Linux study notes: Shell script

Shell script operator

Calculate 4×5÷6
Method 1:
Use $(())

A=$(((4*5)/6))
echo A=$A

Method 2:
Use $[]

B=$[(4*5)/6]
echo B=$B

Method 3:
Use expr
and need to be separated by spaces
Use backticks for assignment·

C=`expr 4 \* 5 / 6 `
echo C=$C 

operation result:

bash mathrl
A=3
B=3
C=3

Variables in shell scripts

Create, output, delete variables

#!/bin/bash 
#声明

#创建一个变量
#中间不能使用空格
A=100

#输出变量,使用$
echo $A
echo A=$A
echo "A=$A"
#删除变量
unset A
echo A

operation result

100
A=100
A=100
 

static variable

#创建静态变量 
readonly B=2
#输出
echo $B
#删除变量
unset B

operation result

2
bash: unset: B：无法取消设定: 只读 variable

Static variables cannot be deleted

System variables
Variables that have been set in the system

echo $HOME
echo $USER
echo $HOME
#还有很多

You can usesetto view system variables

Environment variables
Variables (global variables) that can be used in every shell script

var1.sh
#定义一个环境变量
export EX_VAR=10

Use in another script

#加载变量
source var1.sh
#输出变量
echo $EX_VAR

run

10

Assign command to variable

A=$(ls)
echo $A

operation result

var1.sh var2.sh var.sh

Positional parameter variable
$*: Output all parameters (output as a whole)
$@: Output all parameters (output individually)
$#: Number of output parameters

#!/bin/bash

#输出参数
echo 0=$0 1=$1 2=$2 3=$3
#输出所有参数 方法一
echo 所有参数为$*
#输出所有参数 方法二
echo 所有参数为$@
#输出参数个数
echo $#

run

#传入参数运行，参数之间用空格分开
bash var2.sh 10 20 30
#结果
0=var2.sh 1=10 2=20 3=30
所有参数为10 20 30
所有参数为10 20 30
3

Predefined variables
$$: Current process PID
$!: Output the process number of the last background process
$?: Output the status of the last executed command, 0 is normal

#!/bin/bash

#输出当前进程的PID
echo 当前进程的PID为$$

#后台运行程序
gedit var.sh &
#输出当前最后一个后台的进程号
echo 最后一个后台进程号$!

#输出最后一个执行命令的状态，为0正常
echo 状态为$?

operation result

当前进程的PID为2119
最后一个后台进程号2705
状态为0

Shell script if statement

if [ 条件 ] #中括号里面一定要空格
then 命令
fi

if [ 条件 ] #中括号里面一定要空格
then 命令
elif [ 条件 ]
then 命令
...
else 命令
fi

[]Test condition parameters:
Integer judgment

parameter	Function
-eq	equal
- is	not equal to
-lt	less than
- the	less than or equal to
-gt	more than the
-ge	greater or equal to

String judgment

parameter	Function
=	equal
!=	not equal to
str1 < str2	less than
str1 >str2	more than the
str	True if not empty
- in str	True if the length is greater than 0
- with p	length equals 0 is true

Greater than less than is sorted in lexicographic order

File judgment

parameter	Function
-r	readable
-In	writable
-x	Executable
-f	The file exists and exists
-d	file as directory

logic operation

parameter	Function
!	No
-a	and
-O	or

Case 1: Judging student grades, <60 failed, >60 and <80 passed, >80 excellent, and other grades were wrong.

#!/bin/bash

if [ $1 -lt 60 -a $1 -ge 0 ]
then
        echo "不及格"
elif [ $1 -lt 80 ]
then
        echo "及格"
elif [ $1 -le 100 ]
then 
        echo "优秀"
else 
        echo "成绩错误"
fi

operation result

bash if1 0
不及格
bash if1 100
优秀
bash if1 120
成绩错误

Case 2: Determine file permissions, writable, readable, executable, does not exist. The output file does not exist:

