Shell programming in Linux

Prerequisites: The shell environment pdksh and Bash; the default shell environment in Ubuntu is Bash.

 

Syntax:

<1> Variables

1. There are three types of variables: environment variables, internal variables, and user variables. The environment variables and internal variables can be directly used without user-defined requirements.

Environment variable: defined by the system. You do not need to define but can change the value of the variable.

Internal variables: a special type of variables provided by the system. This variable is used in the program to make a decision. These variables cannot be modified in Shell programs.

Some internal variables are as follows:

$ # Number of location parameters passed to the shell program;

$? The final command completes the Code or the shell program executed in the shell program;

$0 shell program name;

$ * The shell program is called to send a single variable string.

User variables: User-Defined variables that can be modified by users.

Note: 1. variables are non-typed (if any );

2. No need to declare
3. Assignment Example A = 10; "=" cannot have spaces on either side

There are three types of string assignments: "=" and then directly followed by the string; the string is enclosed by single quotation marks; the string is enclosed by double quotation marks. But there are differences.

1. When a string contains spaces, the first or third type cannot be used;

2. Add ''to prevent shell from parsing variables.

3. Add "" to parse all variables in the string

For example, an error occurs when x = ABC def. The correct format is X = 'abc def 'or X = "ABC Def"

 

Double quotation marks

Var = "test string"

Newvar = "value of VaR is $ Var"

Echo $ newvar

Output result: Value of VaR is test string

 

Single quotes

Var = "test string"

Newvar = 'value of VaR is $ var'

Echo $ newvar

The output result is: Value of VaR is $ var.

 

Backslash

A backslash can be used before a character to prevent shell from interpreting subsequent characters as special characters.

Test = 3

Var = $ Test

Echo $ VaR

Output result: 3

Test = 3

Var =/$ Test

Echo $ VaR

The output result is: test.

<2> Expression comparison

The operators that execute logical comparison in different shells are different. In pdksh and bash, a command named test can be used to compare expressions. The test command supports the following four types of comparison:

1. String comparison

= Compare whether two characters are equal

! = Compare whether two strings are not equal

-N: whether the length of a string is greater than zero

-Z: whether the string length is equal to zero

2. digit comparison

-EQ: Compares two numbers for Equality

-Ge: determines whether the first number is greater than or equal to the second number.

-Le: determines whether the first number is less than or equal to the second number.

-Ne: determines whether two numbers are different.

-GT: compare whether the first number is greater than the second number.

-Lt: compare whether the first number is smaller than the second number

3. File Operation comparison

-D: determines whether the file is a directory.

-F: determines whether a file is a common file.

-R: determines whether the read permission is set for the object.

-S: determines whether the length of the file name is greater than 0

-W: Determine whether the file has write permission

-X indicates whether the file has the execution permission.

4. Comparison of logical operators

! Non-logical

-O logic or

-A logic and