bash学习之变量的显示和设置

显示变量：echo $MAIL或者 echo ${MAIL}

[CJP@CJP ~]$ echo $MAIL

/var/spool/mail/CJP

[CJP@CJP ~]$ echo ${MAIL}

/var/spool/mail/CJP

设置变量：用等号（=）赋值就可以，可以实现设置以及修改

[CJP@CJP ~]$ myname=CJP

[CJP@CJP ~]$ echo $myname

CJP

[CJP@CJP ~]$ myname=CJP2

[CJP@CJP ~]$ echo $myname

CJP2

变量设置的一些规则：

a、等号两边不能直接接空格，如 myname = CJP,会出现

[CJP@CJP ~]$ myname = CJP

bash: myname: command not found

[CJP@CJP ~]$ myname=CJP CJP2

bash: CJP2: command not found

b、变量名称只能是英文和数字，但是开头字符不能是数字

[CJP@CJP ~]$ 2myname=CJP

bash: 2myname=CJP: command not found

c、变量内容如果有空格，可以使用双引号或者单引号将内容结合起来

双引号内特殊字符如$，可以保持原有的特性

[CJP@CJP ~]$ var="language is $LANG"

[CJP@CJP ~]$ echo $var

language is zh_CN.utf8

变量内容存在单引号的时候，用双引号

[CJP@CJP ~]$ name=CJP's name

>

> ^C

[CJP@CJP ~]$ name='CJP's name'

> ^C

[CJP@CJP ~]$ name="CJP's name"

[CJP@CJP ~]$ echo $name

CJP's name

单引号内特殊字符是纯文本

[CJP@CJP ~]$ var='language is $LANG'

[CJP@CJP ~]$ echo $var

language is $LANG

d、可用转意字符“\”将特殊字符（[ENTER],$,\,!,空格等）变成一般字符

[CJP@CJP ~]$ var=\$

[CJP@CJP ~]$ echo $var

$

[CJP@CJP ~]$ name=CJP\'s\ name

[CJP@CJP ~]$ echo $name

CJP's name

e、当一串命令中需要得到其他命令提供的信息，可以使用反单引`command`或者$(command)

[CJP@CJP ~]$ path=`echo $PATH`

[CJP@CJP ~]$ echo $path

/home/CJP/qtsdk-2010.05/qt/bin:/usr/lib/qt-3.3/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/CJP/bin

[CJP@CJP ~]$ path=$(echo $PATH)

[CJP@CJP ~]$ echo $path

/home/CJP/qtsdk-2010.05/qt/bin:/usr/lib/qt-3.3/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/CJP/bin

[CJP@CJP ~]$ version=$(uname -r)

[CJP@CJP ~]$ echo $version

2.6.32-220.el6.i686

进入目前内核的模块目录

[CJP@CJP ~]$ cd /lib/modules/$(uname -r)/kernel

[CJP@CJP kernel]$

或者

[CJP@CJP ~]$ cd /lib/modules/`uname -r`/kernel

[CJP@CJP kernel]$

f、为了增加变量内容，可用 “$var” 或者 ${var}累加内容

[CJP@CJP ~]$ var=C

[CJP@CJP ~]$ var="$var"JP

[CJP@CJP ~]$ echo $var

CJP

[CJP@CJP ~]$ var=${var}cjp

[CJP@CJP ~]$ echo $var

CJPcjp

g、变量如果要在其他子进程中执行，需要通过export使变量变成环境变量

[CJP@CJP ~]$ name=CJP

[CJP@CJP ~]$ bash ==进入子进程

[CJP@CJP ~]$ echo $name

==没有变量内容

[CJP@CJP ~]$ exit ==离开子进程

exit

[CJP@CJP ~]$ export name ==设置环境变量

[CJP@CJP ~]$ bash

[CJP@CJP ~]$ echo $name

CJP

[CJP@CJP ~]$ exit

exit

注：子进程（在目前的shell下打开新的shell，新的shell就是子进程）

h、取消变量可以使用unset

[CJP@CJP ~]$ echo $var

CJPcjp

[CJP@CJP ~]$ unset var

[CJP@CJP ~]$ echo $var

[CJP@CJP ~]$