How commands gets executed ?

If you write anything and press enter in the terminal, it will say "command not found."

🕑 23:55  heyashu ~/Desktop/ME/node-js-by-procodrr (master)
🚀 hello
bash: hello: command not found
🕑 00:27  heyashu ~/Desktop/ME/node-js-by-procodrr (master)

There are some types of commands:

1. Alias: Bash first checks if the command matches any defined aliases.
2. Functions: Next, it checks if the command matches any defined shell functions.
3. Built-in commands: Like cd, echo, pwd, etc.
4. Hash Table:
5. Executable: Finally, it searches through the directories listed in the PATH environment variable, in the order they appear, to find an executable file matching the command name.

So, the question is: How do you know which type of command is being used when you type it?

For example, if you type pwd, how do you know what type of command it is?

You can use the which command before the actual command to determine its type.

🕑 00:27  heyashu ~/Desktop/ME/node-js-by-procodrr (master)
🚀 which pwd
/usr/bin/pwd

🚀 cd /
🕑 00:29  heyashu /
🚀 explorer 

If you run cd / in Windows, it will take you to the path where Git is installed. When you run explorer ., it will open that directory, and you can see the pwd.exe file, which is an executable binary file.

type Command

If which gives you any unusual result, try using the type command.

🚀 type cd
cd is a shell builtin
🚀 type pwd
pwd is a shell builtin

How to Create Function Commands

Let's open the .bashrc file and see how we can write functions.

To open .bashrc, just run:

🚀 code ~/.bashrc
🕑 00:36  heyashu /

Nowi bahsdrc wirte tghuis

cd(){
  echo "my function is running"
}

Now, cd is already a built-in command. So, how do you know which one will run? Yes, this will override the main cd command.

If you define a function named cd and type cd, it will execute your function and print an echo. Using this overridden cd, you will not be able to change directories.

🚀 cd
my function is running

🚀 type cd
cd is a function

How poriority gets decides ?

It checks in the following order: Aliases, then Functions, and then continues to other types.

Now, guess what the type of node would be? node is also a command, right?

🚀 type node
node is aliased to `winpty 
node.exe'

winpty node.exe is aliased somewhere. Remember, we saw this winpty in the Performer app?

Check this article.

You can learn that winpty simulates a Unix-like environment in Windows.

After checking functions, it looks for built-in commands, then the hash table, and finally executable files.

Also, remember that the which command only gives you the path.

So, you have a global PATH variable in your system. Try to get this by using:

heyashu ~/Desktop/ME/node-js-by-procodrr 
(master)
🚀 echo $PATH
/c/Users/Ashutosh Tiwari/bin:/mingw64/bin:/usr/local/bin:/usr/bin:/bin:/mingw64/bin:/usr/bin:/c/Users/Ashutosh Tiwari/bin:/c/Python312/Scripts:/c/Python312:/c/Windows/system32:/c/Windows:/c/Windows/System32/Wbem:/c/Windows/System32/WindowsPowerShell/v1.0:/c/Windows/System32/OpenSSH:/c/Program Files (x86)/MAVProxy:/c/Program Files (x86)/MAVProxy:/c/ProgramData/chocolatey/bin:/cmd:/c/Program Files/dotnet:/c/Program Files/nodejs:/c/Python312/Scripts:/c/Python312:/c/Windows/system32:/c/Windows:/c/Windows/System32/Wbem:/c/Windows/System32/WindowsPowerShell/v1.0:/c/Windows/System32/OpenSSH:/c/Program Files/Docker/Docker/resources/bin:/c/Program Files (x86)/MAVProxy:/c/Program Files (x86)/MAVProxy:/c/ProgramData/chocolatey/bin:/c/Program Files/nodejs:/cmd:/c/Program Files/dotnet:/c/Python312/Scripts:/c/Python312:/c/Windows/system32:/c/Windows:/c/Windows/System32/Wbem:/c/Windows/System32/WindowsPowerShell/v1.0:/c/Windows/System32/OpenSSH:/c/Program Files/Docker/Docker/resources/bin:/c/Program Files (x86)/MAVProxy:/c/Program Files (x86)/MAVProxy:/c/ProgramData/chocolatey/bin:/c/Program Files/nodejs:/cmd:/c/Program Files/dotnet:/c/Users/Ashutosh Tiwari/AppData/Local/Microsoft/WindowsApps:/c/Users/Ashutosh Tiwari/AppData/Local/Programs/Microsoft VS Code/bin:/c/Program Files/nodejs/npm:/c/Program Files/nodejs/npm:/c/Users/Ashutosh Tiwari/AppData/Local/Microsoft/WinGet/Packages/Schniz.fnm_Microsoft.Winget.Source_8wekyb3d8bbwe:/c/Users/Ashutosh Tiwari/AppData/Roaming/npm:/c/Users/Ashutosh Tiwari/AppData/Local/Programs/mongosh:/usr/bin/vendor_perl:/usr/bin/core_perl    

You're getting a long string, and to better understand it, let's transform it.

Hehe, you're about to learn a new command now! It basically transforms the output.

