perlvar - PetaPerl Predefined Variables

DESCRIPTION

This document describes the special variables supported by PetaPerl. For full Perl 5 documentation, see perldoc perlvar.

SPECIAL VARIABLES

General Variables

`$_` - The Default Variable

The default input and pattern-searching space. Many functions operate on $_ when no argument is given.

for (1, 2, 3) {
    print;      # prints $_ implicitly
}

$_ = "hello";
print length;   # prints 5

Implementation: src/runtime/pp/globals.rs:95

`@_` - Subroutine Arguments

Within a subroutine, @_ contains the arguments passed to that subroutine. Modifying elements of @_ modifies the caller’s variables (aliasing).

sub double {
    $_[0] *= 2;    # modifies caller's variable
}

my $x = 5;
double($x);
print $x;          # prints 10

Implementation: src/runtime/pp/sub.rs

Global Special Variables

`$0` - Program Name

Contains the name of the program being executed.

print "Running: $0\n";

`$$` - Process ID

The process ID of the Perl process running this script.

print "PID: $$\n";

`$?` - Child Error Status

The status returned by the last pipe close, backtick command, or system() call.

system("false");
print "Exit status: ", $? >> 8, "\n";

`$@` - Eval Error

The error message from the last eval that failed.

eval { die "oops" };
print "Error: $@" if $@;

`$!` - OS Error

When used in numeric context, yields the current value of errno. In string context, yields the corresponding error string.

open(my $fh, '<', '/nonexistent') or print "Error: $!\n";

I/O Variables

`$/` - Input Record Separator

The input record separator, newline by default. Setting to undef enables slurp mode.

{
    local $/;           # slurp mode
    my $content = <$fh>;
}

{
    local $/ = "";      # paragraph mode
    while (<$fh>) { ... }
}

`$\` - Output Record Separator

Appended to the end of every print. Default is undef (nothing appended).

{
    local $\ = "\n";
    print "line1";      # outputs "line1\n"
    print "line2";      # outputs "line2\n"
}

`$,` - Output Field Separator

Printed between arguments to print. Default is undef.

{
    local $, = ", ";
    print "a", "b", "c";   # outputs "a, b, c"
}

`$"` - List Separator

Used when arrays are interpolated into strings. Default is a space.

my @arr = (1, 2, 3);
print "@arr\n";            # outputs "1 2 3"

{
    local $" = ",";
    print "@arr\n";        # outputs "1,2,3"
}

`$|` - Output Autoflush

If set to nonzero, forces a flush after every write on the currently selected output channel. Default is 0.

$| = 1;    # enable autoflush on STDOUT

`$.` - Current Line Number

The current line number of the last filehandle read.

while (<$fh>) {
    print "$.: $_";    # prints line number and content
}

`$;` - Subscript Separator

The subscript separator for multidimensional hash emulation. Default is "\034" (SUBSEP).

$hash{$x, $y} = $value;    # equivalent to $hash{"$x\034$y"}

`$^T` - Basetime

The time at which the program began running, in seconds since the epoch.

print "Started at: ", scalar localtime($^T), "\n";

`$^W` - Warning Flag

The current value of the warning switch. Set by -w.

$^W = 1;    # enable warnings

Regular Expression Variables

`$&` - Match String

The string matched by the last successful pattern match.

"hello world" =~ /wo\w+/;
print $&;      # prints "world"

$` - Prematch String

The string preceding what was matched by the last successful pattern match.

"hello world" =~ /wo\w+/;
print $`;      # prints "hello "

`$'` - Postmatch String

The string following what was matched by the last successful pattern match.

"hello world!" =~ /wo\w+/;
print $';      # prints "!"

`$1`, `$2`, … - Capture Groups

Contain the text matched by capture groups in the last pattern match.

"hello world" =~ /(\w+) (\w+)/;
print "$1, $2\n";    # prints "hello, world"

`%+` - Named Captures

Hash containing the text matched by named capture groups.

"hello world" =~ /(?<first>\w+) (?<second>\w+)/;
print "$+{first}, $+{second}\n";    # prints "hello, world"

Process Variables

`$<` - Real UID

The real user ID of this process.

print "Real UID: $<\n";

`$>` - Effective UID

The effective user ID of this process.

print "Effective UID: $>\n";

`$(` - Real GID

The real group ID of this process.

print "Real GID: $(\n";

`$)` - Effective GID

The effective group ID of this process, followed by supplementary group IDs.

print "Effective GID: $)\n";

System Variables

`%ENV` - Environment Variables

Hash containing the current environment variables.

print $ENV{HOME};
$ENV{MY_VAR} = "value";

`@ARGV` - Command Line Arguments

Contains the command-line arguments passed to the script.

# script.pl arg1 arg2
print "Args: @ARGV\n";    # prints "Args: arg1 arg2"

`@INC` - Include Path

List of directories to search for use and require statements.

push @INC, '/my/lib/path';

`%SIG` - Signal Handlers

Hash used to set signal handlers.

$SIG{INT} = sub { die "Caught SIGINT" };

Version Variables

`$]` - Perl Version (Numeric)

The version number of the Perl interpreter, as a decimal.

print $];    # e.g., 5.042000

`$^V` - Perl Version (String)

The version of Perl as a version string.

print $^V;   # e.g., v5.42.0

`$^O` - Operating System

The name of the operating system.

print $^O;   # "linux" for PetaPerl

`$^X` - Perl Executable Path

The path to the Perl (or PetaPerl) executable.

print $^X;   # e.g., /usr/local/bin/pperl

PetaPerl-Specific Variables

Reserved for future PetaPerl-specific extensions.

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