What happens when you enter [link] in a Linux terminal from the system, network, and OSI layers perspective?

When entering a command with a URL (for example, curl http://example.com) in a Linux terminal, a multi-layered process occurs that can be viewed from the perspectives of the system, network, and OSI model.

1. Processing in the terminal and shell:

   • The terminal accepts user input.
   • The shell parses the command, searches for the executable file (e.g., curl) in the PATH.
   • It launches the curl process with the URL argument.

2. Inside the process (curl):

   • Parsing the URL, determining the protocol (HTTP), host, and port.
   • Resolving the DNS name example.com via a system call (getaddrinfo), which contacts the DNS server.

3. Network level:

   • After obtaining the IP address, a TCP connection is established with the server on port 80 (HTTP).
   • TCP establishes the connection through a three-way handshake (SYN, SYN-ACK, ACK).

4. OSI model:

   • Application layer (7): The HTTP protocol forms a GET request.
   • Presentation layer (6): (if present) encoding, compression.
   • Session layer (5): session management of TCP.
   • Transport layer (4): TCP provides reliable delivery.
   • Network layer (3): IP routes packets to the server.
   • Data link layer (2): Ethernet or Wi-Fi frames are transmitted over the physical medium.
   • Physical layer (1): electrical/optical signals are transmitted over cables or air.

5. Server response:

   • The server accepts the TCP connection, processes the HTTP request.
   • Sends back an HTTP response.

6. Processing the response:

   • curl receives the data and outputs it to the terminal.

Thus, entering the command initiates a chain of system calls, network operations, and interactions between OSI layers, ensuring data is retrieved from the network and displayed.