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Introduction to the Linux Shell

This lecture is based on the [Unix Shell Lesson](http://swcarpentry.github.io/shell-novice/) by [Software Carpentry](https://software-carpentry.org/)

When you go to 127.0.0.1:9999 from your browser for the first time, it brings up the content of your home directory in the VM.
From now on, we will refer to this as the Jupyter Home

The Linux Shell

In the Jupyter Home Page

Click New on the top right corner
Click Terminal

You will be presented with a new tab
This is your Linux terminal into the CentOS VM

A terminal is a command-line interface that captures your keyboard interactions and sends them to the Linux shell.
A Linux shell accepts, inteprets, and sends the appropriate commands, as interpreted from your keyboard inputs, to the Linux operating system for actual execution.

There can be many different terminal programs.
There can be many different shell programs.
They vary in term of capabilities and visual presentations.

Switch back to the terminal tab
Hit Enter
Type ls -F / and hit Enter

$
$ ls -F /
bin@  boot/  dev/  etc/  home/  lib@  lib64@  media/  mnt/  opt/  proc/  root/  run/  sbin@  srv/  sys/  tmp/  usr/  var/
$

The first line is a prompt, indicating that the shell is waiting for input
There might be many different texts for the prompt, depending on individual users' configurations
A common notation for the prompt in literature is the \$ sign

In the second prompt, we entered a statement: ls -F /
This statement has three components:
- ls is the command
- -F is the flag, which represents the option selected for the ls command
- / is the argument for the command
The components are separated by spaces
After the statement is evaluated and the results are printed to the terminal, we are returned to the third prompt.

Each command is a program.
- We will deal with how these programs are located by the Linux OS later
Linux shell inteprets statements by following the REPL (read-evaluate-print-loop) strategy

REPL for ls -F /

Read the entire statement that was typed in (in this case ls -F /)
- The shell uses spaces to separate the components of the statement
Evaluate
- Find a program called ls
- Pass the flag and argument to this program
- Execute the program together with the flag and argument
Print the output produced by the program
Loop back and present a new prompt to the user for the next statement

Excercise

Switch to the terminal tab
Execute git clone https://github.com/linhbngo/Distributed-and-Cluster-Computing.git
Traverse to the Jupyter Home Page and open this notebook

Navigating Files and Directories

The operating system executes the programs.
The actual contents (programming instructions and data) are stored in the file system.
The file system organized your data in directories (or folders) and files.

To execute the right program, you need to let the OS knows there the program is located in the file system (and where the data is located).

Where you (your terminal) currently are in the file system?
Switch to the terminal tab, type the command pwd and hit Enter.
pwd stands for path to working directory

Typically, users start out in their home directory.
Safe and secure space (most of the time!!!).
Users have full modification control over items (files and folders) in their home directory.
/home/<USER_NAME>
A statement (a string) describing the location of files or directories is called a path.

The forward slash (/) has two meanings:
- If it stands alone, or if it starts as the first position in a path, then it represents the root directory.
- The root directory is the master directory of a file system, which holds everything else in the file system.
- If it appears in the middle of a path, then it acts as a separator.

First navigation command: ls
Stands for listing
Shows all files/directories in a location
Switch to the terminal tab and run ls from the prompt.

ls has many options.
To view the options of ls, run ls --help
--help is a standard flag in the majority of standard commands in the Linux OS. It shows the general options available to the commands.

If you want more in-depth discussion of this option, type `man ls.
Stands for manual.

Exercise

What does ls do when used with -l and -h flags?
What flag should I use if I want to
- list the contents of nested directories?
- list the contents of nested directories and sort them by order of time of last change?

Command cd is used to change directory
Shortcuts: ~: your home directory ..: directory one level above the current directory

Relative path: Room 147
Absolute path: Room 147, UNA Building, West Chester University, West Chester, Pennvylvania, USA, Earth, Solar System ...

Working with Files and Directories

mkdir: Make new directory
cp: Copy files or directories
rm: Remove files or directories
mv: Move files or directories to a new location

In-terminal editor: nano
- ^: Ctrl

Anaconda-based editor:
- From Jupyter Home Page, click New and then Text
- The editor is in browser, supports keyword-hightlighting for the majority of programming languages
- The file is created in the directory that the Home Page currently points to.
Remember to save!

Writing and compiling C codes in a Linux environment

Rough analogies:

Java versus C: English language versus German language
GUI IDE versus command line IDE: spoken language versus written language

Create a new text file from Anaconda text editor
Change the file name to hello.c
Copy the C content in the next cell and save:



In [ ]:

    
#include <stdio.h>
int main()
{
   // printf() displays the string inside quotation
   printf("Hello, World!");
   return 0;
}



In [ ]:

    
// This is for comparison against the previous cell only, do not save this into your hello.c file
public class HelloWorld {

    public static void main(String[] args) {
        // Prints "Hello, World" to the terminal window.
        System.out.println("Hello, World");
    }

}

To compile and run:

$ gcc -o hello hello.c
$ ./hello