Operation of shell script programming

First, mathematical operations

1. Expr command

This command can perform mathematical operations on the command line.

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This command is not recommended because it is too cumbersome to use.

2. square brackets and double parenthesis

$[EXPRESSION]

$ ((EXPRESSION))

Expressions with square brackets already appear in the previous instance: $[$a + $b].

3. Let command

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4. Arithmetic Operators

X+=y: Equivalent to X=x+y

X-=y: Equivalent to X=x-y

X*=y: Equivalent to X=x*y

X/=y: Equivalent to x=x/y

x + +: equivalent to X=x+1

x--: Equivalent to X=x-1

Second, string connection

The variable name is added {} When the variable is referenced directly to the string.

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Third, condition test

The expression for a conditional test can use the following three formats:

[Expression]: Note that there is a space between the brackets and the expression;

[[Expression]]: note there is a space between the brackets and the expression;

Test expression: The above two formats are actually the other formats of the test command.

The test command can perform 3 types of conditional judgments: integer test, String test, and file test.

1. Integer Test

-EQ: Tests whether two integers are equal;

-ne: Test whether two integers are unequal, unequal to true, and equal to false;

-GT: Test whether a number is greater than a number, greater than true, otherwise false;

-LT: Test whether a number is less than one number, less than true, otherwise false;

-ge: Test whether a number is greater than one number, greater than or equal to true, otherwise false;

-le: Test whether a number is greater than one number, less than or equal to true, otherwise false;

Example: "Equal" is displayed if two parameters are equal, otherwise "different" is displayed.

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2. Character Test

= =: The test is equal, and so is true, not equal to false;

! =: Test Whether it is unequal, not equal to true, etc for false;

=~: Mode configuration;

-Z String: Tests whether the specified string is empty or null for true

-N String: Tests whether the specified string is empty and empty is true

-S string: Tests whether the specified string is not empty, is not empty, and the null is False

Instance:

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Above is an example of using "-Z", if you change to-N, as shown:

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Note: It is best to add quotation marks when comparing variables (single and double), otherwise if the variable is empty, an error will be given.

3. File Test

-A file: Test file exists as true, otherwise false;

-E File: Tests if file exists, exists as true, otherwise false;

-l/-l file: soft link files;

-B: block equipment;

-C: Character device;

-S: Socket file;

-P: Named pipes;

-S: Non-empty files are true;

-F File: Tests whether the file is a normal document;

-D FILE: Tests whether the specified path is a directory;

-R file: whether the test files are readable to the current user;

-W File: Whether the test file is writable to the current user;

-X file: Tests whether the file is executable to the current user;

-N File: The modification of the file time is more true than the new access time;

Content is redirected to a file, but the file has not been opened

File1-nt file2:file1 than File2 new is true;

File1-ot File2:file1 is more true than the old file2.

Example: If/test is present, rename it to/new-test, otherwise create this directory:

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Example: Given a parameter, first determine whether this parameter exists, if there is a decision whether the file, if the file to ensure that it has execute permissions:

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Verify:

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4. Combination test conditions

Implementing logical operations between multiple conditions

-A: With relation [condition1-a Condition2]

-O: or relationship [Condition1-o Condition2]

!: Non-relationship [! condition] or [-not condition]

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Iv. the logical relationship between commands

&&: Logical AND, the right statement is executed only if the condition on the left is true;

|| : Logical OR, the right statement executes when the left condition is false.

The above example can also be done using logic and:

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Operation of shell script programming