[Linux 文章收集] Unix Shell Programming - Special Variables

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Introduction 
Previous tutorial warned about using certain nonalphanumeric characters in your variable names. This is because those characters are used in the names of special Unix variables. These variables are reserved for specific functions. For example, the $ character represents the process ID number, or PID, of the current shell: 

# echo $$
9178
# ps aux | grep 9178 | grep -v grep
root 9178 0.0 0.0 116680 3544 pts/2 Ss 14:59 0:00 -bash

The following table shows a number of special variables that you can use in your shell scripts: 
 

Command-Line Arguments 
The command-line arguments $1, $2, $3,...$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, ...$9 as the arguments to the command. Following script uses various special variables related to command line − 
- test.sh 

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1. #!/bin/sh  
2.   
3. echo "File Name: $0"  
4. echo "First Parameter : $1"  
5. echo "Second Parameter : $2"  
6. echo "Quoted Values: $@"  
7. echo "Quoted Values: $*"  
8. echo "Total Number of Parameters : $#"  

Execution example: 

# ./test.sh a b
File Name: ./test.sh
First Parameter : a
Second Parameter : b
Quoted Values: a b
Quoted Values: a b
Total Number of Parameters : 2

Special Parameters $* and $@ 
There are special parameters that allow accessing all of the command-line arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, "". Both the parameter specifies all command-line arguments but the "$*" special parameter takes the entire list as one argument with spaces between and the "$@" special parameter takes the entire list and separates it into separate arguments. 

We can write the shell script shown below to process an unknown number of command-line arguments with either the $* or $@ special parameters: 
- test2.sh 

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1. #!/bin/sh  
2.   
3. for TOKEN in $*  
4. do  
5.    echo $TOKEN  
6. done  

Execution example: 

# ./test2.sh a b c 1 2 3
a
b
c
1
2
3

Exit Status 
The $? variable represents the exit status of the previous command. Exit status is a numerical value returned by every command upon its completion. As a rule, most commands return an exit status of 0 if they were successful, and 1 if they were unsuccessful. Some commands return additional exit statuses for particular reasons. For example, some commands differentiate between kinds of errors and will return various exit values depending on the specific type of failure. Below are sample shell to demonstrate this topic: 
- test3.sh 

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1. #!/bin/sh  
2. ls -hl > /dev/null  
3. echo -e "\t[Info] Command ls with Success return $?"  
4.   
5. ls -hl ./notesist > /dev/null 2>&1  
6. echo -e "\t[Info] Command ls with not found return $?"  
7.   
8. lsa -hl > /dev/null 2>&1  
9. echo -e "\t[Info] Command not found return $?"  

Execution result: 

# ./test3.sh
[Info] Command ls with Success return 0
[Info] Command ls with not found return 2
[Info] Command not found return 127

Supplement 
* Prev - Using Shell Variables 
* Next - Using Shell Arrays