#!/bin/bash

if [ -f $1 ]
then
        echo -n "$1文件"
        if [  -r $1 ]
        then
        echo -n "可读"
        fi
        if [ -w $1 ]
        then
        echo -n "可写"
        fi
        if [ -x $1 ]
        then
        echo -n "可执行"
        fi
else    
        echo "文件不存在"
fi
echo

operation result

bash if2 txt #运行
文件不存在  #结果
touch txt #运行
bash if2 txt #运行
txt文件可读可写  #结果

Shell script case statement

The format of the case statement:

case 字符串 in
模式字符串1) 命令 ;;
模式字符串2) 命令 ;;
模式字符串3) 命令 ;;
模式字符串4) 命令 ;;
*）命令 ;;
esac

If the string is equal to the pattern string 1-4, it will be executed. If neither is satisfied, the command of ) will be executed. *

Case: Write a shell to control the file. Enter 1 to output the file content, enter 2 to delete the file, enter 3 to change the file name, and other output errors.
The procedure is as follows:

#!/bin/bash

case $1 in
1) cat $2 ;;
2) rm $2 ;;
3) mv $2 $3 ;;
*) echo "指令错误"
esac

Going as follows：

qx@qx-VirtualBox:~/桌面/Class/ifelse$ bash case 3 txt atxt
qx@qx-VirtualBox:~/桌面/Class/ifelse$ ls
atxt  case  if1  if2
qx@qx-VirtualBox:~/桌面/Class/ifelse$ bash case 1 atxt
abcdefg

qx@qx-VirtualBox:~/桌面/Class/ifelse$ bash case 2 atxt
qx@qx-VirtualBox:~/桌面/Class/ifelse$ ls
case  if1  if2
qx@qx-VirtualBox:~/桌面/Class/ifelse$ bash case 4 atxt
指令错误

Shell script for statement

Formal first:

for 变量 in "值表"
do 
	命令
done

Case 2:

for (( e1;e2;e3 ))
do
	命令
done

Similar to C language, such asfor (( i=0; i<100; i++ ))

Case: Sum all the numbers entered
Use method one to write:

#!/bin/bash

sum=0
for i in " $@ "
do
  sum=$[$sum+$i]
done
echo $sum

Run result:

bash -x for 1 2 3 4 5
+ sum=0
+ for i in " $@ "
+ sum=1
+ for i in " $@ "
+ sum=3
+ for i in " $@ "
+ sum=6
+ for i in " $@ "
+ sum=10
+ for i in " $@ "
+ sum=15
+ echo 15
15

Use method two to write:

#!/bin/bash

num=$#
sum=0
for(( i=1; i<=num; i++ ))
do
  sum=$[$sum+$i]
done
echo $sum

Run result:

bash -x for2 1 2 3 4 5
+ num=5
+ sum=0
+ (( i=1 ))
+ (( i<=num ))
+ sum=1
+ (( i++  ))
+ (( i<=num ))
+ sum=3
+ (( i++  ))
+ (( i<=num ))
+ sum=6
+ (( i++  ))
+ (( i<=num ))
+ sum=10
+ (( i++  ))
+ (( i<=num ))
+ sum=15
+ (( i++  ))
+ (( i<=num ))
+ echo 15
15

Shell script while statement

Format:

while [ 条件 ]
do
	指令
done

Note: there need to be spaces between while and square brackets

Case: Sum the input data
The procedure is as follows:

#!/bin/bash

sum=0
num=0

while [ $# -ne 0 -a $num -lt $# ]
do
  num=$[$num+1]
  sum=$[$sum+$num]
done
echo sum=$sum

Run as follows:

bash -x while 1 2 3 4 5
+ sum=0
+ num=0
+ '[' 5 -ne 0 -a 0 -lt 5 ']'
+ num=1
+ sum=1
+ '[' 5 -ne 0 -a 1 -lt 5 ']'
+ num=2
+ sum=3
+ '[' 5 -ne 0 -a 2 -lt 5 ']'
+ num=3
+ sum=6
+ '[' 5 -ne 0 -a 3 -lt 5 ']'
+ num=4
+ sum=10
+ '[' 5 -ne 0 -a 4 -lt 5 ']'
+ num=5
+ sum=15
+ '[' 5 -ne 0 -a 5 -lt 5 ']'
+ echo sum=15
sum=15