shell 變量

## 引言

shell這門語言，作為與linux互動效率最高的工具，我相信每個code monkey在工作中或多或少都會用到；我今天要講的是這門語言中最基本的部分——變量。shell中的變量與類c語言差異較大，相信大家看完後都會有所收獲。

在shell中，我們可以使用<code>foo=bar</code>這樣的方式聲明變量（注意，這裡不能有空格），當使用這種方式聲明變量時，變量是沒有類型的，或者說變量的類型可以根據上下文自己轉換。比如：

我們可以使用shell内置的<code>declare</code>聲明變量：

option

meaning

-a

variable is an array.

-f

use function names only.

-i

the variable is to be treated as an integer; arithmetic evaluation is performed when the variable is assigned a value (see section 3.4.6).

-p

display the attributes and values of each variable. when -p is used, additional options are ignored.

-r

make variables read-only. these variables cannot then be assigned values by subsequent assignment statements, nor can they be unset.

-t

give each variable the trace attribute.

-x

mark each variable for export to subsequent commands via the environment.

shell支援三種資料類型：字元串、整型、數組。

字元串類型是shell聲明一個變量時預設的類型，當我們執行

時，<code>java_home</code>這個變量的類型是string，

文法

說明

${parameter:-defaultvalue}

get default shell variables value

${parameter:=defaultvalue}

set default shell variables value

${parameter:?”error message”}

display an error message if parameter is not set

${#var}

find the length of the string

${var%pattern}

remove from shortest rear (end) pattern

${var%%pattern}

remove from longest rear (end) pattern

${var:num1:num2}

substring

${var#pattern}

remove from shortest front pattern

${var##pattern}

remove from longest front pattern

${var/pattern/string}

find and replace (only replace first occurrence)

${var//pattern/string}

find and replace all occurrences

<a href="http://www.cyberciti.biz/tips/bash-shell-parameter-substitution-2.html">howto: use bash parameter substitution like a pro</a>

<a href="http://tldp.org/ldp/abs/html/untyped.html">bash variables are untyped</a>

<a href="http://tldp.org/ldp/bash-beginners-guide/html/sect_10_01.html">types of variables</a>