Shell positional parameter variables and predefined variables

Position parameter variable

Position parameter variable	effect
$n	n Is the number ,$0 For the order itself ,$1-$9generationsurfaceThe\; firstOneindividualToThe\; firstNineindividualginsengCount,TenWithOnOfginsengCountNeed\; to\; bewantuseBigenclosedNumberpackagecontain,Such\; as${10}
$*	This variable codes all the arguments in the command line ,$* Treat all parameters as a whole
[email protected]	This variable also represents all the parameters on the command line , however [email protected] Treat each parameter differently
$#	This variable codes the number of all parameters in the command line

$\ast and$@ The difference between

#$* All parameters in the are considered as a whole , So this for Loop only once 
#!/bin/sh

for i in "$*"
   do 
     echo "The parma is : $i"
   done

Running results ：

The parma is : 1 2 3 4 5 6

#[email protected] Each parameter in is treated as independent , therefore ,[email protected] There are several parameters in , It's going to cycle a couple of times 
#!/bin/sh

x=1
for x in "[email protected]"
   do
       echo "The param $x: is $x"
       x=$(($x+1))
   done

Running results ：

The param 1: is 1
The param 2: is 2
The param 3: is 3
The param 4: is 4
The param 5: is 5
The param 6: is 6

Predefined variables

Predefined variables	effect
$?	Return status of last executed command , If the value of this variable is 0, Prove that the last command was executed correctly , If the value of this variable is not 0, Prove that the last command was not executed correctly
$$	The process number of the current process （PID）
$!	The process number of the last process running in the background （PID）