Linux Shell script variables and process control

[ Simple use of shell script creation ]

All of the following operations are based on an already-established 1.shscript

Variable declaration

The declaration of Shell variables is similar to the declaration of variables in Python, ie

变量名=值
a=123

a=123
b=456
c="hello world !"

It is worth noting that

• No spaces before and after the equal sign
• Variable naming rules, to be more standardized, can not be underlined, what starts with a question mark
• The default type of the variable is a string , even if a = 123, a is actually “123”, as for how to add and subtract, detailed below

Variable use

When using a variable, you cannot directly type the variable name, because we can think of the script as typing on the command line. We directly type an a. How does the system know whether to type the a command or call the a variable?

Variable references need to be prepended with a $dollar sign to tell the system that this is a variable. In addition, if you use ${变量名}parentheses, the name of the variable will be further refined and less prone to errors, but this bracket is not necessary

Write a simple script to verify:

#!/bin/bash

a=123
b=456
c="hello world !"

echo $a+$b
echo $c

It can be seen that a + b does not produce an addition operation, because the default variable type is a string , and our + sign is also a string, which is similar to the replacement of macro definition in C ++ . . .

Formatted output

Sometimes we want to output a description string in front of the variable. For example “这个字符串的值是: abc”, abc is the value of a variable. At this time, when thinking of printf , you can use a method similar to printf. You can also remove the quotation marks . Definition replacement

echo "说明语句 $变量名"
echo 说明语句 $变量名

Run this script

#!/bin/bash

a=123
b=456
c="Hello World"

echo "a+b=$a+$b"
echo a+b=$a+$b
echo c is :$c
echo "c is :$c"

Addition, subtraction, multiplication and division of variables

expr statement

Use expr This is a statement to declare an expression, use the expr note 反引号enclosed, and ~is a key, press the shift is ~, it does not follow that 反引号, under the ESC

`expr $变量a 运算符 $变量b`

Modify the script just now

#!/bin/bash

a=123
b=456
c=`expr $a + $b`

echo $a+$b
echo $c

Verifying the above statement again, you can see that the first line is direct $a+$b, which is equivalent to a simple replacement of the macro definition string. The second line uses the expr statement to really add and subtract. It is worth noting that the operator must have Space

Other operators

Add, subtract, multiply and divide

#!/bin/bash

a=123
b=10

echo a + b = `expr $a + $b`
echo a - b = `expr $a - $b`
echo a \* b = `expr $a \* $b`
echo a / b = `expr $a / $b`
echo *

Escape of multiplication

It is noteworthy that, the multiplication needed \*to escape a direct hit *, through all the files in the current folder name , and java is import java.xxx.*;similar to the following example demonstrates the use of \*importance to escape

Relationship judgment

[ $变量1 == $变量2 ]  ：变量1是否  [等于]	变量2, 返回true or false
[ $变量1 != $变量2 ]  ：变量1是否 [不等于]	变量2, 返回true or false
[ $变量1 -eq $变量2 ] ：变量1是否  [等于]  	变量2, 返回true or false
[ $变量1 -ne $变量2 ] ：变量1是否 [不等于]	变量2, 返回true or false
[ $变量1 -gt $变量2 ] ：变量1是否  [大于]	变量2, 返回true or false
[ $变量1 -lt $变量2 ] ：变量1是否  [小于]	变量2, 返回true or false
[ $变量1 -ge $变量2 ] ：变量1是否 [大于等于] 变量2, 返回true or false
[ $变量1 -le $变量2 ] ：变量1是否 [小于等于] 变量2, 返回true or false

解释：
eq : equal 等于
ne : not equal 不等于
gt : greater than 大于
lt : less than 小于
ge : greater equal 大于等于
le : less equal 小于等于

if judgment statement

The if statement is similar to most of the statements, except that the end of the} becomes fi, which can be understood as final, the meaning of the end, remember to add fi at the end

if 条件语句
then
	语句1
	语句2
	...
	语句n
fi

You can also add an else statement

if 条件语句
then
	语句1
	语句2
	...
	语句n
else
	语句1
	语句2
	...
	语句n
fi

Demo

Execute the following script and produce the following output

#!/bin/bash

a=123
b=124

if [ $a == $b ]
then
        echo a=b
        echo a=b hhhhh
else
        echo a!=b
        echo a!=b AAhhhh
fi

Another form

It is worth noting that it is surrounded by two brackets , and then judged like a general programming language, that is, you can directly write the greater than and less than symbols

Executing the following script will produce the following output

#!/bin/bash

a=123
b=124

if (($a < $b))
then
        echo a=b
        echo a=b hhhhh
else
        echo a!=b
        echo a!=b AAhhhh
fi

loop statement

while statement

Similar to if, the ending} is replaced with done

while 条件
do
	语句1
	语句2
	...
	语句n
done

Remember that you just created 5 txt, we delete them in a loop, print the files in the current directory while deleting, execute the following script, and produce the following output

#!/bin/bash

ls

i=1

while (($i <= 5))
do
        rm $i.txt
        i=`expr $i + 1`
        ls
done

About self-increasing, other detailed methods

i=`expr $i + 1`
let i+=1
let "i++"
((i++))

for statement

Enumerate iteration variables

By enumerating v1, v2, to achieve loop

for 迭代变量 in v1 v2 v3 ... vn
do
	语句
	...
	语句
done

#!/bin/bash

for i in 1 2 3
do
        echo $i
        let "i++"
done

for + seq sequence to achieve loop

seq statement: (don't forget the backticks)

`seq 首项 公差 末项`

Execute the following script and produce the following output

#!/bin/bash

for i in `seq 0 2 8`
do
        echo $i
done