🚀 echo $PATH | tr ":" " " /"
> 
> ^C
🕑 00:53  heyashu ~/Desktop/ME/node-js-by-procodrr 
(master)
🚀 echo $PATH | tr ":" "\n"                        
/c/Users/Ashutosh Tiwari/bin
/mingw64/bin
/usr/local/bin
/usr/bin
/bin
/mingw64/bin
/usr/bin
/c/Users/Ashutosh Tiwari/bin
/c/Python312/Scripts
/c/Python312
/c/Windows/system32
/c/Windows
/c/Windows/System32/Wbem
/c/Windows/System32/WindowsPowerShell/v1.0
/c/Windows/System32/OpenSSH
/c/Program Files (x86)/MAVProxy
/c/Program Files (x86)/MAVProxy
/c/ProgramData/chocolatey/bin
/cmd
/c/Program Files/dotnet
/c/Program Files/nodejs
/c/Python312/Scripts
/c/Python312
/c/Windows/system32
/c/Windows
/c/Windows/System32/Wbem
/c/Windows/System32/WindowsPowerShell/v1.0
/c/Windows/System32/OpenSSH
/c/Program Files/Docker/Docker/resources/bin       
/c/Program Files (x86)/MAVProxy
/c/Program Files (x86)/MAVProxy
/c/ProgramData/chocolatey/bin
/c/Program Files/nodejs
/cmd
/c/Program Files/dotnet
/c/Python312/Scripts
/c/Python312
/c/Windows/system32
/c/Windows
/c/Windows/System32/Wbem
/c/Windows/System32/WindowsPowerShell/v1.0
/c/Windows/System32/OpenSSH
/c/Program Files/Docker/Docker/resources/bin       
/c/Program Files (x86)/MAVProxy
/c/Program Files (x86)/MAVProxy
/c/ProgramData/chocolatey/bin
/c/Program Files/nodejs
/cmd
/c/Program Files/dotnet
/c/Users/Ashutosh Tiwari/AppData/Local/Microsoft/WindowsApps
/c/Users/Ashutosh Tiwari/AppData/Local/Programs/Microsoft VS Code/bin
/c/Program Files/nodejs/npm
/c/Program Files/nodejs/npm
/c/Users/Ashutosh Tiwari/AppData/Local/Microsoft/WinGet/Packages/Schniz.fnm_Microsoft.Winget.Source_8wekyb3d8bbwe
/c/Users/Ashutosh Tiwari/AppData/Roaming/npm       
/c/Users/Ashutosh Tiwari/AppData/Local/Programs/mongosh
/usr/bin/vendor_perl
/usr/bin/core_perl

So basically, these are the steps added to the PATH variables.

Now, the hash also plays a big role in how things are managed. Search online to learn more about these commands. The hash prepares a table to quickly search, find, and run commands—similar to a cache. It’s a very interesting topic, so try to explore it online.

Before checking for an executable file, Bash looks in its hash table for previously looked-up executables to quickly locate them.

I know it’s tough, and sometimes I’m not able to understand it fully. These are deep dives, and they will take time to comprehend.

I have just read and created these notes for the first time, but for better understanding, it needs proper revision and study.

Process Objects

Now lets understand what are process objects

Process.argv

// write this in app.js
console.log(process.argv);
// do node app.js
//o/p
[
  'C:\\Program Files\\nodejs\\node.exe',      
  'C:\\Users\\Ashutosh Tiwari\\Desktop\\ME\\node-js-by-procodrr\\process\\app'
]
🕑 01:14  heyashu ~/Desktop/ME/node-js-by-procodrr/process (master)

🚀 node app.js 123243 345345 353 5 35 3 5
[
  'C:\\Program Files\\nodejs\\node.exe',      
  'C:\\Users\\Ashutosh Tiwari\\Desktop\\ME\\node-js-by-procodrr\\process\\app.js',
  '123243',
  '345345',
  '353',
  '5',
  '35',
  '3',
  '5'
]

See magic its like arguments see above code

Now I will suggest you to try logging this and check what are these process objects

// Accessing Process Properties

// Command-line arguments
process.argv;

// Environment variables
process.env;

// Current process ID
process.pid;

// Parent process ID
process.ppid;

// Operating system platform
process.platform;

// Node.js version
process.version;

// Node.js and dependencies versions
process.versions;

// Processor architecture
process.arch;

// Using Process Methods
// Current working directory
process.cwd();

// Change working directory
process.chdir("/tmp");

// Memory usage
process.memoryUsage();

// Process uptime
process.uptime();

// Exiting the process
// process.exit(0);

// Kill the process
process.kill(process.pid);

// Emit warning
process.emitWarning("This is a custom warning message!", {
  code: "MY_WARNING_CODE",
  detail: "This is some additional warning detail.",
});

// Interacting with stdin, stdout, and stderr streams
process.stdout.write("Hello, stdout!\n");
process.stderr.write("Hello, stderr!\n");

// Next Tick demonstration
process.nextTick(() => {
  // This will run on the next tick of the event loop.
});

// Registering event listeners
process.on("exit", (code) => {
  // Process is about to exit with code
});

process.on("warning", (warning) => {
  // Handle warning
});

process.stdin.on("data", (data) => {
  // Process input data
});

Dont thank me, Thanks to Anurag sir