Linux使用 -- 变量

命名规则

  1. 字母数字下划线
  2. 不以字母开头

赋值

  1. 基本概念
    • Shell变量是弱类型的变量
  2. 方式
    • 变量名=变量值
      • a=123,等号左右不能出现空格
    • 使用let为变量赋值
      • let a=10+20,Shell是解释性语言,尽量不要进行运算
    • 将命令赋值给变量
      • l=ls
    • 将命令结果赋值给变量,使用$()或者``
      • letc=$(ls -l /etc)
    • 变量值有空格等特殊字符可以包含在""或者''
      • a='hello world'
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[root@localhost ~]# a=123
[root@localhost ~]# a =123
-bash: a: 未找到命令
[root@localhost ~]# a = 123
-bash: a: 未找到命令
[root@localhost ~]# a= 123
-bash: 123: 未找到命令
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[root@localhost ~]# l=ls
[root@localhost ~]# $l
anaconda-ks.cfg a.sh a.txt b.txt combine.sh error.txt ls.txt
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[root@localhost ~]# cmd1=`ls /root`
[root@localhost ~]# echo $cmd1
anaconda-ks.cfg a.sh a.txt b.txt combine.sh error.txt ls.txt

[root@localhost ~]# cmd2=$(ls /root)
[root@localhost ~]# echo $cmd2
anaconda-ks.cfg a.sh a.txt b.txt combine.sh error.txt ls.txt
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[root@localhost ~]# string1=hello bash
[root@localhost ~]# echo $string1
hello
[root@localhost ~]# string1='hello bash'
[root@localhost ~]# echo $string1
hello bash
[root@localhost ~]# string2="I'm bash"
[root@localhost ~]# echo $string2
I'm bash
[root@localhost ~]# string3='Hello "zhongmingmao"'
[root@localhost ~]# echo $string3
Hello "zhongmingmao"

引用

  1. ${变量名}是对变量的引用 – 最正式的用法
  2. echo ${变量名}查看变量的值
  3. ${变量名}在部分情况下可以省略为$变量名
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[root@localhost ~]# string1='hello bash'
[root@localhost ~]# echo $string1
hello bash
[root@localhost ~]# echo ${string1}
hello bash
[root@localhost ~]# echo $string123

[root@localhost ~]# echo ${string1}23
hello bash23

作用范围

  1. 默认作用范围
    • 当前Shell
  2. 变量导出
    • export
  3. 变量的删除
    • unset
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[root@localhost ~]# a=1
[root@localhost ~]# bash # 进入子进程
[root@localhost ~]# echo $a

[root@localhost ~]# a=2
[root@localhost ~]# exit
[root@localhost ~]# echo $a
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[root@localhost ~]# b='hello subshell'
[root@localhost ~]# vim subshell.sh
[root@localhost ~]# cat subshell.sh
#!/bin/bash

# demo subshell

echo $b
[root@localhost ~]# chmod u+x subshell.sh
[root@localhost ~]# bash subshell.sh

[root@localhost ~]# ./subshell.sh

[root@localhost ~]# source ./subshell.sh
hello subshell
[root@localhost ~]# . ./subshell.sh
hello subshell
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[root@localhost ~]# export b='hello subshell'
[root@localhost ~]# ./subshell.sh
hello subshell
[root@localhost ~]# bash ./subshell.sh
hello subshell
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[root@localhost ~]# unset b
[root@localhost ~]# echo $b

[root@localhost ~]#

系统环境变量

  1. 环境变量:每个Shell打开都可以获得到的变量
    • 命令:envset(更详细)
    • $PATH
    • PS1
    • $?(上一条命令是否正常执行)、$$(PID)、$0(执行的文件名)
  2. 位置变量
    • $1$2$n

$PATH

$PATH被export过,对子Shell生效,但对平行Shell不生效

Session A

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[root@localhost ~]# cat 5.sh
#!/bin/bash

echo 'hello bash'
du -sh
[root@localhost ~]# chmod u+x 5.sh
[root@localhost ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
[root@localhost ~]# ./5.sh
hello bash
80K .
[root@localhost ~]# 5.sh
bash: 5.sh: 未找到命令
[root@localhost ~]# PATH=$PATH:/root
[root@localhost ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/root
[root@localhost ~]# 5.sh
hello bash
80K .
[root@localhost ~]# which 5.sh
/root/5.sh
[root@localhost ~]# bash # 进入子进程
[root@localhost ~]# echo $PATH # PATH依然有效
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/root
[root@localhost ~]# 5.sh
hello bash
80K .

Session B

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[root@localhost ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin

$PS1

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[root@localhost ~]# echo $PS1
[\u@\h \W]\$

$?、$$、$0

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[root@localhost ~]# ifconfig > /dev/null
[root@localhost ~]# echo $?
0
[root@localhost ~]# ifconfig nonn1 > /dev/null
nonn1: error fetching interface information: Device not found
[root@localhost ~]# echo $?
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[root@localhost ~]# echo $$
3143
[root@localhost ~]# bash # 进入子进程
[root@localhost ~]# echo $$
3227
[root@localhost ~]# ps -f
UID PID PPID C STIME TTY TIME CMD
root 3143 3139 0 19:05 pts/1 00:00:00 -bash
root 3227 3143 0 19:24 pts/1 00:00:00 bash
root 3238 3227 0 19:24 pts/1 00:00:00 ps -f
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[root@localhost ~]# echo $0
bash
[root@localhost ~]# cat 6.sh
#!/bin/bash

echo $$
echo $0
[root@localhost ~]# chmod u+x 6.sh
[root@localhost ~]# bash 6.sh
3243
6.sh
[root@localhost ~]# . 6.sh
3227
bash

$1、$n

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[root@localhost ~]# cat 7.sh
#!/bin/bash

# $1 $2 ${10}
echo $1
echo $2
echo ${2-X} # 参数替换
[root@localhost ~]# chmod u+x 7.sh
[root@localhost ~]# ./7.sh -a -b
-a
-b
-b
[root@localhost ~]# ./7.sh -a
-a

X

环境变量配置文件

  1. /etc/profile
  2. /etc/bashrc – nologin shell
  3. /etc/profile.d/
  4. ~/.bash_profile
  5. ~/.bash_rc – nologin shell
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[root@localhost ~]# head -n 1 /etc/profile
echo /etc/profile
[root@localhost ~]# head -n 1 /etc/bashrc
echo /etc/bashrc
[root@localhost ~]# head -n 1 ~/.bashrc
echo ~/.bashrc
[root@localhost ~]# head -n 1 ~/.bash_profile
echo ~/.bash_profile

[root@localhost ~]# su - root
上一次登录:一 10月 18 19:05:34 CST 2019从 192.168.206.1pts/1 上
/etc/profile
/root/.bash_profile
/root/.bashrc
/etc/bashrc

[root@localhost ~]# su root
/root/.bashrc
/etc/bashrc

[root@localhost ~]# bash
/root/.bashrc
/etc/bashrc

[root@localhost ~]# source /etc/profile
/etc/profile
[root@localhost ~]# source /etc/bashrc
/etc/bashrc

数组

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[root@localhost ~]# IPTS=( 10.0.0.1 10.0.0.2 10.0.0.3 )
[root@localhost ~]# echo $IPTS # 只会显示第一个元素
10.0.0.1
[root@localhost ~]# echo ${IPTS[@]}
10.0.0.1 10.0.0.2 10.0.0.3
[root@localhost ~]# echo ${#IPTS[@]}
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[root@localhost ~]# echo ${IPTS[1]}
10.0.0.2
0%