Perl with backticks - I do not understand the operation inside the backticks

Question

I'm a perl novice. What's happening inside the backticks of the following?

$selfext and $banner are previously defined. The statement below is executed in Linux and I don't understand where the banner function is coming from? When I print $_ I get a null string.

$_=`banner $selfext $banner`;

Can you show the contents of `$selfext` and `$banner`? In Perl, an expression in backticks is executed as a system call and its output is returned as a string. Here, you are assigning that output to `$_`. If it is empty, then the `banner` command probably didn't produce any output. — joanis, Jul 12 '19 at 20:26
Note that the code appears to suffer from [code injection](https://en.wikipedia.org/wiki/Code_injection) bugs. — ikegami, Jul 12 '19 at 21:11

score 2 · Answer 1 · edited Jul 13 '19 at 07:29

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This is invoking a shell which is executing the banner command. banner is a normal UNIX program like uniq, sort or even perl itself, i.e. this is not some perl function. It is not installed on all systems though (same as with perl) and therefore the execution might fail. For more see man banner.

edited Jul 13 '19 at 07:29

melpomene

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answered Jul 12 '19 at 20:19

Steffen Ullrich

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Steffen, Thanks! the script is running on RedHat7.1 and "banner" does not exist. I should've mentioned the script is about 20 years old and began its life runnning on Unix machines like AIX. – BilgeRat Jul 12 '19 at 21:18
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`banner` is also part of [Perl Power Tools](http://perlpowertools.com) should you need it for something. – brian d foy Jul 12 '19 at 22:57
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@BilgeRat There's probably a system package for it (`yum install banner`?). – melpomene Jul 13 '19 at 07:27
Package name is `bsd-games`. – daxim Jul 15 '19 at 08:04

Perl with backticks - I do not understand the operation inside the backticks

1 Answers1