Published Book on Amazon
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All of IOT Starting with the Latest Raspberry Pi from Beginner to Advanced – Volume 1 |
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All of IOT Starting with the Latest Raspberry Pi from Beginner to Advanced – Volume 2 |
출판된 한글판 도서
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최신 라즈베리파이(Raspberry Pi)로 시작하는 사물인터넷(IOT)의 모든 것 – 초보에서 고급까지 (상) |
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최신 라즈베리파이(Raspberry Pi)로 시작하는 사물인터넷(IOT)의 모든 것 – 초보에서 고급까지 (하) |
Original Book Contents
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.
| RUN_MODE="daemons" PIDDIR=/var/run/samba |
10.2.1.2 Types of Variables
There are system variable and local variable according to the range to which they are applied. They have the following features:
● System variable
System variable is defined as a pre-reserved name at the Raspberry Pi system level and has the same variable name in any script, and can not be used for any other purpose in the script. Variables such as $ PATH are system variables.
These system variables are as follows:
■ Environment variable
| variable name | description |
| BASH | Shell name |
| BASH_VERSION | Shell version |
| COLUMNS | column |
| LINES | line |
| HOME | home directory |
| LOGNAME | login name |
| OSTYPE | OS type |
| PATH | path setting value |
| PS1 | prompt setting value |
| PWD | working directory |
| SHELL | Shell name |
| USERNAME | current login user id user ID |
Table 10‑1 Environment variable
■ Other special System variable
| variable name | description |
| $$ | PID value of the corresponding process |
| $? | return value of the most recently executed command |
| $# | Number of arguments passed to the program |
| $0 | Variable value to remember command name when executing the command |
| $n | The n-th passed argument (ex: $1, $2) |
| $* | Marks all passed arguments as a single string |
| $@ | List all passed arguments as a list of strings |
| $! | PID value of the last executed background process |
Table 10‑2 Special System variable
● Local variable
On the other hand, the local variable is the variable that is used only when the script is executed. As the variable itself disappears and the value is also lost when the execution of the corresponding script is terminated, the local variable is restarted with the new value when the script is executed next time. Variables of the same name can be used for other purposes in other scripts,, and there is no correlation between them.
10.2.1.3 Declaring Data Types of Variable
In the Shell, variables are not defined before use. Values stored in variables are treated as strings by default and are case-sensitive. Numbers alos are basically treated as strings, but calculations are allowed for arithmetic operations in the same way as ordinary numbers.
If you want to restrict the use of data type for a variable, you may use it after explicitly defining it with "declare" or "typeset" statements.
[Command Format]
| declare/ typeset <변수> <Type> |
[Command Overview]
■ This specifies the data type that can be used for a variable.
■ User privilege -- Normal user.
[Detail Description]
■ The following data types can be specified:
■ -i -- Integer
■ -a -- Array
■ -f -- Function
■ -r -- Read only
■ -x -- Available variable outside the Shell
10.2.1.4 "unset" Command - Delete Variable Definition
The "unset" command is to remove previously defined variables and make them undefined.
[Command Format]
| unset <variable> |
[Command Overview]
■ This deletes the previously defined variable.
■ User privilege -- Normal user.
[Detail Description]
■ After unset, the variable does not exist.
[Used Example]
| pi@raspberrypi ~ $ var="1234" |
| pi@raspberrypi ~ $ echo $var |
| 1234 |
| pi@raspberrypi ~ $ unset var |
| pi@raspberrypi ~ $ echo $var |
| -bash: var: unbound variable |
10.2.1.5 "set" Command - Set Shell Call Parameter
When you call the Shell script file, you can pass parameters. When you refer to this parameter in the script, You can refer to it as a variable named $1, $2, .... The "set" command sets the value of this parameter given when the script is called.
[Command Format]
| set value1 value2 ….. |
[Command Overview]
■ This sets the value of the parameter to be passed when the script is called.
■ User privilege -- Normal user.
[Detail Description]
■ The value to be set is assigned to $1, $2, $ 3 ... in order.
[Used Example]
| pi@raspberrypi ~ $ set "This is 1" "This is 2" |
| pi@raspberrypi ~ $ echo $1 |
| This is 1 |
| pi@raspberrypi ~ $ echo $2 |
| This is 2 |