Function description of shell command parser

1. shell What is it?

shell It's just an application , We can use the keyboard 、 Serial port sends commands to it , After entering the car, it will execute these commands .

With “ls  -l” Command as an example ,

① Receive keyboard data and echo

such as ： We use the keyboard to shell The program sends letters l, When it is received, it will be displayed on the screen

② Parse the input string , Find program , Execution procedure ：

When we use the keyboard to shell When the program sends a carriage return , It knows that character input is over

a. shell Can parse strings , These strings are divided into several parts with spaces

The first 1 Part is the program name 、 Command name ,

Other parts are parameters

b. shell Will look for the program , Where to find ？ Go to PATH Find the location indicated by the environment variable .

   Of course, you can also specify absolute 、 Relative paths ,shell You will go directly to these paths to find the program

c. Find the program , Will start the program , And pass in parameters

2. How to set PATH

In the PATH Add /home/book For example, catalog ：

2.1 Permanent setting method 1, This works for all users ：

modify /etc/environment, such as ：sudo gedit /etc/environment , Then add the downlink red envelope ：

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/xue"

Then restart the system or log in again .

2.2 Permanent setting method 2, Valid only for the current user ：

modify ~/.bashrc, Add or modify... At the end of the line ：

export PATH=$PATH:/home/xue

Then restart the system or log in again .

2.3 Temporary settings ：

Execute the following command at the terminal , This is only valid for the current terminal ：

export PATH=$PATH:/home/xue