So my question is, how can I check when I open a process, if the command really exists?

use strict;
use warnings;

my $run_cmd = "dira";

if ( open(my $CMD, "$run_cmd |") ) {
  while( <$CMD> ) {
      print "$_";
  }
  close($CMD);
} else {
  die "Problem executing command $run_cmd - $!";
  # ... or something non-fatal
}
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Update:

Another possibility would be to loop through all pathes in the environment variable PATH and then checking if the command exists.

Update2:
The program above was tested and worked fine under Linux. Since I am not a Windows-expert, I can only do wild guesses on what the problem could be:

• cmd is started successfully but then fails to run dira?
• open() might return something that Perl thinks is true (e.g. -1)?
• dira exists somewhere, but prints an error...?
• copy/paste error?
• ...

Thanks. Problem is that even if I take the not-existing command "dira" I'm not going into the else block. I verified it with the debugger.

Do you enter the else block on your machine if you choose a not-existing command?

On GNU/Linux (line 12 is test in "if"; second message was generated in "else" body) & perl 5.8.8 ...

Can't exec "dira": No such file or directory at p.pl line 12.
Problem executing command dira - No such file or directory at p.pl lin
+e 18.
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...

Of course my goal would it be that the open command dies the program because the command "dira" does not exist.

Your program did exactly that. You told it to die with $!, which is the German text you see. You can make it die with another text if you do not like the German one.

I tried the following:

#!/usr/bin/perl

use strict;
use warnings;

open(my $CMD, "dira |") || die "another text";
# ...
close($CMD);
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But the program still produces the same error message:

Der Befehl "dira" ist entweder falsch geschrieben oder
konnte nicht gefunden werden.
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Not for me:

#!/usr/bin/perl

use strict;
use warnings;

open(my $CMD, "dira |") || die "something else\n";
while( <$CMD> ) {
    print "$_";
}

close($CMD);
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Output:

Can't exec "dira": No such file or directory at ./test.pl line 6.
something else
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This form of open starts a child process. The return result is the child process ID. Thus, your open(...) || die ... will show the error if the child can't be created. You need to check $? to check for errors from the child. See this for an example of checking for errors in this situation.

#!/usr/bin/perl

use strict;
use warnings;
use SVN::Notify;

my $command = 'xmllint --version';

open(my $CMD, "$command |") or die $!;

my $exe = SVN::Notify->find_exe($command);
while (<$CMD>) {
    print $_;
}
close $CMD;
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#!/usr/bin/perl

use strict;
use warnings;
use SVN::Notify;

my $command = 'dir';

my $exe = SVN::Notify->find_exe($command);
system($exe);
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Thank you. Sounds good. I tried it. But my problem is now that I always get the message "Cannot locate command" for a existing command and for a not-existing command.

Remember that "dir" is not an executable. It's a shell built-in. So try "cmd /c dir" as your command (and thus your executable is "cmd"). Though, really, dir is a bad example because it's so trivial to do inside perl. Other built-ins may be more difficult/impossible to emulate in perl, so the question is still more or less valid as a general question, just not so much for this particular example.

Thanks. Now it works fine after your update. Here a sample code snippet:

#!/usr/bin/perl

use strict;
use warnings;
use SVN::Notify;

my $command = 'perl';

my $exe = SVN::Notify->find_exe($command);
if( defined $exe )
{
    print "Command \"$command\" exists!";
}
else
{
    print "Command \"$command\" does NOT exist!";
}
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Only problem is that it is not able to handle parameters. If I use the command "perl --version" then I receive the answer that this command does not exist. If I use only "perl" then I get the answer that it exists.

Fazit: Thank you a lot. This is a very good method to find out whether a command exists or not. But it is necessary to cut away the parameters of the command before giving it to the SVN::Notify->find_exe function.

FWIW: The only way that you could try to run a non-existant command, is if the text of that command were accepted as input from outside the program. And that is a highly dubious practice without you perform some kind of pre-check that the command supplied as input is something that you are prepared to run on behalf of the user.

I tried File::Which. It is working fine and at the moment I use it to check if a command exists or not.