Perl special variables

Author：Eve Cole Update Time：2024-12-26 10:48:01

Some special variables are defined in the Perl language, usually prefixed with $, @, or %, for example: $_.

Many special variables have a long English name. The operating system variable $! can be written as $OS_ERROR.

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

The most commonly used special variable is $_, which contains default input and pattern matching content. Examples are as follows:

Example

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

Executing the above program, the output result is:

 GoogleCoderctoTaobao

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

Example

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

Executing the above program, the output result is:

 GoogleCoderctoTaobao

In the example, "Google" is output first, then "Codercto" is output, and finally "Taobao" is output.

In an iterative loop, the string of the current loop will be placed in $_, and then output through print. In addition, print does not specify an output variable, and $_ is used by default.

Here are a few places where Perl will assume $_ even if it's not specified:

Various unary functions, including functions like ord() and int() and all file test operations ("-f", "-d") except "-t", which operates on STDIN by default.

Various list functions such as print() and unlink().

Pattern matching operations "m//", "s///" and "tr///" without the "=~" operator.

The default iteration variable for "foreach" loops when no other variables are given.

Implicit iteration variables for grep() and map() functions.

When "while" has only one condition, and that condition tests the result of the "" operation, $_ is the default location for the input record. This does not happen except in "while" test conditions. (Mnemonic: The underscore can be omitted in certain operations.)

Special variable types

According to the nature of use of special variables, they can be divided into the following categories:

Global scalar special variable.

Global array special variables.

Global hash special variable.

Global special file handle.

Global special constants.

Regular expression special variables.

File handle special variable.

global scalar special variables

The following lists all scalar special variables, including variables with special characters and English forms:

$_	Default input and pattern matching content.
$ARG
$.	The current line number of the file handle last read
$NR
$/	Enter the record separator, which defaults to a new line character. If you use the undef variable, the end of the file will be read.
$RS
$,	Output field separator
$OFS
$	Output record separator
$ORS
$"	This variable is similar to $, but should be used to interpolate array and slice values into double-quoted strings (or similar interpolated strings). Defaults to one space.
$LIST_SEPARATOR
$;	Delimiter used when simulating multidimensional arrays. The default is "34".
$SUBSCRIPT_SEPARATOR
$^L	Paper feed sent to the output channel. Default is "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	Variable to hold formatted data before printing
$ACCUMULATOR
$#	Default number output format when printing numbers (deprecated).
$OFMT
$?	Returns the status of the previous external command
$CHILD_ERROR
$!	The numerical 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 of the command eval. If it is empty, it means that the last eval command was executed successfully.
$EVAL_ERROR
$$	The process number running the current Perl script
$PROCESS_ID or $PID
$<	The actual user number of the current process
$REAL_USER_ID or $UID
$>	The effective user ID of the current process
$EFFECTIVE_USER_ID or $EUID
$(	The actual 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 file name of the script being executed
$PROGRAM_NAME
$[	The subscript of the first element of the array, the default is 0.

$]	Perl version number
$PERL_VERSION
$^D	debug flag value
$DEBUGGING
$^E	Operating system extended error messages in non-UNIX environments
$EXTENDED_OS_ERROR
$^F	Maximum file bundle descriptor value
$SYSTEM_FD_MAX
$^H	Syntax checking status activated by the compiler

$^I	Built-in control editor values
$INPLACE_EDIT
$^M	Spare memory pool size

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

Global array special variables

@ARGV	List of command line arguments passed to the script
@INC	List of directories to search when importing a module
@F	Array input from command line

Global hash special variables

%INC	The hash table %INC contains all files included with do or require statements. The key is the file name and the value is the path to the file.
%ENV	Contains current environment variables
%SIG	List of signals and how to handle them

global special file handle

ARGV	Special file handles for iterating through all file names in the array variable @ARGV
STDERR	Standard error output handle
STDIN	standard input handle
STDOUT	stdout handle
DATA	The special file handle refers to anything in the file after the __END__ flag, including script content. Or reference everything after the __DATA__ flag in an include file. As long as you read data in the same package, __DATA__ exists.
_ (underscore)	Special file handles are used to cache file information (fstat, stat, and lstat).

Global special constants

__END__	The logic of the script ends, and subsequent text is ignored.
__FILE__	current file name
__LINE__	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 last successfully matched substring
$PREMATCH	The content before the last successfully matched substring
$'	The content after the previous successfully matched substring
$POSTMATCH
$+	The last bracket matching the previous regular expression search pattern. For example: /Version: (.)\|Revision: (.)/ && ($rev = $+);
$LAST_PAREN_MATCH

File handle special variables

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