimestamp.pl > File_1 

the contents of the addtimestamp.pl:

!/usr/bin/perl
while (<>) { print localtime() . ": $_"; }

So why doesn’t the output get redirected to the “File_1” file?

It works perfectly when I don’t, it prints out the output perfectly every second with no issues at all.

Answer 1

When outputting to the terminal, perl’s output is line buffered, so you will see every line as it is output. When its output is a file, it will be block buffered so you will not see any output until a full block is ready to write (4k I think, but variable and system-defined).

You need to set stdout to use line buffering:

$|=1;

Search for [perl line buffered output] and you’ll see plenty of results about this.