Shell operator, $((expression)) "or" $[expression], expr method, condition judgment, test condition, [condition], comparison between two integers, judgment according to file permission, judgment accor

1. Operator

1.1 Basic grammar

“$(( Arithmetic expression ))”  or  “$[ Arithmetic expression ]”

1.1.1expr Method

 use expr Output +、-、*、%
 Symbol (+、-、*、%) Space is required on both sides 
 use * When you need this \, because * There are many uses for , So we need to distinguish 

1.1.2 use expr to a assignment

You need to replace

a=$( Arithmetic expression ) Or use a=` Arithmetic expression `

1.1.3$(( Arithmetic expression )) or $[ Arithmetic expression ]

1.1.4 Calculate the sum of two numbers with script

stay add.sh Write the following code in

2. conditional

2.1 Basic grammar

1）test condition
2）[ condition ]（ Be careful condition Space before and after ）
Be careful ： If the condition is not empty, it is true,[ atguigu ] return true,[ ] return false.

2.2 Common judgment conditions

2.2.1 Compare two integers

-eq be equal to （equal） -ne It's not equal to （not equal）
-lt Less than （less than） -le Less than or equal to （less equal）
-gt Greater than （greater than） -ge Greater than or equal to （greater equal）
notes ： If it's a comparison between strings , Use the equal sign “=” Judge equal ; use “!=” Judgment is unequal .

2.2.2 Judge according to the file authority

-r Have read permission （read）
-w Have the right to write （write）
-x Have the authority to execute （execute）

2.2.3 Judge according to the document type

-e File exists （existence）
-f The file exists and is a regular file （file）
-d The file exists and is a directory （directory）

2.2.4 test

2.2.4.1test condition

Test whether his output is correct or wrong $? To judge , Output 0 Then the expression is true , Output 1 Expression is false

2.2.4.2[ condition ]（ Be careful condition Space before and after ）

use [] When , The equals sign inside must be left blank on both sides , Otherwise, it will be recognized as a whole. If the condition is not empty, it is true

When [] There's something in it , It is true , False when there are only spaces , If there is no space before and after, the command will not be found

2.2.4.3 stay [] It uses = and ！=

2.2.4.4-eq be equal to （equal） -ne It's not equal to （not equal）

If it's a comparison between strings , Use the equal sign “=” Judge equal ; use “!=” Judgment is unequal

2.2.4.5-lt Less than （less than） -le Less than or equal to （less equal）

Out-of-service < and <=, Because in linux In indicates input redirection

2.2.4.6-gt Greater than （greater than） -ge Greater than or equal to （greater equal）

Out-of-service > and >=, Because in linux in > Indicates output redirection

2.2.4.7 Determine file permissions 、 -r Have read permission （read）、 -w Have the right to write （write） 、 -x Have the authority to execute （execute）

2.2.4.8 Determine file type 、-e File exists （existence）、-f The file exists and is a regular file （file）、-d The file exists and is a directory （directory）

2.2.4.9 Multi condition judgment （&& Indicates that the previous command is executed successfully , To execute the last command ,|| Indicates the last After a command fails to execute , To execute the next command ）