-2

So, I came across a PERL script while working on an already implemented project. Since its confidential, I cannot reveal much details but the code is somewhat like this :

if(defined($x)){
    exit(0);
}
#...........
#.some other code.
#...........

if(!defined($y)){
      print "ERROR !";
      exit(1);
}

I am assuming exit(1) is in error conditions and exit(0) is for success cases. But since we are already printing the error message, what is the difference here between exit(0) and exit(1)?

Ashlesha Sunil Agate
  • 43
  • 1
  • 5
  • 2
    This is not limited to Perl. [Exit codes](https://en.wikipedia.org/wiki/Exit_status) are a general concept to have a program signal whether it could do what it was asked or whether something went wrong, so that other programs (usually the calling program) can react to it. – Robert Aug 14 '20 at 20:50
  • @CharlesDuffy, PERL and linux work the same way , is that what it is? – Ashlesha Sunil Agate Aug 14 '20 at 21:07
  • The name of the language is Perl, not PERL; it's not an acronym. – ikegami Aug 14 '20 at 21:16
  • @AshleshaSunilAgate, `exit` in perl is the same as exiting in C or any other language, so yes, normal operating system conventions apply as to the meaning of that exit status and how other programs are going to interpret it. – Charles Duffy Aug 14 '20 at 21:23
  • @AshleshaSunilAgate, ...whereas `exit 0` is how one writes "exit with a status of success" in shell, and `exit(0)` is how one writes it in perl, the meaning is the same both ways. – Charles Duffy Aug 14 '20 at 21:25

1 Answers1

6

The process that spawned the program can check what number you returned. 0 typically means "no error" (and it's the default exit status for Perl scripts). Other values are program specific.

For example, grep exits with

  • 0, when it found a match
  • 1, when it found no matches
  • 2, when an error occurred

so you can do

if grep -q foo file; then
   echo 'match'
else
   echo 'no match (or error)'
fi

You probably should have used die instead of print and exit. die("$msg\n") outside of an eval is roughly equivalent to

print STDERR "$msg\n";
exit( $! || ( $? >> 8 ) || 255 );

So it's simpler, the message is printed to STDERR (not STDOUT) as it should, and it's guaranteed to exit with a non-zero code (and hopefully one that is meaningful when debugging).

ikegami
  • 367,544
  • 15
  • 269
  • 518
  • Can you explain the difference in what is printed or happens internally in my case ? – Ashlesha Sunil Agate Aug 14 '20 at 21:07
  • What do you mean by "your case", and difference between your case and what? – ikegami Aug 14 '20 at 21:15
  • @AshleshaSunilAgate: From the perspective of your code, there is no difference. But something is calling your program (whether that's another program or the user, via a command shell). If whatever is calling your program checks the return value, then it will get 0 in the first case and 1 in the second. It is a Unix tradition that 0 means the program ran successfully and any non-zero return values indicate errors. The meaning of specific non-zero return values is specific to the program in question. – Dave Cross Aug 16 '20 at 18:53