Shulou(Shulou.com)06/02 Report--

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

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