Special variables in the shell

$ represents the current process when using the Echo $$ is the PID that will output the current shell

1 Echo $$

List of special variables

variables	meaning
$	File name of the current script
$n	Arguments passed to the script or function. N is a number that represents the first few parameters. For example, the first parameter is $ $, and the second argument is $ A.
$#	The number of arguments passed to the script or function.
$*	All parameters passed to the script or function.
[Email protected]	All parameters passed to the script or function. When enclosed by double quotation marks (""), it is slightly different from $*, as will be mentioned below.
$?	The exit state of the last command, or the return value of the function.
$$	The current shell process ID. For Shell scripts, this is the process ID where the scripts are located.

Script File varlist.sh

1#!/bin/Bash2 3 Echo "File Name: $"4 Echo "First parameter: $"5 Echo "Second Parameter: $"6 Echo "Quoted Values: [email protected]"7 Echo "Quoted Values: $*"8 Echo "total number of Parameter: $#"

Execute the varlist.sh Hello World result output as follows:

$ sh./varlist.sh Hello worldfile name:./varlist.shfirst parameter:hellosecond parameter:worldquoted Values:hello worl dquoted Values:hello worldtotal Number of Parameter:2

The difference between $* and [email protected]:

When using "" is going out for them difference, "$*" will all the parameters as a whole, in the form of "$ $ ... $n" Output all parameters, "[email protected]" will separate the various parameters to "$" "$" ... All parameters are output in the form "$n". Look at the following example:

1#!/bin/Bash2 #description: Use variable3 #Author: Blacksonny4 #Copyright (c)5 #Script follows here:6 7 Echo "\$*="$*8 Echo "\"\$*\"=" "$*"9 Ten Echo "\[email protected]="[email protected] One Echo "\ "\[email protected]\" =" "[email protected]" A  - Echo "print each param from \$*" -  forVarinch$* the  Do -     Echo "$var" -  Done -  + Echo "print each param from \[email protected]" -  forVarinch[email protected] +  Do A     Echo "$var" at  Done -  - Echo "print each param from \ "\$*\"" -  forVarinch "$*" -  Do -     Echo "$var" in  Done -  to Echo "print each param from \ "\[email protected]\"" +  forVarinch "[email protected]" -  Do the     Echo "$var" *  Done

Output Result:

1$ sh./varcompare.sh"a" "b" "C" "D"2$*=a b c D3 "$*"=a b c D4[Email protected]=a b c D5 "[email protected]"=a b c D6Print eachparamFrom $*7 a8 b9 CTen D OnePrint eachparamFrom [email protected] A a - b - C the D -Print eachparamFrom"$*" - a b c D -Print eachparamFrom"[email protected]" + a - b + C AD

Exit Status:

$? You can get the exit status of the previous command. The so-called exit status is the return result after the last command was executed.
Exit status is a number, in general, most of the command execution succeeds returns 0, and the failure returns 1.

Special variables in the shell