[Shell] Common shell variable usage, $n, $#, $*, $@, $?, $HOME, $PWD, $SHELL, $USER

Variables in Shell

1. System variables

$HOME

Get the current user's working directory

[root@jiangnan ~]# echo $HOME
/root
[root@jiangnan ~]# 

$PWD

Get the current directory

[root@jiangnan tomcat]# echo $PWD
/root/tomcat
[root@jiangnan tomcat]# 

Difference: We can also use pwd to return the current directory, but pwd is a command and PWD is a system variable.

$SHELL

Get the currently used shell parser

[root@jiangnan tomcat]# echo $SHELL
/bin/bash
[root@jiangnan tomcat]# 

$USER

get current user

[root@jiangnan tomcat]# echo $USER
root
[root@jiangnan tomcat]# 

set

Get all variables in shell

[root@jiangnan tomcat]# set
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:expand_aliases:extquote:force_fignore:histappend:hostcomplete:interactive_comments:login_shell:progcomp:promptvars:sourcepath
BASH_ALIASES=()
......
_=clear
colors=/root/.dircolors
[root@jiangnan tomcat]# 

env

Get all environment variables

[root@jiangnan ~]# env
XDG_SESSION_ID=13289
HOSTNAME=jiangnan
TERM=xterm
......
_=/usr/bin/env
OLDPWD=/root/tomcat
[root@jiangnan ~]# 

2. Custom variables

2.1 Basic syntax

define variable: variable = value

Undo variable: unset variable

Declare static variables: readonly variables.

Note: Static variables cannot be unset

2.2 Variable Definition Rules

• The variable name can consist of letters, numbers and underscores, but cannot start with a number. It is recommended to capitalize the environment variable name.
• No spaces on either side of the equal sign
• In bash, the default type of variables is string type, and numerical operations cannot be performed directly.
• If the value of the variable has spaces, it needs to be enclosed in double or single quotation marks.
• Use export to promote variables to environment variables for global use

2.3 Examples

# 定义变量A
[root@jiangnan ~]# A=5
[root@jiangnan ~]# echo $A
5
# 给变量A重新赋值
[root@jiangnan ~]# A=6
[root@jiangnan ~]# echo $A
6
# 撤销变量A
[root@jiangnan ~]# unset A
[root@jiangnan ~]# echo $A  # 变量A被撤销了，所以不能打印任何数据

[root@jiangnan ~]# 

# 定义静态变量B
[root@jiangnan ~]# readonly B=6
[root@jiangnan ~]# echo $B
6
# 不能对静态变量撤销
[root@jiangnan ~]# unset B
-bash: unset: B: cannot unset: readonly variable
[root@jiangnan ~]# 

# 变量默认类型都是字符串类型，不能直接运算
[root@jiangnan ~]# C=1+2
[root@jiangnan ~]# echo $C
1+2
[root@jiangnan ~]# D=1
[root@jiangnan ~]# E=2
[root@jiangnan ~]# C=D+E
[root@jiangnan ~]# C=$D+$E
[root@jiangnan ~]# echo $C
1+2
[root@jiangnan ~]# 

# 变量的值如果有空格，需要使用双引号或单引号括起来
[root@jiangnan ~]# H=I love China
-bash: love: command not found
[root@jiangnan ~]# H='I love China'
[root@jiangnan ~]# echo $H
I love China
[root@jiangnan ~]# 

# 提升H为环境变量，可以在脚本中使用了
[root@jiangnan tomcat]# H='I love China'
[root@jiangnan tomcat]# export H
[root@jiangnan tomcat]# vi hello.sh
[root@jiangnan tomcat]# cat hello.sh 
#!/bin/bash 
echo $H
[root@jiangnan tomcat]# sh hello.sh 
I love China
[root@jiangnan tomcat]# 

3. Special variables

$n

n is a number, 0 represents the script name, and 1-9 represent the first to ninth parameters. More than ten parameters need to be enclosed in curly brackets, such as ${10}

[root@jiangnan tomcat]# vi variable1.sh 
[root@jiangnan tomcat]# cat variable1.sh 
#!/bin/bash
echo "脚本名称为:"$0
echo "第1个参数为:"$1
echo "第10个参数为:"${10}
[root@jiangnan tomcat]# sh variable1.sh a b c d e f g h i g k l m n  # 参数之间空格隔开
脚本名称为:variable1.sh
第1个参数为:a
第10个参数为:g
[root@jiangnan tomcat]# 

$#

Get the number of all input parameters

[root@jiangnan tomcat]# cat variable2.sh 
#!/bin/bash
echo "参数个数为:"$#
[root@jiangnan tomcat]# sh variable2.sh a b c
参数个数为:3
[root@jiangnan tomcat]# 

This is usually used for loops.

$ and $@

All represent all parameters, but $* treats all parameters as a whole, $@ treats each parameter separately

[root@jiangnan tomcat]# vi variable3.sh
[root@jiangnan tomcat]# cat variable3.sh 
#!/bin/bash
echo $*
echo $@
[root@jiangnan tomcat]# sh variable3.sh a b c
a b c   # 由$*获取
a b c   # 由$@获取
[root@jiangnan tomcat]#  

From here, it can be seen that all parameters are obtained, there is no difference, but the difference can be seen in the for loop.

[root@jiangnan tomcat]# vi variable4.sh 
[root@jiangnan tomcat]# cat variable4.sh 
#!/bin/bash
for parameter1 in "$*"
do
	echo "I love "$parameter1
done
for parameter2 in "$@"
do
	echo "I love "$parameter2
done
[root@jiangnan tomcat]# sh variable4.sh beijing shanghai tianjin
I love beijing shanghai tianjin  # $*把参数当成一个整体
I love beijing                   # $@把每个参数区别对待
I love shanghai
I love tianjin
[root@jiangnan tomcat]# 

$* and $@ use double quotes to become the whole, see the difference.

$?

The return status of the last command executed. If the value of this variable is 0, it proves that the previous command was executed correctly; if the value of this variable is non-zero, it proves that the previous command was executed incorrectly.

[root@jiangnan tomcat]# sh variable4.sh a b c
I love a b c
I love a
I love b
I love c
[root@jiangnan tomcat]# echo $?
0  # 返回0证明我的上一条命令sh variable4.sh a b c 执行成功了
[root@jiangnan tomcat]# 

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