Operating Systems

There are 2 common approaches to interact with a computer. The first one is GUI (Graphical User Interface) and the second one is CLI (Command Line Interface). GUI is a visual interface that you can use to interact with your computer. Command line interface is a text-based interface that you can use to interact with your computer. In CLI, you are giving a command to your computer and the computer runs it and gives a result to you by printing it to the screen. There are a lot of commands that you can use in CLI. Here are some of them:

A cli command is nothing more than a program that you can run from your terminal or shell. Each cli command is an executable file located in your PATH. When you run a command, your shell will search for the executable file in your PATH and if it finds it, it will run it. So, you need to put your executable file in your PATH in order to run it from anywhere. You can check your PATH by using echo $PATH command. It is a list of directories separated by :. You can put your executable file in one of these directories.

Let's look at how ls command works. When you write ls in your terminal, it will try to find an executable file named ls in your PATH. It will check each directory in your PATH one by one. If it finds it in the first directory, it will run it directly. If it does not find it in the first directory, it will check the second directory and so on. If it does not find it in any of these directories, it will give an error. You can check where ls command is located by using which ls command. It will give you the path of the executable file. Also, terminal will check the file permissions. If the file is not executable, it will give an error. For this reason you need to know the concept of users, user groups and file permissions.

There are two types of executable files. The first one is a binary ELF file. The second one is a script file. A script file is a text file that contains shebang at the beginning of the file. Shebang is a special comment that starts with #!. It tells the shell which program to use to run the script. For instance, if you write #!/bin/bash at the beginning of your script file, shell will use bash to run your script. If you write #!/usr/bin/env node, terminal will run env command with node as the first argument. And as a second argument, it will give your script file's path. So, env command will run node with your script file's path as the first argument. Please use shebang at the beginning of your script file. Otherwise, your script will not run.

In the challenge, you will implement an argument parser in order to parse the arguments that are given to your program. You can get these arguments by using process.argv array. It is an array that contains the arguments that are given to your program. You need to follow these subcommands in order to implement your TODO app:

list: mytodo list also mytodo (default subcommand) (--done and --undone flags should be supported)
add: mytodo add "A sample task description"
done: mytodo done 1
undone: mytodo undone 1
delete: mytodo delete 1
update: mytodo update 1 "A new task description"

Standard IO, Exit Code, Environment Variables and Arguments

Each program ran from the terminal has an exit code. This exit code is just a number between -128 and 127. It is used to indicate the status of the program. If the program runs successfully, it will return 0. If it fails, it will return a non-zero number (You can use any non zero number between -128 and 127). You can check the exit code of the last ran program by using echo $? command. It will print the exit code of the last ran program. Exit codes are very important to check the status of a command.

Also, each program has 3 standard streams in order to interact with the user. A stream is a just a file that you can write to or read from. There are 3 standard streams in a program. These are stdin, stdout and stderr. stdin is the standard input stream. It is used to read input from the user (keyboard). stdout is the standard output stream. It is used to write output to the user (screen). stderr is the standard error stream. It is used to write error messages to the user (screen).

However, also a command can be started with some arguments. These arguments are called command line arguments. Arguments are just an array of strings. And your arguments should be separated by a space. For instance, if you write ls -l -a in your terminal, ls command will be started with 3 arguments. The first argument will be ls, the second argument will be -l and the third argument will be -a.

Basic Commands

Users and Groups

File System

Permissions

Processes

Package Managers

Shell Scripting

Commands

Environment Variables

Also, there is a concept of environment variables. Environment variables are just key-value pairs that are stored in your shell. These variables are used to get some common configuration values across different programs. For instance, you can use EDITOR environment variable to get the editor that you want to use. You can use PATH environment variable to get the directories that you want to search for executable files. Do you see the pattern? Your shell uses PATH environment variable to search for executable files. You can use process.env object to access these environment variables in your program. In this challenge, you will use TODO_USERNAME environment variable to get the assignee of the TODOs.

TODO App Done Right

Operating Systems

What is Kernel?

What is a program?

Command Line Interface