Detailed explanation of bash command usage

On Linux, bash is adopted as the standard, which basically describes the processing and execution of text with a ".sh" extension like the vi editor.

As with programming, it has many features like variables, functions, and arithmetic processing, so if you are making a small program, you can write it in bash.

Also, since bash is executed by the shell, it is also called a shell script.

Create a shell script

We start by creating a simple script that prints "Hello World!!" to the console.

Use the vi command to create a new file.

$ vi hello.sh

Once the editor opens, write as shown below.

#!/usr/bin/bash
echo "Hello World!!"
exit 0

The "#!/usr/bin/bash" on the first line indicates that it is a shell script using bash.

The second line describes the statement to be executed.

Finally, exit bash using "exit 0". Parameter 0 indicates normal termination.

After creating the file, execute the shell script using the bash command.

$ bash hello.sh

Execution Result:

Hello World!!

Hello World!! is output

In addition, when executing shell scripts, in addition to bash, there are also commands that change execution permissions and run with "./".

$ chmod 755 hello.sh
$ ./hello.sh

There is a way to execute it using sh command.

$ sh hello.sh

Shell scripts can be used for commenting as well as programming.

Comments can be written after "#".

#!/usr/bin/bash
echo "Hello World!!"
#End processing.
exit 0

Shell scripts can define variables and assign values.

#!/usr/bin/bash
 
num=100
PI=3.14
STR1="Hello"
str_2="World!!"
 
echo ${num}
echo ${PI}
echo ${STR1}
echo ${str_2}
 
exit 0

Variables can be alphanumeric characters, such as uppercase and lowercase letters, numbers, and underscores (_).

When assigning a value to a variable, write it as "variable = value".

Note that if you put spaces before and after the "=", it will cause an error.

In addition, when accessing a variable, you need to add "$" before the variable name, such as "${variable}", and enclose the variable in "{}".

Input and Output

#!/usr/bin/bash
 
read AGE
echo "ege=$AGE"
 
exit 0

Execution Result:

30
ege=30

read stores the content input from the console into the variable specified in the parameter.

The variables specified by read can be called ordinary variables.