Shell tutorial 3-shell special Variables

As mentioned above, variable names can only contain numbers, letters, and underscores, because some variables that contain other characters have special meanings. Such variables are called special variables.

For example, $ indicates the ID of the current shell process, that is, PID. See the following code:

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1. $ Echo $

$echo $$

Running result

29949

Special Variable list

Variable	Description
$0	File Name of the current script
$ N	Parameters passed to the script or function. N is a number that represents the number of parameters. For example, the first parameter is $1, and the second parameter is $2.
$ #	The number of parameters passed to the script or function.
$ *	All parameters passed to the script or function.
[Email protected]	All parameters passed to the script or function. The double quotation marks ("") are slightly different from $.
$?	The exit status of the previous command or the return value of the function.
$	The ID of the Current Shell Process. Shell scripts are the process IDs of these scripts.

The parameter passed to the script when the command line parameter is run is called the command line parameter. The command line parameters are represented by $ n. For example, $1 indicates the first parameter, $2 indicates the second parameter, and so on.

See the following script:

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1. #! /Bin/bash
2.  
3. Echo "File Name: $0"
4. Echo "first parameter: $1"
5. Echo "first parameter: $2"
6. Echo "quoted values: [email protected]"
7. Echo "quoted values: $ *"
8. Echo "Total number of parameters: $ #"

#!/bin/bashecho "File Name: $0"echo "First Parameter : $1"echo "First Parameter : $2"echo "Quoted Values: [email protected]"echo "Quoted Values: $*"echo "Total Number of Parameters : $#"

Running result:

$./test.sh Zara AliFile Name : ./test.shFirst Parameter : ZaraSecond Parameter : AliQuoted Values: Zara AliQuoted Values: Zara AliTotal Number of Parameters : 2

$ * And [email protected] $ * and [email protected] indicate all parameters passed to the function or script. If they are not included in double quotation marks, all use "$1" "$2 "... All parameters are output in the form of "$ N.

But when they are enclosed by double quotes (""), "$ *" takes all parameters as a whole, with "$1 $2... All parameters are output in the form of $ n. "[email protected]" separates parameters and uses "$1" "$2 "... All parameters are output in the form of "$ N.

The following example shows the difference between $ * and [email protected:

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1. #! /Bin/bash
2. Echo "\ $ * =" $ *
3. Echo "\" \ $ * \ "=" "$ *"
4.  
5. Echo "\ $ @ =" [email protected]
6. Echo "\" \ $ @ \ "=" "[email protected]"
7.  
8. Echo "print each Param from \ $ *"
9. For VaR in $ *
10. Do
11. Echo "$ Var"
12. Done
13.  
14. Echo "print each Param from \ $ @"
15. For VaR in [email protected]
16. Do
17. Echo "$ Var"
18. Done
19.  
20. Echo "print each Param from \" \ $ *\""
21. For VaR in "$ *"
22. Do
23. Echo "$ Var"
24. Done
25.  
26. Echo "print each Param from \" \$ @\""
27. For VaR in "[email protected]"
28. Do
29. Echo "$ Var"
30. Done

#!/bin/bashecho "\$*=" $*echo "\"\$*\"=" "$*"echo "\[email protected]=" [email protected]echo "\"\[email protected]\"=" "[email protected]"echo "print each param from \$*"for var in $*do    echo "$var"doneecho "print each param from \[email protected]"for var in [email protected]do    echo "$var"doneecho "print each param from \"\$*\""for var in "$*"do    echo "$var"doneecho "print each param from \"\[email protected]\""for var in "[email protected]"do    echo "$var"done

Run./test. Sh "A" "B" "C" "D". The following result is displayed:

$*=  a b c d"$*"= a b c d[email protected]=  a b c d"[email protected]"= a b c dprint each param from $*abcdprint each param from [email protected]abcdprint each param from "$*"a b c dprint each param from "[email protected]"abcd

Exit Status $? You can get the exit status of the previous command. The exit status is the result returned after the previous command is executed.

The exit status is a number. Generally, if most commands are successfully executed, 0 is returned. If the command fails, 1 is returned.

However, some commands return other values, indicating different types of errors.

In the following example, the command is successfully executed:

$./test.sh Zara AliFile Name : ./test.shFirst Parameter : ZaraSecond Parameter : AliQuoted Values: Zara AliQuoted Values: Zara AliTotal Number of Parameters : 2$echo $?0$

$? It can also represent the return value of the function, which will be explained later.

Shell tutorial 3-shell special Variables