Perl Special Variables

The Perl language defines some special variables, usually prefixed with $, @, or %, for example: $_.

Many special variables have a long English name; the OS error variable $! can be written as $OS_ERROR.

If you want to use special variables with English names, you need to add the following at the top of the program:use English;This allows you to use descriptive English special variable names.

The most commonly used special variable is $_, which contains the default input and pattern matching content. Example:

Example

#!/usr/bin/perl foreach ('Google','Example','Taobao') { print $_; print "\n"; }

Running the above program produces the output:

Google
Example
Taobao

In the following example, we do not use $_ to output content:

Example

#!/usr/bin/perl foreach ('Google','Example','Taobao') { print; print "\n"; }

Running the above program produces the output:

Google
Example
Taobao

In the example, it first outputs "Google", then outputs "Example", and finally outputs "Taobao".

In an iteration loop, the current loop's string is placed in $_, then output via print. Additionally, print uses $_ by default when no output variable is specified.

The following are places where Perl assumes $_ is used even if it is not explicitly written:

Various unary functions, including functions like ord() and int(), as well as all file test operators ("-f", "-d") except "-t"; "-t" operates on STDIN by default.
Various list functions, such as print() and unlink().
Pattern matching operations "m//", "s///", and "tr///" when the "=~" operator is not used.
The default iteration variable for a "foreach" loop when no other variable is given.
The implicit iteration variable for the grep() and map() functions.
When "while" has only a single condition and that condition tests the result of a "<>" operation, $_ is the default location for storing the input record. This only happens in a "while" test condition. (Mnemonic: the underscore can be omitted in certain operations.)

Special Variable Types

According to the usage characteristics of special variables, they can be divided into the following categories:

Global scalar special variables.
Global array special variables.
Global hash special variables.
Global special file handles.
Global special constants.
Regular expression special variables.
File handle special variables.

Global Scalar Special Variables

The following lists all scalar special variables, including both special character and English forms:

$_	Default input and pattern matching content.
$ARG
$.	The current line number of the last file handle read.
$NR
$/	Input record separator, defaults to the newline character. If this variable is set to undef, it will read until the end of the file.
$RS
$,	Output field separator.
$OFS
$\	Output record separator.
$ORS
$"	This variable is similar to $,, but applies when interpolating array and slice values into double-quoted strings (or similar interpolated strings). Defaults to a space.
$LIST_SEPARATOR
$;	The separator used when emulating multidimensional arrays. Defaults to "\034".
$SUBSCRIPT_SEPARATOR
$^L	The form feed character sent to the output channel. Defaults to "\f".
$FORMAT_FORMFEED
$:	The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format. Default is "\n"".
$FORMAT_LINE_BREAK_CHARACTERS
$^A	The variable used to hold formatted data before printing.
$ACCUMULATOR
$#	The default number output format when printing numbers (deprecated).
$OFMT
$?	Returns the status of the last external command.
$CHILD_ERROR
$!	The numeric value of this variable is the value of errno, and the string value is the corresponding system error string.
$OS_ERROR or $ERRNO
$@	The error message from the eval command. If empty, it means the last eval command executed successfully.
$EVAL_ERROR
$$	The process number of the currently running Perl script.
$PROCESS_ID or $PID
$<	The real user number of the current process.
$REAL_USER_ID or $UID
$>	The effective user number of the current process.
$EFFECTIVE_USER_ID or $EUID
$(	The real group user number of the current process.
$REAL_GROUP_ID or $GID
$)	The effective group user number of the current process.
$EFFECTIVE_GROUP_ID or $EGID
$0	Contains the filename of the script being executed.
$PROGRAM_NAME
$[	The subscript of the first element of an array, defaults to 0.

$]	The version number of Perl.
$PERL_VERSION
$^D	The value of the debug flag.
$DEBUGGING
$^E	Operating system extended error information in non-UNIX environments.
$EXTENDED_OS_ERROR
$^F	The maximum file descriptor number.
$SYSTEM_FD_MAX
$^H	The syntax check status activated by the compiler.

$^I	The value of the built-in controlling editor.
$INPLACE_EDIT
$^M	The size of the alternate memory pool.

$^O	The operating system name.
$OSNAME
$^P	An internal variable specifying the current debug value.
$PERLDB
$^T	The time the script started running in seconds counted from the new century.
$BASETIME
$^W	The current value of the warning switch.
$WARNING
$^X	The name of the Perl binary executable.
$EXECUTABLE_NAME
$ARGV	The current filename when reading from the default file handle.

Global Array Special Variables

@ARGV	The list of command-line arguments passed to the script.
@INC	The list of directories to search when importing modules.
@F	The array input of the command line.

Global Hash Special Variables

%INC	The hash %INC contains all files included with do or require statements. The key is the filename, and the value is the path to the file.
%ENV	Contains the current environment variables.
%SIG	The signal list and how they are handled.

Global Special File Handles

ARGV	A special file handle that iterates over all filenames in the array variable @ARGV.
STDERR	Standard error output handle.
STDIN	Standard input handle.
STDOUT	Standard output handle.
DATA	The special file handle references any content after the __END__ marker in the file, including the script content. Or it references all content after the __DATA__ marker in a file; __DATA__ exists as long as you read data in the same package.
_ (underscore)	A special file handle used to cache file information (fstat, stat, and lstat).

Global Special Constants

__END__	The logical end of the script; the following text is ignored.
__FILE__	The current filename.
__LINE__	The current line number.
__PACKAGE__	The current package name; the default package name is main.

Regular Expression Special Variables

$n	Contains the nth substring of the last pattern match.
$&	The string of the previous successful pattern match.
$MATCH	The string of the previous successful pattern match.
$`	The content before the substring of the previous successful match.
$PREMATCH
$'	The content after the substring of the previous successful match.
$POSTMATCH
$+	The last bracket matching the format of the previous regular expression search. For example: /Version: (.)\|Revision: (.)/ && ($rev = $+);
$LAST_PAREN_MATCH

File Handle Special Variables

$\|	If set to zero, the function fflush is automatically called after each call to write or print, flushing the written content back to the file.
$OUTPUT_AUTOFLUSH
$%	The current output page number.
$FORMAT_PAGE_NUMBER
$=	The current page length. Defaults to 60.
$FORMAT_LINES_PER_PAGE
$-	The number of lines remaining on the current page.
$FORMAT_LINES_LEFT
$~	The name of the current report output format. The default value is the file handle name.
$FORMAT_NAME
$^	The name of the current report header format. The default value is the file handle name with the suffix "_TOP".
$FORMAT_TOP_NAME

Other Extensions