Conditional statements for shell programming
shell conditional statement
I. Conditional testing
Test command:
Tests whether a specific expression holds true. When the condition holds, the return value of the test statement is 0, otherwise it is another value.
Format 1: test conditional expression
Format 2:[Conditional Expression] There should be at least one space between the conditional expression and the square brackets.
II. Document testing
[operator file or directory]
Common test operators
-d: Test whether it is Directory
-e: Test whether the directory or file exists (Existing)
-f: Test whether it is a file
-r: Tests whether the current user has permission to read (Read)
-w: Tests whether the current user has permission to write (Write)
-x: Test whether the current user has permission to execute (eXcute)
III. Comparison of integer values
[integer 1 operator integer 2 ]
Common test operators
-eq =(Equal)
-ne: not equal
-gt: Greater Than
It is (less Than)
-le: Less or equal
-ge: Greater or Equal
IV. String Comparison
[String 1 = String 2 ]
[String 1 != String 2 ]
format 2
[ -z string]
Common test operators
=: String content identical
!=：String content is different,! The sign means the opposite.
-z: string content is empty
logic test
Format 1:[Expression 1 ] operator [Expression 2 ]
Format 2: Command 1 Operator Command 2
Common test operators
-a or &&: logical and, meaning "and"
-o or|| logical OR logical OR
!：logical no
Unary operator: i=1
i++: i=$i+1 (i=++i first operation then assignment;i=i++ first assignment in operation)
binary operator
a+b=c
ternary operator
Conditions && Results 1|| results 2
Structure of the if statement
1. single branch structure
2. double branch structure
3. multibranched structure
