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
10.1 Command Overview
10.1.1 Standard Input and Output
In the Shell, a standard device is already specified for the input device that receives the input data for execution and the output device for which the processing result is displayed. Therefore, unless there is a specific designation, data is always input from the standard input device, and the processing result is always output to the standard output device.
Chapter 10 Understanding Shell Command
Chapter’s Main Topics
This chapter describes the Shell command. It will first explain the basics of the Shell command, explain how to use the Shell command, and then describe the most basic Shell commands. We will not cover all the Shell commands here, and commands associated with a particular topic will be discussed together when dealing with the topic.
It includes descriptions on the following topics:
■ Command overview
■ Shell environment
■ Shell and profile
■ Basic command
■ Operation processing related commands
■ Selective execution related commands
■ Program execution related commands
■ Input and output adjustments related commands
■ Data manipulation related commands
■ System information related commands
9.9.2 Setting Hostname
To set the host name for the Raspberry Pi system, use the [Advanced Options] item as shown below. Select the item and press the [Enter] button.
9.9 Network Related Setting
This section describes the configuration of the network related environment:
■ Setting hostname
Hostname is the name given to the computer itself, which is the name that other computers will use to refer to the current computer when communicating with other computers on the network. The Raspberry Pi system is named “raspberrypi” by default at the time it is installed. If you want to change it, you can specify it as a different name when you set the booting item.