Shulou(Shulou.com)06/03 Report--

Shell script Theory part 2-conditional Test & IF judgment condition Test

In the last blog post, we mentioned that there is a $in the predefined variable. Variables are used to determine whether the command was executed successfully. So how do you use it?

At this point, you need a special testing tool, the test command, to test specific conditions and use $? To judge the return value, the command form is as follows:

Test conditional expression

&

[conditional expression] / / more commonly used

Note: brackets "[" and "]" need to be separated by a space from the conditional expression, otherwise an error will occur!

I. document testing

Format:

[operator file or directory]

Common operators:

-d: test whether it is a directory

-e: test whether the file / directory exists

-f: test whether it is a file

-r: test whether the current user has read permissions

-w: test whether the current user has write permission

-x: test whether the current user has execute permissions

How to use it:

Second, integer value test

Format:

[integer 1 Operand integer 2]

Common operators

-eq: equal to

-ne: not equal to

-gt: greater than

-lt: less than

-le: less than or equal to

-ge: greater than or equal to

How to use it:

Third, string testing

Format:

[string 1 operator string 2]

[- z string] / / determine whether the content of the string is empty

Common operators:

=: the string content is the same

! =: the string content is different

How to use it:

IV. Logic testing

Format:

[expression 1] operator [expression 2]

&

Command 1 operator Command 2

Common test characters:

-an or &: logic and the meaning of "and"

-o or | |: logical OR, meaning "or"

! The logic is no, the conclusion is reversed.

How to use it:

IF statement to judge single branch structure

Format:

If conditional test operation

​ then command sequence

Fi

Structure flow chart:

Double branch structure

Format:

If conditional test operation

​ then Command sequence 1

​ else Command sequence 2

Fi

Structure flow chart:

Multi-branch structure

Format:

If conditional Test Action 1

​ then Command sequence 1

Elif conditional Test Action 2

​ then Command sequence 2

Else command sequence 3

Fi

Structure flow chart:

Unfinished to be continued ~

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