what does echo $$, $? $# mean

gokulagiridaran · #1 07-30-2008

Hi I am not good in scripting, can anyone of you please help me, with these echo $$, $? $# mean, any good guide / doc on these basics would be much more helpfull.

Thanks in advance.

era · #2 07-30-2008

The manual page for your shell documents these.

$$ is the PID of the current process.

$? is the return code of the last executed command.

$# is the number of arguments in $*

$* is the list of arguments passed to the current process

yesmani · #3 07-30-2008

Hi gokulagiridaran,
There are many other that would help you scriptiong, here are few that would really help you.

$_- 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.
$$-- This UNIX-based variable holds the process number of the process running the Perl interpreter.
$?-- 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.
$<number>-- 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 $<number> 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.

gokulagiridaran · #4 07-30-2008

Thanks a lot.!

era · #5 07-30-2008

Note that yesmani's response is the documentation for these variables in Perl. I was assuming you meant the shell. (In particular, I don't think $# alone makes any sense in modern Perl. Looks like the table above was for Perl 4, maybe.)

gokulagiridaran · #6 07-30-2008

Hi era,
thank you, if you have those variables which applies for BASH can you pls fwd me ?

Thank you .
Gokul.

era · #7 07-30-2008

Use Google.

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