Shell编程中echo $$,$是什么?

原创 陈宝佳  2018-08-27 11:14  阅读 452 次 评论 0 条
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shell的手册页记录了这些




$$ 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




$$是当前进程的PID

$?是最后执行的命令的返回码。

$#是$ * 

$ *中的参数数量,是传递给当前进程的参数列表

更多特性

$_- 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. 

(完)

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