Why doesn't the output get redirected into the file?

Question

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.

There's a utility to do this: ts, part of the moreutils package. — donothingsuccessfully, Dec 11 '12 at 07:51

score 3 · Accepted Answer · answered Dec 11 '12 at 07:56

3

You fell into the buffering Gotcha. Perl buffers its output and only writes out to disk when the buffer is full. Buffering is a good thing performance-wise, but at low data rates can be confusing. If you wait long enough you'll notice your file being written (check with tail -F File_1). I believe standard buffer is 4kB in size.

Add $|=1 like below to disable buffering in Perl:

vmstat 1 | perl -e '$| = 1; while (<>) { print localtime() . ": $_"; }' > /tmp/fileetje

answered Dec 11 '12 at 07:56

jippie

14,086

I just add '$|=1' to the start of the script and it worked like a Charm!
Wait, that's actually correct, why am I using two scripts when I can just merge both!, Awesome Dude!

Thanks!
– I AM L Dec 11 '12 at 08:03

Why doesn't the output get redirected into the file?

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