Reference for shell scripting

• ${var} 和 $var的区别

http://stackoverflow.com/questions/8748831/when-do-we-need-curly-braces-in-variables-using-bash

正常情况下都可以，特殊情况下需要用花括号来区分变量名和其他的字符

• “==”和 “=” 的区别

http://stackoverflow.com/questions/2600281/what-is-the-difference-between-operator-and-in-bash

正常情况下都可以使用，最好使用 “==”

• []  、 [[]] 、  test 、  ((number compare only))  都可以用来判断条件

• set / setenv / export /declare 的区别

export (in ksh/sh)：exported var is accessible by all the shells or processes invoked by the original shell  (an environment value)

set (in csh): set a local variable

setenv (in csh): set an environment variable

http://www.theunixschool.com/2010/04/what-is-difference-between-export-set.html 

declare is used to declare variables and give them attribute

• boolean expressions

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html 

[ -a File ]

[ -d dir ]

[ -z String ]

[ -n String ]

[ Integer1 OP Interger2 ]  "OP" is one of -eq, -ne, -lt, -le, -gt or -ge

test exit status:

 [ $? -eq 0 ]

Regular expressions may also be used in comparisons:

if [[ "$gender" == f* ]]

• shell expansion    

http://ss64.com/bash/syntax-expand.html

 

• Arithmatic Operation

http://www.tldp.org/LDP/abs/html/arithexp.html

z=$((z+3))

let z=z+3