变量（variables）¶

引用变量¶

$var - 表示引用变量var。
${} - 一种消除歧义的机制，也用来引用变量。比如${var}test表示变量var的内容跟上test，而$vartest则表示vartest变量。

变量作用域（scope）¶

局部（local）- 在shell函数中加上local关键字的变量为局部变量。
全局（global）- shell中变量默认为全局可见（当前shell可见）。
环境（environment）

Example：

#!/bin/bash
# Global and local variables inside a function.

func ()
{
local loc_var=23       # Declared as local variable.
echo                   # Uses the 'local' builtin.
echo "\"loc_var\" in function = $loc_var"
global_var=999         # Not declared as local.
                        # Defaults to global. 
echo "\"global_var\" in function = $global_var"
}  

func

# Now, to see if local variable "loc_var" exists outside function.

echo
echo "\"loc_var\" outside function = $loc_var"
                                    # $loc_var outside function = 
                                    # No, $loc_var not visible globally.
echo "\"global_var\" outside function = $global_var"
                                    # $global_var outside function = 999
                                    # $global_var is visible globally.
echo                      

exit 0
#  In contrast to C, a Bash variable declared inside a function
#+ is local *only* if declared as such.

特殊变量¶

参考链接。

Special Variable	Description
`$0`	Gets the name of the current script.
`$#`	Gets the number of arguments passed while executing the bash script.
`$*`	Gives you a string containing every command-line argument.
`$@`	It stores the list of every command-line argument as an array.
`$1-$9`	Stores the first 9 arguments.
`$?`	Gets the status of the last command or the most recently executed process.
`$!`	Shows the process ID of the last background command.
`$$`	Gets the process ID of the current shell.
`$-`	It will print the current set of options in your current shell.
`!$`	the "end" of the previous command.1
`[@]`	与`$`类似，比如`${LIST[]}`将变量`LIST`的内容作为单个参数。2
`[*]`	与`$@`类似，比如`${LIST}[@]`将变量`LIST`的内容作为一个参数列表。2

Example:

LIST=(1 2 3)
for i in "${LIST[@]}"; do
    echo "example.$i" # print three times
done

LIST=(1 2 3)
for i in "${LIST[*]}"; do
    echo "example.$i" # print only once
done

测试环境变量是否存在：参考链接