Linux bash/shell 是如何读取我们给它的命令的？

Question

For example, if I send bash -c " echo "$S" "

How does it read the command, from left to right, or read from left and right together to middle? or else?

If it's not too much, can you list a step by step, like how the linux bash/shell read and treat every single character and symbol it receives?

Answer 1

Assuming that you are issuing this command from a bash shell, and the variable S contains the value xy :

First, the variable is expanded, which would leave you with

bash -c " echo "x      y" " 

Then, the quotes are expanded. Therefore, you are left with 4 token:

• bash
• -c
• echo x (one leading space)
• y (ie y followed by one space)

Next, another bash is started, with the arguments -c , echo x and y .

This bash-subshell would now execute the echo command and print xy .