Re^2: How to get screenshort using PERL?

#!/usr/local/bin/perl

use strict;
use warnings;

my $file = '/root/Desktop/';
sleep 5;
$file =  system("import -window root MyScreenshot.png");
print $file, "\n";
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Ouch! That really hurts my eyes!

Why do you sleep before taking a screenshot?
Why do you assign a useless constant to $file?
Are you sure system() returns a file or a filename? Hint: It doesn't!
So why do you assign the return value to a variable named $file?
Why do you mess with the notoriously problematic default shell instead of using the multiple-argument form of system?
Why do you print the return value of system to STDOUT?

At least, you use strict and warnings.

use strict;
use warnings;

system("import -window root MyScreenshot.png");
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Better, but still problematic: You still invoke the default shell, begging for trouble. All error checks are missing. Yes, system() may fail. For example, because there is no import utility -- e.g. when ImageMagick is not installed. Or because there is some other tool named import, with completely different parameters a and a completely different purpose.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

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Re^3: How to get screenshort using PERL? by almut (Canon) on Apr 28, 2010 at 15:13 UTC
You still invoke the default shell Actually, this particular example wouldn't invoke a shell, because there are no shell metacharacters in the command. $ strace -f -eexecve perl -e 'system("import -window root MyScreenshot +.png")' execve("/usr/local/bin/perl", ["perl", "-e", "system(\"import -window +root MySc"...], [/* 31 vars /]) = 0 Process 20945 attached (waiting for parent) Process 20945 resumed (parent 20944 ready) [pid 20945] execve("/home/almut/bin/import", ["import", "-window", "ro +ot", "MyScreenshot.png"], [/ 31 vars /]) = -1 ENOENT (No such file +or directory) [pid 20945] execve("/usr/local/sbin/import", ["import", "-window", "ro +ot", "MyScreenshot.png"], [/ 31 vars /]) = -1 ENOENT (No such file +or directory) [pid 20945] execve("/usr/local/bin/import", ["import", "-window", "roo +t", "MyScreenshot.png"], [/ 31 vars /]) = -1 ENOENT (No such file o +r directory) [pid 20945] execve("/usr/sbin/import", ["import", "-window", "root", " +MyScreenshot.png"], [/ 31 vars /]) = -1 ENOENT (No such file or dir +ectory) [pid 20945] execve("/usr/bin/import", ["import", "-window", "root", "M +yScreenshot.png"], [/ 31 vars */]) = 0 [download] As you can see, the `import` is being run directly. (Add a `';'` (metacharacter) at the end of the command, and it will run the command via a shell...)	[reply] [d/l] [select]
Re^4: How to get screenshort using PERL? by afoken (Chancellor) on Apr 29, 2010 at 12:18 UTC
Actually, this particular example wouldn't invoke a shell ... Right. But the next logical step will probably cause both invoking a shell and opening a security hole: Depending on the beginners experience, the call may change to `system("import -window root $filename")` (very naive) or `system("import -window root '$filename'")` (a little bit of experience, but still wrong). With a proper multi-argument `system()` (i.e. `system('import','-window','root','MyScreenshot.png')`), it is very unlikely to happen. We may see some cargo cult, like putting the filename in quotes (`system('import','-window','root',"$filename")`), but that does not cause any problems (except perhaps some wasted CPU cycles). And for extra bonus points, the `import` utility would be called with an absolute path, to avoid the dependancy on `$ENV{'PATH'}`. That would also save a lot of CPU cycles trying to find `import`. Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]