Description of the Shell's conditional expressions

Determine if b.txt This file exists, there is output 1, there is no output 0

[-F B.txt]&& echo 1| | Echo 0

-F: Determines whether the file is

-E: Determine if the file exists

-D: Determine if the directory

-R: Judging whether it is readable

-W: Determine if writable

-X: Determine if executable

Double quotation marks are required to test a single file or directory variable to avoid errors

file=/etc/services[-F "$file"]&& echo 1| | Echo 0

Conditional expression judgment condition after executing multiple command statement wording

#!/bin/bash[$1-eq 2]&&{echo "true"}| | {echo "false"}# && executes the following statement after it is established; | | Do not set up to execute the following statement # If the input value equals 2 print true# otherwise print false#sh test.sh 2: Print True

Common String test operators:

-Z "string"	True if string length is 0
-N "String"	True if string length is not 0
"String 1" = "String 2"	String 1 equals string 2 is True
"String 1"! = "String 2"	String 1 is not equal to string 2 is True

Ps:

①, the string test operation symbol in the table above must be caused by ""

②, compare symbols must have spaces at both ends

#字符串长度为0所以输出1 [-N ""]&& echo 1| | Echo 0

Description of the Shell's conditional expressions