Shell programming of conditional statements

Conditional test

The Shell environment judges whether the execution is successful according to the return status value " $? " after the command is executed. When the return value is 0, it means success, otherwise it means failure or exception (non-zero value).

Using the special test tool test command, you can test a specific condition and judge whether it is true or not according to the return value (value is 0).

test conditional expression

[Conditional Expression]           #Both sides of the conditional expression need to have spaces, no spaces will report an error, and this method is more commonly used

[[ conditional expression ]]

Options:

-a Whether the file exists (-a will be a problem if it is negated, it is recommended to use -espan>)

-e file exists

-f to judge whether it is a normal file

-d determines whether it is a directory file

-r determines whether the current user can read the file

-w determines whether the current user can write to the file

-x determines whether the current user can execute the file

ps: The difference between [ ] and [[ ]] [] does not support regular nuclear wildcards [[]] supports

All permissions for super administrators:Except x, other discomforts are limitedp>

1. Others do not have rw super admins do

2. Others don't have x super administrators either, as long as other users have x super administratorsMembers also have

Integer comparison

General formula: [ integer1 operator integer2 ]

Operator:

-eq equals

-ne is not equal to

-lt is less than

-gt greater than

-le less than or equal to

-ge greater than or equal to

String comparison

Format:

[string1=string2]

[string1!=string2]

[ -z string ]

Common test operators:

=: The first string is the same as the second string

!=: The first string is not the same as the second string, where the "!" symbol means negation

-z: empty string content

Logic Test

Logical testing refers to determining the dependencies between two or more conditions.When the system task depends on several different conditions, it is necessary to have a testing process to judge whether these conditions are established at the same time or only one of them is established.

Format:

[ expression 1 ] operator [ expression 2 ]

Command 1 Operator Command 2

Common test operators:

&&: Logical AND, which means that the return value of the entire test command is 0 (the result is true) only when both conditions are true.
When using the Test command to test, "&&" can be changed to "-a".

||: Logical OR, indicating OR, as long as one of the two conditions before and after is established, the return value of the entire test command is 0 (the result is established).
When using the Test command to test, "||" can be changed to "-o".

!: Logical NO, indicating "no", only when the specified condition is not met, the return value of the entire test command is 0 (the result is established).

Short-circuit and (and): &&

All true is true, and false is false

cmd1 && cmd2

1. The first command is false and there is no need to execute the command 2 should be positivefalse

2. The first command is true and I need to execute command 2 before I can judge whether it isTrue

Short or (or): ||

​​​​​​​A true is true, all false is false

cmd1 || cmd2

1. The first command is true, no need to execute command 2, it should be affirmativetrue

2. The first command is false, and the whole formula will be judged after executing command 2Is the child true

if:Conditional judgment statement

​​​​​​​To be continued