Linux学习之shell 编程基础（二）

一、bash环境变量

HOME、MAIL、SHELL、PATH 等，环境变量大都用大写字母组成

[[email protected] dalianmao]# echo $SHELL
/bin/bash
[[email protected] dalianmao]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/dalianmao/bin
[[email protected] dalianmao]# echo $MAIL
/var/spool/mail/dalianmao
[[email protected] dalianmao]# echo $HOME
/root

二、变量的取用与设定：echo，变量设定规则，unset

（1）echo $ variable

[[email protected] dalianmao]# echo $HOME
/root

（2）变量设定规则

  1、等号两边不能直接接空格符，如下所示为错误

[[email protected] dalianmao]# myname = dalianmao
bash: myname: command not found
[[email protected] dalianmao]# myname= dalianmao
bash: dalianmao: command not found

2、变量名称只能是英文字母与数字，但是开头字符不能是数字，如下为错误：

[[email protected] dalianmao]# 2myname=dalianmao
bash: 2myname=dalianmao: command not found

3、变量内容若有空格符可使用双引号或者单引号将变量内容结合起来，但

• 双引号内的特殊字符如$等，可以保有原本的特性，如下所示：

  [[email protected] ~]$ clear
  [[email protected] ~]$ name=dalianmao
  [[email protected] ~]$ name="my name is $name"
  [[email protected] ~]$ echo $name
  my name is dalianmao

   

• 单引号内的特殊字符则仅为一般字符（纯文本），如下所示：  

[[email protected] ~]$ name=dalianmao
[[email protected] ~]$ name='my name is $name'
[[email protected] ~]$ echo $name
my name is $name

4、可用跳脱字符  \ 将特殊符号（如Enter，$,\,空格符，' 等）变成一般字符；

5、在一串指令中，还需要籍由其他的指令提供的信息，可以使用反单引号` 或$。

[[email protected] ~]$ clear
[[email protected] ~]$ myname=`ll|grep wzc`
[[email protected] ~]$ echo $myname
drwxr-xr-x. 3 root root 4096 Sep 21 09:13 wzc
[[email protected] ~]$ name=$(ll|grep wzc)
[[email protected] ~]$ echo $name
drwxr-xr-x. 3 root root 4096 Sep 21 09:13 wzc

6、若该变量为扩增内容时，则可用“$变量”或${变量}累加内容。

[[email protected] ~]$ myname=dalianmao
[[email protected] ~]$ echo $myname
dalianmao
[[email protected] ~]$ myname=${myname}' is my name!'
[[email protected] ~]$ echo $myname
dalianmao is my name!
[[email protected] ~]$ myname="$myname"' is myname!hahahhaha'
[[email protected] ~]$ echo $myname
dalianmao is my name! is myname!hahahhaha

7、若该变量需要在其他子程序执行，则需要以export来使变量变成环境变量。

[[email protected] ~]$ name=dalianmao
[[email protected] ~]$ echo $name
dalianmao
[[email protected] ~]$ bash
[[email protected] ~]$ echo $name

[[email protected] ~]$ exit
exit
[[email protected] ~]$ export name
[[email protected] ~]$ bash
[[email protected] ~]$ echo $name
dalianmao
[[email protected] ~]$ exit
exit
[[email protected] ~]$ unset name
[[email protected] ~]$ echo $name

[[email protected] ~]$ 

8、取消变量的方法为使用unset。

[[email protected] ~]$ myname=dalianmao
[[email protected] ~]$ echo $myname
dalianmao
[[email protected] ~]$ unset myname
[[email protected] ~]$ echo $myname

[[email protected] ~]$