New to Perl

soupmagnet · 02-18-2013, 09:58 PM

I'm making the move from Bash to Perl and I'm trying to port a Bash script to get a better understanding of it as I go. I'm looking at subroutines right now and I want to get the output of subroutine and I know I can do something like this:

Code:

#!/usr/bin/perl

sub onStatus {
print "ONLINE\n";
}

print onStatus();

But the output is always:

ONLINE
1

What does that mean? I haven't read about anything like that anywhere and I'm wondering if it's something similar to when you have to chomp a variable to get rid of the newline or if it's something completely different.

pan64 · 02-19-2013, 12:10 AM

just think about the code you wrote: there are two print statements. One of them prints the word ONLINE, and the second? You have a function call after print, so perl will print the result of the function call.

bigearsbilly · 02-19-2013, 04:37 AM

better is:

Code:

#!/usr/bin/perl

sub onStatus {
    "ONLINE\n";
}

print onStatus;

or

Code:

#!/usr/bin/perl

sub onStatus {
    print "ONLINE\n";
}

onStatus;

joe_2000 · 02-19-2013, 03:03 PM

Quote:

Originally Posted by soupmagnet

I'm making the move from Bash to Perl and I'm trying to port a Bash script to get a better understanding of it as I go. I'm looking at subroutines right now and I want to get the output of subroutine and I know I can do something like this:

Code:

#!/usr/bin/perl

sub onStatus {
print "ONLINE\n";
}

print onStatus();

But the output is always:

ONLINE
1

What does that mean? I haven't read about anything like that anywhere and I'm wondering if it's something similar to when you have to chomp a variable to get rid of the newline or if it's something completely different.

What's happening in perl subroutines is that the last statement in it also defines the sub routine's return value. So the line

Code:

print "ONLINE\n";

does two things:
1) Print the word ONLINE. (Note that if you intended to print the value of a variable named ONLINE you should write $ONLINE as in bash scripts.
2) Assign the sub's return value to 1.

Then, the line

Code:

print onStatus();

again does two things:
1) Call the sub that actually prints ONLINE (see item 1 above)
2) Print the sub routine's return value, which is 1.

soupmagnet · 02-19-2013, 07:42 PM

Quote:

Originally Posted by joe_2000

What's happening in perl subroutines is that the last statement in it also defines the sub routine's return value. So the line

Code:

print "ONLINE\n";

does two things:
1) Print the word ONLINE. (Note that if you intended to print the value of a variable named ONLINE you should write $ONLINE as in bash scripts.
2) Assign the sub's return value to 1.

Then, the line

Code:

print onStatus();

again does two things:
1) Call the sub that actually prints ONLINE (see item 1 above)
2) Print the sub routine's return value, which is 1.

So is that always the case with subroutines? Can that effect the overall outcome? Obviously my example is quite minimal and based on very little knowledge Perl, but in the end what I would like to do would probably best be described with psudocode of a bash script I'm trying to emulate.

Code:

#!/bin/bash

onStatus(){
(command)
if [ $? -eq 0 ]; then
    echo "ONLINE"
  else 
    echo "OFFLINE"
fi
}

echo "Status: $(onStatus)"

Which I would then use later on in the script with something like this...

Code:

if [ "$(onStatus)" == "ONLINE" ]; then
    (commands)
fi

Will the output of the Perl subroutine that includes the exit status effect how my code will eventually work?

Sergei Steshenko · 02-19-2013, 07:51 PM

Quote:

Originally Posted by soupmagnet

So is that always the case with subroutines? Can that effect the overall outcome? Obviously my example is quite minimal and based on very little knowledge Perl, but in the end what I would like to do would probably best be described with psudocode of a bash script I'm trying to emulate.

Code:

#!/bin/bash

onStatus(){
(command)
if [ $? -eq 0 ]; then
    echo "ONLINE"
  else 
    echo "OFFLINE"
fi
}

echo "Status: $(onStatus)"

I think you are confused between the desired string output value and outputting a string value to an I/O channel.

E.g.

Code:

sub status_to_text
  {
  $? ? 'status is non-zero' : 'status is 0'
  }

my $string_status = status_to_text;
print "\$string_status=$string_status\n";
print 'string status by calling the sub directly: ', status_to_text, "\n";

bigearsbilly · 02-20-2013, 01:58 AM

Quote:

Originally Posted by soupmagnet

I'm making the move from Bash to Perl

Incidentally, they are completely different tools.
Shell scripting is good for certain things, Perl is good for others.

I wouldn't prefer one to the exclusion of t'other.

joe_2000 · 02-20-2013, 03:18 AM

Quote:

Originally Posted by soupmagnet

So is that always the case with subroutines? Can that effect the overall outcome?

Yes.

Quote:

Originally Posted by soupmagnet

Code:

#!/bin/bash

onStatus(){
(command)
if [ $? -eq 0 ]; then
    echo "ONLINE"
  else 
    echo "OFFLINE"
fi
}

echo "Status: $(onStatus)"

Which I would then use later on in the script with something like this...

Code:

if [ "$(onStatus)" == "ONLINE" ]; then
    (commands)
fi

If you want to stay as close as possible to this snippet just assign a variable to the string ONLINE or OFFLINE rather then using the print command. The string should then become the return value.

Code:

#!/usr/bin/perl

sub onStatus {
$tmp = "ONLINE\n";
}

print onStatus();

chrism01 · 02-21-2013, 12:09 AM

Can i just recommend using warnings and strict; they'll save you a lot of grief later.
Also, Perl is compiled on the fly, so you can code like eg C ie you can call a fn BEFORE you've defined it (lexically)

Code:

#!/usr/bin/perl -w
use strict;

print onStatus();

sub onStatus 
{
    my $tmp = "ONLINE\n";
}


#### Alternate version
#!/usr/bin/perl -w
use strict;

my $tmp=onStatus();
print "$tmp\n";

sub onStatus 
{
    return "ONLINE";
}

http://perldoc.perl.org/
http://www.perlmonks.org/?node=Tutorials
http://www.tizag.com/perlT/index.php

bigearsbilly · 02-21-2013, 01:25 AM

Quote:

Originally Posted by chrism01

Also, Perl is compiled on the fly,

That's not strictly true chrism, it's compiled before it runs.

c.f. Tcl is compiled on the fly, which means you can run a script for a year and then suddenly
come across a syntax error.

Suic · 02-21-2013, 04:18 AM

Code:

#!/usr/bin/perl

sub onStatus {
print "ONLINE\n";
return;
}

print onStatus();

Sergei Steshenko · 02-21-2013, 08:24 AM

Quote:

Originally Posted by Suic

Code:

#!/usr/bin/perl

sub onStatus {
print "ONLINE\n";
return;
}

print onStatus();

And the recommendation to use

Code:

use strict;

is still correct - without it you are metaphorically throwing yourself on a mine filed.

pan64 · 02-21-2013, 12:59 PM

not to speak about
#!/usr/bin/perl -w
see http://perldoc.perl.org/warnings.html

chrism01 · 02-26-2013, 07:59 PM

@Billy; yeah, that's what I meant. Didn't know about TCL, so didn't realise it was ambiguous.

bigearsbilly · 03-01-2013, 04:58 AM

@chrism

Ok I will forgive you this one time