Variable

赋值

在shell 里面赋值用"="，但是要注意不能有空格，VAR=value是OK的，因为shell识别到"="以后就会执行赋值操作; 但是VAR = value不行，因为shell会认为VAR是一个程序从而去寻找相应的同名程序来执行

MY_MESSAGE="Hello World"
variable=value
variable='value'
variable="value"
echo $MY_MESSAGE

• 如果 value 包含了空白符，那么就必须使用引号包围起来。
• 单引号： 原样输出，里面是什么就是什么
• 双引号： 解析输出，先解析变量名和命令

nuts="NUTS!!!"
var1='JOJO:${nuts}'
var2="JOJO:${nuts}"
echo $var1
echo $var2

The shell does not care about types of variables; they may store strings, integers, real numbers - anything you like. (但实际上还是以string的形式存储的)

# like printf() in C
MYVAR="BIZZARE ADVENTURE"
echo "JOJOs: $MYVAR"

• 声明变量不用$, 调用变量得用$
• 以上是显式赋值(Explicitly),本篇文末还有隐式赋值的介绍

最后如果希望删除变量，我们可以：

unset variable_name

Some Special Vairables

首先我们来看看带数字类的保留的变量：$0, $1, .. $9

• The variable $0 is the basename of the program as it was called (相当于argv[0])
• $1 .. $9 are the first 9 additional parameters the script was called with(相当于argv[1], argv[2],...)

另外还有几个符号类型的保留变量：$@, $* $#

• $@ is all parameters $1 .. whatever (除了$0以外所有的数字型, 所有argument), 相当于argv
• $* is similar to $@ only $* does not preserve white space (As a general rule, use $@ and avoid $*)
• $# is the number of parameters the script was called with argc

#!/bin/sh
echo "I was called with $# parameters" 
echo "My name is $0"
echo "My first parameter is $1"
echo "My second parameter is $2"
echo "All parameters are $@"

除了以上这些和当前run的环境相关的variables以外，还有

• $? contains the exit value of the last run command
• $$ is the PID (Process IDentifier) of the currently running shell
• $! is the PID of the last run background process

一般debug复杂程序的时候可以用

#!/bin/sh
/usr/test_Dir/test_program
if [ "$?" -ne "0" ]; then
  echo "Sorry, we had a problem there!"
fi

Wild Cards

• The asterisk character (*, also called "star") matches zero or more characters

  For example, doc* matches doc and document but not dodo. 一切皆有可能

• The question mark ? matches exactly one character.

For example the pattern 123? will match 123 and 1234, but not 12345. 一个皆有可能

• ranges of characters enclosed in square brackets ([ and ]) match a single character within the set

For example, [A-Za-z] matches any single uppercase or lowercase letter

Command in Shell

我们还可以再shell里面直接使用shell command 从而得到隐式赋值： 有两种等价的形式：

variable=`command`
variable=$(command)
longFileList=`ls -l`
date=`date +%H:%M:%S`