What Happens When You Connect to a Server via SSH and Run a Command?

Let’s say you’re using a Mac. You open the Terminal app — that’s your terminal emulator. It’s the program that gives you a window to type commands and see output.

Now, you type something like:

ssh ubuntu@1.2.3.4

This runs the SSH client, a program on your computer that creates a secure connection to a remote machine (in this case, your server at 1.2.3.4).

Once you authenticate, your SSH client connects to the SSH server (also called the SSH daemon or sshd) running on the remote machine. Now you have a live SSH session — a secure tunnel between your local terminal and the server’s shell.

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🧠 Flow of a Command — Visual Diagram

[ Your Terminal App ]
         |
         v
[ SSH Client (your machine) ]
         |
      === SSH CONNECTION ===
         |
         v
[ SSH Daemon (on server) ]
         |
         v
[ Shell (e.g. bash) on server ]
         |
         v
[ OS executes command like ls ]
         |
         v
[ Output travels back through SSH ]
         |
         v
[ Your Terminal displays it ]

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🔍 What Happens When You Type a Command?

• If you type a built-in command like cd or pwd, the shell handles it directly — no external program needed.
• If you type a program like ls or grep, the shell tells the OS to find and run the actual binary (e.g. /bin/ls).

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🧪 Try These Commands to Explore

See what kind of command cd is:

type cd
# cd is a shell builtin

See where the ls command lives:

which ls
# /bin/ls

See which shell you’re using:

echo $SHELL
# /bin/bash (or /bin/zsh, etc.)

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🔁 TL;DR

Your terminal is the window,
SSH is the secure tunnel,
The shell on the server is the interpreter,
And the OS is what actually executes the command.

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