How do I use backticks to capture the elapsed time output from time(1)?

perl, time

Solution

The redirection needed is just a bit tricky.

my $result = `/usr/bin/time -f %e "./"$PROG 2>&1 >/dev/null`;

`time` outputs its measurement on the standard error (file descriptor 2), but Perl backticks capture the standard output (file descriptor 1). Read the redirection sequence above as “first send the standard error to the standard output’s destination and then throw away the original standard output.”

The answer to How can I capture `STDERR` from an external command? in the Perl FAQ addresses the issue you’re seeing.

To capture a command’s `STDERR` but discard its `STDOUT`:

$output = `cmd 2>&1 1>/dev/null`;           # either with backticks
$pid = open(PH, "cmd 2>&1 1>/dev/null |");  # or with an open pipe
while (<PH>) { }                            #    plus a read

… Ordering is important in all these examples. That’s because the shell processes file descriptor redirections in strictly left to right order.

system("prog args 1>tmpfile 2>&1");
system("prog args 2>&1 1>tmpfile");

The first command sends both standard out and standard error to the temporary file. The second command sends only the old standard output there, and the old standard error shows up on the old standard out.

Note that `$result` will contain a trailing newline, which you can remove with `chomp` as in

chomp $result;

or all in a single line with

chomp(my $result = `/usr/bin/time -f %e "./"$PROG 2>&1 >/dev/null`);

Problem

Original source