Understanding the Execution of `ls -l` in Linux

understanding_ls_l_linux.md

Understanding the Execution of ls -l in Linux

1. Command Input:

   • The user types ls -l in the terminal, and the shell reads this input.

2. Command Parsing & PATH Variable:

   • The shell parses the command and its arguments.
   • The shell searches for the ls binary in the directories listed in the PATH environment variable.

3. Permission & Access Check:

   • The kernel checks if the current user has execute permissions on the ls binary.
   • If the user doesn’t have the appropriate permissions, the shell returns a permission denied error.

4. Forking:

   • The shell performs a fork() system call to create a new child process.

5. Child Process & Executing ls:

   • The child process replaces its image with the ls command using an exec() family system call, such as execve().
   • The kernel loads the ls binary into memory and parses the binary’s header to set up the memory image.

6. Loading Shared Libraries:

   • The ls binary is linked to shared libraries (like libc), and the kernel, with the help of the dynamic linker/loader, maps these libraries into the process’s address space.
   • Dependencies of the shared libraries are also loaded if necessary.

7. Running ls & System Calls:

   • The ls command starts executing and makes various system calls to read the contents of the current directory and retrieve information about files/directories.
   • The kernel handles these system calls and performs operations on behalf of the ls command.
   • The kernel also checks file and directory permissions to determine if the ls command can access and read the information.

8. Gathering & Outputting Information:

   • The ls command formats the gathered information and writes the output to standard output (stdout) using the write() system call.
   • The kernel writes the data to the terminal device.

9. Returning Control:

   • After the ls command completes its execution, it exits, and control is returned to the kernel.
   • The kernel cleans up resources and signals the parent shell process about the completion of the child process.
   • The shell becomes active again and is ready to accept the next command.