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This article is about how to use judgment statements in Shell scripting. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

1. Single branch if statement

Format: if judgment condition; then

Statement1statement2...fi

Second, double-branched if statement

Format: if judgment condition; then

Statement1statement2...elsestatementN...fi

Use a previously used script to illustrate this structure.

III. Multi-branch if statement

Format: if judgment condition 1 then

Statement1...elif judgment condition 2; thenstatement2...elif judgment condition 3; thenstatement3...elsestatement4...fi

IV. Case statement

Format: case variable in

PATTERN1) statement...;;PATTERN2) statement... ;; *) statement...;;esac

Because judging the structural statement is relatively simple, there is not too much explanation, just remember the structural usage of each sentence.

Supplement shell basic syntax

1.1.The beginning of shell file

The shell file must start with the following line (must be on the first line of the file):

#! / bin/sh

Symbol #! It is used to tell the system that the parameters behind it are the program used to execute the file. In this example, we use / bin/sh to execute the program.

When you edit a script, you must also make it executable if you want to execute it.

To make the script executable:

Run chmod + x filename so that you can run it with. / filename

1.2 comments

In shell programming, sentences that begin with # indicate comments until the end of the line. We sincerely recommend that you use comments in your program.

If you use comments, you can understand how the script works and how it works in a short period of time, even if you haven't used the script for a long time.

1.3 variables

In shell programming, all variables are made up of strings, and you don't need to declare them, just assign them, and apply them in the form of $+ variable names.

To assign a value to a variable, you can write:

A = "hello world"

Now print the contents of the variable a:

Echo "An is:" echo $a

Sometimes variable names are easily confused with other words, such as:

Num=2 echo "this is the $numnd"

This does not print out "this is the 2nd", but only "this is the", because shell searches for the value of the variable numnd, but this variable has no value. You can use curly braces to tell shell that we are printing the num variable:

Num=2 echo "this is the ${num} nd"

This will print: this is the 2nd

1.4 Environmental variables

Variables that have been processed by the export keyword are called environment variables. We don't talk about environment variables, because usually we just log in

Environment variables are used in the script.

Thank you for reading! This is the end of this article on "how to use the judgment sentence of Shell scripting programming". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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