通过重定向捕获Perl系统命令的返回码

Question

I am trying to capture the return code of a system call. To make it simple, consider a command that you know returns code 1, for example the following bash script t.sh :

#! /bin/bash
exit 1

Then consider:

use warnings;
use strict;
use feature qw(say);

my $res=system("t.sh");
say ($?>>8);

$res=system("t.sh &>/dev/null");
say ($?>>8);

which prints

1
0

Why does the second system call (the one with error redirection) give me a zero return code?

Answer 1

I can't duplicate your issue with Bash v4.1.2:

$ perl -wE 'system( q{/bin/bash -c "true &> /dev/null"} ); say $? >> 8'
0
$ perl -wE 'system( q{/bin/bash -c "false &> /dev/null"} ); say $? >> 8'
1

However, the &> redirection operator is not portable and should generally be avoided . I'm not sure, but it seems like it may not have worked properly in earlier releases. ^*

The following syntax is semantically equivalent and much more portable:

>file 2>&1

(note the & in 2>&1 )

Use it instead.

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* According to the Advanced Bash Scripting Guide , "This operator is now functional, as of Bash 4, final release."