Bash中的Parameter Expansion

Bash中的Parameter Expansion 

parameter expansion的基本形式是${xxxx},常见的有：
${parameter:-word}
    如果parameter为null或者被unset，那么取word来扩展结果  
${parameter:=word}
    如果parameter为null或者被unset，那么将word的值赋值给parameter，其值作为扩展结果。
${parameter:offset:length}
    取parameter从offset开始，length长度个字符作为扩展结果

${!xxx}形式，就是indirect expansion，即${!var}会取var变量的值作为一个变量的名字，然后取这个 变量的值来作为扩展结果。但也有例外：
${!prefix*} 和 ${!prefix@} 会扩展为以prefix开头的所有的变量名
${!name[@]} 和 ${!name[*]} 如果name是数组，会扩展为该数组中包含合法元素的下标。如果不是数组，一般来结果就是0或者null.
还有许多不常用的参见man手册。

以下是一个parameter expansion的例子代码：
#!/bin/bash
# below are some examples for Parameter expansion of bash
set -ex
A1=a1
A2=a2
STR=1122334455
# RAY is an array
RAY[1]=10
RAY[2]=10
RAY[5]=10

# A's value is a var name 'F'
F=haha
A=F

# exceptions to indirection expansion, ie: A* and A@ are expanded to be
# all var names with first character is 'A'. the below two commands are
# identical
echo ${!A*}
echo ${!A@}

# exceptions to indirection expansion, ie: the keys of the valid elements
# in array RAY are printed, here result is 1 2 5
echo ${!RAY[@]}
echo ${!RAY[*]}

# indirection expansion, A's value is F, and var F's value haha is the
# result, take a var's value as a var name and get the new var's value as
# the final result.
echo ${!A}

# take substrings from index 2 based 0, with length 2
echo ${STR:2:2}