What is the meaning of the number displayed by echo $$?

$$

The PID of the current process.

More examples of different uses listed below:

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$_

The default parameter for a lot of functions.

$.

Holds the current record or line number of the file handle that was last read. It is read-only and will be reset to 0 when the file handle is closed.

$/

Holds the input record separator. The record separator is usually the newline character. However, if $/ is set to an empty string, two or more newlines in the input file will be treated as one.

$,

The output separator for the print() function. Nor-mally, this variable is an empty string. However, setting $, to a newline might be useful if you need to print each element in the parameter list on a separate line.

$\

Added as an invisible last element to the parameters passed to the print() function. Normally, an empty string, but if you want to add a newline or some other suffix to everything that is printed, you can assign the suffix to $.

$

The default format for printed numbers. Normally, it's set to %.20g, but you can use the format specifiers covered in the section "Example: Printing Revisited" in Chapter 9to specify your own default format.

$%

Holds the current page number for the default file handle. If you use select() to change the default file handle, $% will change to reflect the page number of the newly selected file handle.

$=

Holds the current page length for the default file handle. Changing the default file handle will change $= to reflect the page length of the new file handle.

$-

Holds the number of lines left to print for the default file handle. Changing the default file handle will change $- to reflect the number of lines left to print for the new file handle.

$~

Holds the name of the default line format for the default file handle. Normally, it is equal to the file handle's name.

$^

Holds the name of the default heading format for the default file handle. Normally, it is equal to the file handle's name with _TOP appended to it.

$|

If nonzero, will flush the output buffer after every write() or print() function. Normally, it is set to 0.

$?

Holds the status of the last pipe close, back-quote string, or system() function.

$&

Holds the string that was matched by the last successful pattern match.

$`

Holds the string that preceded whatever was matched by the last successful pattern match.

$'

Holds the string that followed whatever was matched by the last successful pattern match.

$+

Holds the string matched by the last bracket in the last successful pattern match. For example, the statement /Fieldname: (.)|Fldname: (.)/ && ($fName = $+); will find the name of a field even if you don't know which of the two possible spellings will be used.

$*

Changes the interpretation of the ^ and $ pattern anchors. Setting $* to 1 is the same as using the /m option with the regular expression matching and substitution operators. Normally, $* is equal to 0.

$0

Holds the name of the file containing the Perl script being executed.

$

This group of variables ($1, $2, $3, and so on) holds the regular expression pattern memory. Each set of parentheses in a pattern stores the string that match the components surrounded by the parentheses into one of the $ variables.

$[

Holds the base array index. Normally, it's set to 0. Most Perl authors recommend against changing it without a very good reason.

$]

Holds a string that identifies which version of Perl you are using. When used in a numeric context, it will be equal to the version number plus the patch level divided by 1000.

$"

This is the separator used between list elements when an array variable is interpolated into a double-quoted string. Normally, its value is a space character.

$;

Holds the subscript separator for multidimensional array emulation. Its use is beyond the scope of this book.

$!

When used in a numeric context, holds the current value of errno. If used in a string context, will hold the error string associated with errno.

$@

Holds the syntax error message, if any, from the last eval() function call.

$<

This UNIX-based variable holds the read uid of the current process.

$>

This UNIX-based variable holds the effective uid of the current process.

$)

This UNIX-based variable holds the read gid of the current process. If the process belongs to multiple groups, then $) will hold a string consisting of the group names separated by spaces.

$:

Holds a string that consists of the characters that can be used to end a word when word-wrapping is performed by the ^ report formatting character. Normally, the string consists of the space, newline, and dash characters.

$^D

Holds the current value of the debugging flags. For more information.

$^F

Holds the value of the maximum system file description. Normally, it's set to 2. The use of this variable is beyond the scope of this book.

$^I

Holds the file extension used to create a backup file for the in-place editing specified by the -i command line option. For example, it could be equal to ".bak."

$^L

Holds the string used to eject a page for report printing.

$^P

This variable is an internal flag that the debugger clears so it will not debug itself.

$^T

Holds the time, in seconds, at which the script begins running.

$^W

Holds the current value of the -w command line option.

$^X

Holds the full pathname of the Perl interpreter being used to run the current script.

Source:

• http://www.unix.com/302219737-post3.html

It's the process id of the current shell.

Source:

Expands to the decimal process ID of the invoked shell. In a subshell (see Shell Execution Environment ), '$' shall expand to the same value as that of the current shell.