simple shell.md

How shell executes ls -l *.c

In this post we'll talk about how shell executes command ls -l *.c. Moreover, we'll also discus how it is implemented under the hood.

We strongly advise you to have at least fundamental knowledge of C programming language and Linux system programming before you continue.

Note that we skipped many error check and memory management for brevity which are must be included in working version of the code. Refer here for implemention.

When we open a shell(users perspective) it prompts us to enter a line of command and when we enter the command it executes our command and it prompts us to enter another command.

But there is more to this story. The following are simplified version of steps the shell takes.

1. Getline

The shell prints the prompt and waits for input. Let us assume we entered ls -l *.c. This is preformed using getline().

while(1) {
   char *line = NULL;
   size_t len = 0, nline;
   printf("$ ");
   nline = getline(&line, &len, stdin);
   if(nline == -1) return 0;
   // continues here
}

2. Tokenize

Then The shell splits this line of command into tokens. In our case ls, -land *.c. this uses strtok(command, DELIM) where DELIM is string consisting whitespaces character(spaces, tabs and etc.).

// continued
const char *DELIM =  " \t\a\r"
token = strtok(line, DELIM);
while(token != NULL) {
	tokens[pos++] = strup(token); // push to array of strings
	token = strtok(NULL, DELIM);
}

3. Expansion

After that the shell looks for expansion in our case token *.c matches every file ending in .c. So tokens will became :

tokens = {"ls", "-l", "main.c", "util.c", "test.c"};

4. Alias

The shell checks if the first token is aliased. if it is it replaces it with the original expanded version. In our case ls is aliased to ls --color=tty.

ls: aliased to ls --color=tty
/usr/bin/ls
/bin/ls

5. Builtins

Then checks if the command is builtin(like exit, cd, help ... etc). ls is not builtin command.

6. PATH

If the command is not builtin command it searches the command in the PATH enviromental variable. ls is found in path /bin/ls.

7. New Process

After that Creates new process using syscall fork().

int status;
pid_t child_pid = fork();
if (child_pid == -1)
	return (1);

if (child_pid == 0) // if this is child process execute
{
	if (execve(tokens[0], tokens, __environ) == -1)
		return (1);
} else // if it is parent wait for child
	wait(&status);

8. Execution

Finaly it executes it. It uses execve(tokens[0], tokens, env) system call. See step 7. and continues at step 1 again.

Conclusion

This is just simple illustration of how shell goes on executing commands like ls -l *.c. But now days shells has evolved to do much complex tasks like history, conditional statements, loops and any other things programming languages supports. It was quite journey learning shell and we do believe you can benfit be implementing those concepts in yourself. You can find our implementation in C here. Thanks 👍