Bash operations, tests, if statements, and for loop statements

I. logical operations

Boolean: true or false

And operation:

True & True = true

True & false = false

False & True = false

False & false = false

Or operation:

True | true = true

True | false = true

False | true = true

False | false = false

Non-calculation:

False = false

Non-false = true

Ii. Status Return Value

Echo $?

Success: 0 is returned.

Failed: 1-255 is returned. Failure is not equal to false.

Custom: exit [N]

Iii. Arithmetic Operations

1. Define Integer Variables

Let var_name = integer_value

Declare-I var_name = integer_value

If no integer variable is defined, the numeric type will be converted by Bash to participate in arithmetic operation.

2. Calculation Method

Let var_name = arithmatic_expression

Var_name = $ [arithmatic_expression]

Var_name = $ (expression ))

Var_name = $ (expr expression)

3. Arithmetic Operators

+,-, *,/, %, **, + =,-+, * =,/=, % =, ++

Iv. Comparison Test

1. Integer comparison test

-GT: greater

-Lt: less

-Ge: greater than or equal

-Le: less than or equal

-EQ: equal

-Ne: not equal

2. String comparison test. compare strings based on the size of the ascll code.

Binary operators:

>:

<:

>=:

<=:

=:

! =:

= ~ : Pattern matching, ["$ Var" = ~ Pattern]

Single Object OPERATOR:

-N string: whether it is null. If it is null, it is true. If it is null, it is false.

-Z string: whether it is null. If it is null, it is true. If it is not null, it is false.

3. file comparison test

-A/-E file: if the file exists, it is true. Otherwise, it is false.

-F file: if a common file exists, it is true. Otherwise, it is false.

-D file: the directory file that exists and is true; otherwise, the file is false.

-S file: an existing and non-empty file is true. Otherwise, the file is false.

-R file: If a readable file exists, it is true. Otherwise, it is false.

-W file: an existing and writable file is true. Otherwise, the file is false.

-X file: if an executable file exists, it is true. Otherwise, it is false.

-L/-H: indicates that an existing file with a symbolic link is true. Otherwise, the file is false.

-B file: If the block device already exists, it is true. Otherwise, it is false.

-C file: If a character device exists, the device is true. Otherwise, the device is false.

-S file: The file that exists and is a Socket socket is true. Otherwise, the file is false.

File1-nt file2: The mtime of file1 is true if it is newer than file2; otherwise, it is false.

File1-ot file2: The mtime of file1 is true if it is earlier than file2; otherwise, it is false.

V. Test Methods

1. Test expr

2. [expr]

3. [[expr]

Test comparison is usually performed only between the same type

6. For Loop statements

Views a limited list of elements. The statements are separated by commas (,) in the same row.

Syntax:

For var_name in list

Do

Loop body

Done

List: List, which contains one or more elements.

Exit condition: the traversal ends.

VII. If Condition Statement

Single branch if statement:

If test condition; then

Select Branch

Fi

If statement with two branches:

If test condition; then

Select branch 1

Else

Select branch 2

Fi

Multi-branch if statement:

If test condition 1; then

Select branch 1

Elif test condition 2; then

Select branch 2

Elif test condition 2; then

Select Branch 3

...

Elif test condition n; then

Select branch n

Else

Select branch {n + 1}

Fi

This article from the "Xiao Yang" blog, please be sure to keep this source http://princepar.blog.51cto.com/1448665/1653459

Bash operations, tests, if statements, and for loop statements