shell 与 环境变量

1. 查看系统可用的shell 类型 ：

[arthur@localhost ~]$ cat /etc/shells 
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash
/usr/bin/tmux
/bin/tmux
/usr/bin/zsh
/bin/zsh

2. 查看用户登陆时获得的shell类型：

[arthur@localhost ~]$ cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
...
rngd:x:973:973:Random Number Generator Daemon:/var/lib/rngd:/sbin/nologin

[arthur@localhost ~]$ cat /etc/passwd | grep arthur
arthur:x:1000:1000:arthur:/home/arthur:/bin/bash

3. shell 的变量功能：

3.1 可以使用echo来显示变量内容：

[arthur@localhost ~]$ echo $PATH
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${HOME}
/home/arthur

3.2 设置和修改变量内容：

可对变量赋值、修改、增添、取消

[arthur@localhost ~]$ vname1="This's is variable 1"
[arthur@localhost ~]$ echo $vname1
This's is variable 1
#单引号识别错误
[arthur@localhost ~]$ vname2='This\'s\ is\ variable\ 2' | echo $vname2
> ^C
#单引号识别错误
[arthur@localhost ~]$ vname2='This\'s\ is\ variable\ 2' 
> ^C
[arthur@localhost ~]$ vname2=This\'s\ is\ variable\ 2
[arthur@localhost ~]$ echo $vname2
This's is variable 2
[arthur@localhost ~]$ echo $vname1 $vname2
This's is variable 1 This's is variable 2
#修改变量
[arthur@localhost ~]$ vname=variable 
[arthur@localhost ~]$ echo $vname
variable
[arthur@localhost ~]$ vname=variables 
[arthur@localhost ~]$ echo $vname
variables
#取消变量
[arthur@localhost ~]$ unset vname1 vname2
[arthur@localhost ~]$ echo $vname1 $vname2

变量内容删除、取代与替换：

[arthur@localhost ~]$ echo $PATH
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#变量path 定义
[arthur@localhost ~]$ path=${PATH}  
[arthur@localhost ~]$ echo $path
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#######################################################################
#######################################################################
**删除**
##将首个含  /local/bin  的路径移除，`#`代表从变量内容的最前面开始向右删除，且仅删除最短的那个； `/`代表要被删除的部分，`#`代表由最前面开始向右删除，所以这里由`/`开始写起；  
[arthur@localhost ~]$ echo ${path#/*local/bin:}
/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo $PATH
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#可以发现，上述删除操作并未改变变量`path`内容
[arthur@localhost ~]$ echo $path
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#######################################################################
##删除前面所有目录，仅保留最后一个目录.   一个`#`代表仅删除最短的目录，两个`#`代表删除最长的目录
[arthur@localhost ~]$ echo ${path#/*:}
/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path##/*:}
/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo $path
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path#home*}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin:home/arthur/bin
[arthur@localhost ~]$ echo ${path}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin:home/arthur/bin


#########################################################################
#从后往前删除变量名称 `%`

[arthur@localhost ~]$ echo $path
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#删除最后一个目录，从`:` 到`bin`为止
[arthur@localhost ~]$ echo ${path%:*bin}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin
#只保留第一个目录, `%%` 代表最长的符合字串
[arthur@localhost ~]$ echo $path
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path%%:*bin}
/home/arthur/.local/bin
#########################################################################
#########################################################################
**取代**
#将 `path` 变量内容 `sbin` 取代成大写 `SBIN`
[arthur@localhost ~]$ echo ${path}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#两条斜线中间的为旧字符串
[arthur@localhost ~]$ echo ${path/sbin/SBIN}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/SBIN:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#仅一条斜线时，仅首个变量内容会发生取代
[arthur@localhost ~]$ echo ${path/bin/BIN}
/home/arthur/.local/BIN:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
#两条斜线，所有匹配的内容都会发生取代
[arthur@localhost ~]$ echo ${path}
/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin
[arthur@localhost ~]$ echo ${path//bin/BIN}
/home/arthur/.local/BIN:/home/arthur/BIN:/home/arthur/.local/BIN:/home/arthur/BIN:/usr/local/BIN:/usr/local/sBIN:/usr/BIN:/usr/sBIN:/var/lib/snapd/snap/BIN

3.3 环境变量

查看当前shell环境下的环境变量： env 或者export

[arthur@localhost ~]$ env
LANG=en_US.UTF-8
...
USER=arthur
PWD=/home/arthur
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
HOME=/home/arthur
LOGNAME=arthur
PATH=/home/arthur/.local/bin:/home/arthur/bin:/home/arthur/.local/bin:/home/arthur/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/var/lib/snapd/snap/bin

自定义变量转为环境变量：export

#export 变量
export PATH=${PATH}:/dir/to/des/bin

3.4 变量的有效范围

环境变量等价于全局变量(global variable); 自定义变量等价于局部变量(local variable)