shell变量及操作符

linux中shell变量$#,$@,$0,$1,$2的含义解释，因为有些不常用所以经常忘记
$$
Shell本身的PID（ProcessID）
$!
Shell最后运行的后台Process的PID
$?
最后运行的命令的结束代码（返回值）
$-
使用Set命令设定的Flag一览
$*
所有参数列表。如"$*"用「"」括起来的情况、以"$1 $2 … $n"的形式输出所有参数。
$@
所有参数列表。如"$@"用「"」括起来的情况、以"$1" "$2" … "$n" 的形式输出所有参数。
$#
添加到Shell的参数个数
$0
Shell本身的文件名
$1～$n
添加到Shell的各参数值。$1是第1参数、$2是第2参数…。
我们先写一个简单的脚本，执行以后再解释各个变量的意义

Character	Where	Meaning
ESC	csh	Filename completion.
RETURN	csh, sh	Execute command.
space	csh, sh	Argument separator.
TAB	csh, sh	Argument separator.
TAB	bash	Filename completion.
#	csh, sh	Start a comment.
`	csh, sh	Command substitution (backquotes).
"	csh, sh	Weak quotes.
'	csh, sh	Strong quotes.
\	csh, sh	Single?character quote.
$var	csh, sh	Variable.
${var}	csh, sh	Same as $var.
$var:mod	csh	Edit var with modifier mod
${var?default}	sh	If var not set, use default.
${var=default}	sh	If var not set, set it to default and use that value.
${var+instead}	sh	If var set, use instead. Otherwise, null string.
${var?message}	sh	If var not set, print message (else default). If var set, use its value.
${var#pat}	ksh, bash	Value of var with smallest pat deleted from start.
${var##pat}	ksh, bash	Value of var with largest pat deleted from start.
${var%pat}	ksh, bash	Value of var with smallest pat deleted from end.
${var%%pat}	ksh, bash	Value of var with largest pat deleted from end.
|	csh, sh	Pipe standard output.
|&	csh	Pipe standard output and standard error.
^	sh	Pipe character (obsolete).
^	csh, bash	Edit previous command line.
&	csh, sh	Run program in background.
?	csh, sh	Match one character.
*	csh, sh	Match zero or more characters.
;	csh, sh	Command separator.
;;	sh	[/td]End of case statement.[/tr]
~	csh, ksh, bash	Home directory.
~user	csh, ksh, bash	Home directory of user.
!	csh, bash	Command history.
-	Programs	Start of optional argument.
-	Programs	Read standard input. (Only certain programs.)
$#	csh, sh	Number of arguments to script.
"$@"	sh	Original arguments to script.
$*	csh, sh	Arguments to script.
$-	sh	Flags set in shell.
$?	sh	Status of previous command.
$$	csh, sh	Process identification number.
$!	sh	Process identification number of last background job.
$<	csh	Read input from terminal.
cmd1 && cmd2	csh, sh	Execute cmd2 if cmd1 succeeds.
cmd1 || cmd2	csh, sh	Execute cmd2 if cmd1 succeeds.
$(..)	ksh, bash	Command substitution.
((..))	ksh, bash	Arithmetic evaluation.
\. file	sh	Execute commands from file in this shell.
:	sh	Evaluate arguments, return true.
:	sh	Separate values in paths.
:	csh	Variable modifier.
[]	csh, sh	Match range of characters.
[]	sh	Test.
%job	csh, ksh, bash	Identify job number.
(cmd;cmd)	csh, sh	Run cmd;cmd in a subshell.
{}	csh, bash	In?line expansions.
{cmd;cmd;}	sh	Like (cmd;cmd) without a subshell.
>file	csh, sh	Redirect standard output.
>>file	csh, sh	Append standard output.
	csh, sh	Redirect standard input.
<	csh, sh	Read until word, do command and variable substitution.
<<\word	csh, sh	Read until word, no substitution.
	sh	Read until word, ignoring leading TABs.
>>! file	csh, sh	Append to file, even if noclobber set and file doesn't exist.
>! file	csh	Output to file, even if noclobber set and file exists.
>| file	ksh, bash	Output to file, even if noclobber set and file exists.
>& file	csh	Redirect standard output and standard error to file.
m> file	sh	Redirect output file descriptor m to file.
m>> file	sh	Append output file descriptor m to file.
m< file	sh	Redirect input file descriptor m from file.
<&m	sh	Take standard input from file descriptor m.
<&?	sh	Close standard input.
>&m	sh	Use file descriptor m as standard output.
>&-	sh	Close standard output.
m<&n	sh	Connect input file descriptor n to file descriptor m.
m<&-	sh	Close input file descriptor m.
n>&m	sh	Connect output file descriptor n to file descriptor m.
m>&-	sh	Close output file descriptor m.