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.
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.
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.
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.
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.
10.10.2 “cal” Command and “ncal” Command
This command performs the function to print the calendar. While the “cal” command shows it in the traditional calendar format with a simpler type, the “ncal” command can display it in a variety of formats and use various options.
The following describes the usage format for the “cal” command.
10.10 Commands Related to System Information
10.10.1 “date” Command- Confirm & Change Time Information
The time zone set in the current system can be changed using “raspi-config” command. For details, refer to the description of [9.2 How to Configure the System].
If the system is connected to the Internet, it will connect to the time server on the network and set the time automatically. This configuration is handled automatically each time the system boots. Therefore, even if you change the time manually, the next time you boot, it will be reset to the time of time server.
On Linux, use the “date” command to directly query or adjust the system date and time.
10.1.4 Utilizing Command Execution History
When working at the Terminal, you may work a task repeatedly on multiple files, or work multiple tasks on a specific file. In addition, there are times when you need to reconfirm what tasks you have done previously in multiple steps. In some cases, you may want to correct partially and rework the command in case the input is wrong and an error occurs. When you execute various commands, it is useful to know the history of commands that you have executed before.
10.1.3 Quoting Rule
10.1.3.1 Overview of Quoting Rule
Quoting is to enclose a string with quotation marks. The reason for doing this is to prevent the special characters from being reinterpreted or expanded by the Shell or Shell script when special characters are included in the string. For reference, “special character” refers to a character that can be interpreted differently from the meaning of a character. For example, the wildcard character “*” is a special character.
10.1.2 Special Key of Shell
In Shell, special keys are defined in advance to perform special functions.
key
function
description
ctrl+d
terminate input
Same as logout or exit. Log out of the shell (similar to exit).
“EOF” (End of file, end the foreground job).
End incoming input from standard input.
ctrl+c
terminate program
terminate a running program
ctrl+z
switch to background
Switch the running job to background and pause.
ctrl+s
halt screen output
halt screen output
ctrl+q
resume screen output
Restart screen output interrupted by ctrl + s