freebsd/share/man/man7/style.perl.7

.\" Copyright (c) 2000 Josef Karthauser <joe@FreeBSD.org>
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.Dd October 16, 2000
.Dt STYLE.PERL 7
.Os FreeBSD
.Sh NAME
.Nm style.perl 7
.Nd "FreeBSD Perl source file style guide"
.Sh DESCRIPTION
This file specifies the preferred style for perl scripts in the
.Tn FreeBSD
source tree.
.Bd -literal -offset 0i
 #
 # Style guide for Perl.  Based on the kernel style guide.
 #

 #
 # VERY important single-line comments look like this.
 #

 # Most single-line comments look like this.

 # Multi-line comments look like this.  Make them real sentences.
 # Fill them so they look like real paragraphs.
.Ed
.Pp
All scripts should follow the copyright block at the start of the
script with a comment block that describes what the script does.
.Bd -literal -offset 0i
 #!/usr/bin/perl -w

 # COPYRIGHT
 # BLOCK

 # This script processes an old kernel config file, which it gets on
 # stdin, and outputs a new style hints file to stdout.
.Ed
All scripts should use the
.Fa strict
module and run without warnings. For example:
.Bd -literal -offset 0i
 #!/usr/bin/perl -w

 # Copyright, description of what the script does, etc

 use strict;
 ...
.Ed
.Pp
Where possible run the script with taint mode switched on.  This
is documented in
.Xr perlsec 1 .
.Bd -literal -offset 0i
 #!/usr/bin/perl -wT
.Ed
.Pp
The main program should be placed in a block labeled MAIN:.  This
makes it easier to identify the entry point in a large perl script,
and provides a scope for variables which are used in the main
program but nowhere else.
.Bd -literal -offset 0i
 MAIN:{
	print(foo("/usr/bin/man", "7", "style.perl"));
	exit(0);
 }
.Ed
.Pp
All subroutines should be defined using argument prototypes as defined in
.Xr perlsub 1 .
.Bd -literal -offset 0i
 sub foo($@) {
	my $cmd = shift;
	my @args = @_;
 }
.Ed
.Pp
All variables should be defined before use; this is enforced if operating
under
.Fa use strict .
.Pp
Scope local variables should be defined using
.Fa my
.Va $variable
and not
.Fa local
.Va $variable .
The
.Fa local
declaration should only be used when it is required, and not by
default.  Lots of perl4 scripts use
.Fa local
because the
.Fa my
definition didn't exist prior to perl5.
.Pp
In most cases globals should be defined at the top of the code
using a
.Fa vars
definition block:
.Bd -literal -offset 0i
 use vars qw($globalscalar @globalarray %globalhash);
.Ed
.Pp
In some cases it may be appropriate to use
.Fa my
statements at the top of the script as an alternative to using
.Fa vars
declarations.
.Pp
All variables should be commented.
.Bd -literal -offset 0i
 sub foo($@) {
	my $cmd = shift;	# Command to run
	my @args = @_;		# Arguments to $cmd
 }
.Ed
.Pp
Local variables should be separated from function arguments by a
blank line:
.Bd -literal -offset 0i
 sub foo($@) {
	my $cmd = shift;	# Command to run
	my @args = @_;		# Arguments to command

	my $pid;		# Child PID
	local *PIPE;		# Pipe
	my $output;		# Output from command
 }
.Ed
.Pp
Whenever possible code should be run through the code checker
.Nm perl
.Ar -wc
.Ar script.pl
or
.Nm perl
.Ar -wcT
.Ar script.pl
and produce no warnings.
.Pp
Indentation is an 8 character tab.  Second level indents are four spaces.
.Bd -literal -offset 0i
 while (cnt < 20) {
	z = a + really + long + statement + that + needs +
	    two lines + gets + indented + four + spaces +
	    on + the + second + and + subsequent + lines.
 }
.Ed
.Pp
Do not add whitespace at the end of a line, and only use tabs
followed by spaces to form the indentation.  Do not use more spaces
than a tab will produce and do not use spaces in front of tabs.
.Pp
Opening braces should be at the end of the controlling line.  Else
and elsif belong on the same line as the closing brace for the
previous if or elsif block:
.Bd -literal -offset 0i
 sub foo($@) {
	my $cmd = shift;            # Command to run
	my @args = @_;              # Arguments to command

	my $pid;                    # Child PID
	local *PIPE;                # Pipe
	my $output;                 # Output from command

	unless (defined($pid = open(PIPE, "-|"))) {
		die("open(): $!\\n");
	} elsif ($pid == 0) {
		exec($cmd, @args);
		die("exec(): $!\\n");
	}
	$output = "";
	while (<PIPE>) {
		$output .= $_;
	}
	waitpid($pid, 0);
	if ($? & 0xff) {
		die("$cmd caught a signal " . ($? & 0x7f) . "\\n");
	} elsif ($?) {
		die("$cmd returned exit code " . ($? >> 8) . "\\n");
	}
	return $output;
 }
.Ed
.Pp
Where possible scripts should use standard modules instead of
rewriting the code inline.  It may be appropriate in some cases to
import a CPAN module into the base system to facilitate this.
.Pp
Use
.Fa chomp
instead of
.Fa chop
where appropriate.
.Pp
Use
.Fa unless
instead of
.Fa if (! ... )
where it improves readability.
.Pp
Where it doesn't conflict with this guide read
.Xr perlstyle 1
and adopt Larry Wall's style recommendations.

.Sh SEE ALSO
.Xr perlsec 1 ,
.Xr perlstyle 1 ,
.Xr style 9
.Sh HISTORY
This man page is largely based on the
.Xr style 9
man-page in
.Tn FreeBSD .