Ruby | Pre-define Variables & Constants

Ruby Predefine Variables

Ruby contains a wide range of predefined variables. Every predefined variable has its own specification. You can use predefine variables to perform a specific task like when dealing with interpreter parameters or regular expressions. The list of predefined variables in Ruby as shown below:

Variables	Description
$!	It holds the exception information message set by the last ‘raise’. Alias of $ERROR_INFO.
$@	It holds an array of the backtrace of the last exception raised. Alias of $ERROR_POSITION.
$/	The input record separator, by default newline. If it is set to nil then the whole file will be read at once. Alias of $INPUT_RECORD_SEPARATOR.
$\	The output separator for the print and IO#write, nil by default. Alias of $OUTPUT_RECORD_SEPARATOR
$,	The output field separator for the print and default separator for Array#join. Alias of $OUTPUT_FIELD_SEPARATOR.
$;	It is the default separator for String#split. Alias of $FIELD_SEPARATOR.
$.	It holds the current input line number read from the last file. Alias of $INPUT_LINE_NUMBER.
$<	An object that gives access to the concatenation of the content of all the files given as a command line argument or $stdin. Alias of $DEFAULT_INPUT.
$>	It is the destination of output for kernel.print and kernel.printf, the default value is $stdout. Alias of $DEFAULT_OUTPUT.
$&	The string matched by the last pattern match. Alias of $MATCH.
$`	The string to the left of the last pattern match. Alias of $PREMATCH.
$’	The string to the right of the last pattern match. Alias of $POSTMATCH.
$+	The string correlated to the last matched group in the last successful pattern matched. Alias of $LAST_PAREN_MATCH.
$1-$9	The string matched in the nth group of the last successful pattern matched.
$_	The last input line read by get or readline in the current scope. It is a local variable. Alias of $LAST_READ_LINE.
$~	It holds the information about the last match in the current scope. It is a local variables. Alias of $LAST_MATCH_INFO.
$-p	It is true if option -p is set (loop mode is on). It is read-only variable.
$-l	It is true if option -l is set (line-ending process is on). It is read-only variable.
$-i	This variable hold the extension if in-place-edit mode is set otherwise nil.
$-a	It is true if option -a is set (autosplit mode is one). It is read-only variable.
$-d	The level of -d is switch. Alias of $DEBUG.
$-v	The verbose flag. It is set by the -v switch. Alias of $VERBOSE.
$-K	The character encoding of the source code. Alias of $KCODE.
$0	It contains the name of the script being executed.
$$	The process number of the current Ruby program being executed. Alias of $PROCESS_ID.
$?	The status of the last child process terminated. Alias of $CHILD_STATUS.
$:	Load paths for programs and binary module by load or required. Alias of $LOAD_PATH.
$FILENAME	The name of current input file reads from $<. Same as $<.filename.
$stderr	Current standard error output.
$stdin	Current standard input.
$stdout	Current standard output.
$=	Flag for case-sensitive, nil by default.Alias of $IGNORECASE
$*	Command line argument given for the program, also known as ARGV.Alias of ARGV.
$”	Array contains the module name loaded by require.Alias of $LOAD_FEATURES.

Example:

# Ruby program to illustrate the
# use of pre-defined Variables

# Using '$0' To know about the
# script name
puts "Script_name: ",$0;

# Using ' #{$$}' to know about the total
# number of process in the script
puts "Total number of process in this script: #{$$}"

# Using $; as default separator
# for String#split.
a = "1,2,3,4,5,6,7"
$; = ","
p a.split

# Pattern matching
"Welcome to GeeksforGeeks Portal!" =~ /Geeks/

# use to print the string to the
# left of the last pattern match
p $`

# use to print the string matched
# by the last pattern match
p $&

# use to print the string to the
# right of the last pattern match
p $'

Output:

Script_name: 
/var/www/service/usercode/969513068/source.rb
Total number of process in this script: 3950
["1", "2", "3", "4", "5", "6", "7"]
"Welcome to "
"Geeks"
"forGeeks Portal!"

Ruby Predefined Constants

Ruby contains many predefined constants. The list of predefined constants are shown below:

Constant Name	Description
TRUE	Equivalent to true.
FALSE	Equivalent to false.
NIL	Equivalent to nil.
STDIN	Standard input and default value of $stdin.
STDOUT	Standard output and default value of $stdout.
STDERR	Standard output error and default value of $stderr.
RUBY_VERSION	A string shows the version of Ruby interpreter.
RUBY_PLATFORM	A string shows the platform of Ruby interpreter.
RUBY_RELEASE_DATE	A string shows the release date of Ruby interpreter.
DATA	The file object of the program, pointing just after the __END__. And not defined if __END__ is not present in the program.
ARGV	An array holds the command-line arguments passed to the program. Alias of $*.
ARGF	An object that gives the access to the virtual concatenation of files passed as command-line arguments. Alias of $<.
ENV	It is a hash-like object, contains current environment variables.

Note: It is recommended to use true, false, and nil because TRUE, FALSE, and NIL are backward-compatible.

Example:

# Ruby program to illustrate
# pre-defined Constants

# To know about Ruby Version
a = RUBY_VERSION
puts "Current Version: #{a}"

# To Know about Ruby Platform
b = RUBY_PLATFORM
puts "Platform of Ruby: #{b}"

# To know about Ruby Release Date
c = RUBY_RELEASE_DATE
puts "Release date of Ruby: #{c}"

Output:

Current Version: 2.3.1
Platform of Ruby: x86_64-linux-gnu
Release date of Ruby: 2016-04-26