Raspberry Pi_Eng_10.6.1 “&&” Command – Execute Command by “and” Method

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10.6 Commands Related to Selective Execution

Normally, when a command is processed normally, the exit status is set to zero, which is ㅅtreated as “True”. On the other hand, if the command is not processed normally, the exit status is set to a non-zero value, which is treated as “False”.

10.6.1 “&&” Command – Execute Command by “and” Method

This command processes the next command if the first command is processed normally, and sets the exit status by “and” operation according to the final processing result of the both commands.

Raspberry Pi_Eng_10.4.4 “alias” Command

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10.4.4 “alias” Command

On Linux, there is a feature that allows you to specify an alias name for a certain string in advance, and then use the defined alias name later instead of the string. The command to use at this time is the “alias” command.

Raspberry Pi_Eng_10.4.3 “man” Command – Get Help on Commands

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10.4.3 “man” Command – Get Help on Commands

Most of the commands used in Linux can perform a variety of functions in a single command. In order to correctly execute these various functions provided by a specific command, the command must be input in various formats. Also, the execution result of the command is displayed in various types and various forms. In reality, for each command, we can hardly remember all necessary knowledge about what functions the command can do, and how to instruct the command, and how to understand the outcome of the execution.

So Linux provides detailed help for each command. With the help, users can search various informations that they can not remember when they need it, and solve the problem. The “man” command is used in this case.

Raspberry Pi_Eng_10.4.2 “echo” Command

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10.4.2 “echo” Command

This command displays the specified text on standard output. It is not necessary for common task, but is often used to display information about various progresses when writing bash scripts.

Raspberry Pi_Eng_10.4.1 “clear” Command – Clear Screen

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10.4 Basic Command

10.4.1 “clear” Command – Clear Screen

When you work on the Shell Terminal, various contents are displayed on the screen. It can be inconvenient if the content of the screen is large or complex. In this case, you want to clear the current Terminal screen and start a new one. When you want to erase all the contents of the screen and start a new one, use the following command.

Raspberry Pi_Eng_10.3.2 Applied Priority of Profile

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10.3.2 Applied Priority of Profile

When the system is started and a particular user logs on, what is applied to the system is the final result of the various profiles integrated according to their priority. By default, the local profile has priority over the global profile, and the application profile has a higher priority than the user profile.

The applied priority is that the above has a higher priority as shown below.

Raspberry Pi_Eng_10.3.1 Type of Profile

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10.3 Shell and Profile

10.3.1 Type of Profile

There are many files and programs installed in the Raspberry Pi system. When working on the system, these items mutually interact with each other under certain control, and the necessary work is done. The results will vary depending on which files and which programs is used when you perform certain tasks. Therefore, in order for the system to operate consistantly, it is necessary to control the user’s working environment by instructing the system which program to use, which data to use, and how to operate.

Raspberry Pi_Eng_10.2.2 Environment Variable

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10.2.2 Environment Variable

When the system is booted and a particular user logs on, the user is presented with a predefined screen and if the user issues a certain command, the command is processed in a predefined manner. At this time, the user confirms the contents defined in advance and determines the concrete processing method. When making important decisions that is necessary for the system to works, the system refers to a number of predefined data. Among these data, the important variables that the system frequently uses are stored in a special place, which is called an environment variable.

Raspberry Pi_Eng_10.2.1 Variable

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10.2 Shell Environment

10.2.1 Variable

10.2.1.1 What is Variable?

Variable is a place where data can be temporarily stored in the computer’s memory. The difference with the file is that when the computer is shut down, the data in the file is retained when the computer is turned on again, and the data in the variable is lost when the computer is shut down.

If you use multiple words to define variable,, you can use “_” to connect them together.