Question

I can thought that it will open a shell, execute the parameter (shell command) and return the result in a scalar.
But, execute system function in a Perl script is faster than a shell command.
It will call this command in C?
If yes, what’s the difference between

rmdir foo

and

system('rmdir foo');

Answer 1

1. The difference between the two is that the second one will open (fork) a child process (which will be the rmdir command) while the first one will make a direct Unix system call using the API without opening a process. Opening child process is expensive resource wise.

2. system() call will always open a child process to execute, BUT, it may either open a shell which will in turn fork off the desired program as its own child process (thus resulting in 2 child processes), or fork off the program as a child process directly.

   The choice of when Perl will open a shell during a system() call is spelled out in perldoc -f system. The short (and not 100% accurate) version is:

   • If there is only one parameter to system call, and the parameter evaluates to a string that contains shell meta-characters, a shell will be forked first.

   • If there’s only one parameter and it evaluates to no metacharacters; or there’s a >1 element list of parameters, the program is forked directly bypassing shell.

Thus:

system("rmdir foo"); # forks off rmdir command directly

system("rmdir", "foo"); # forks off rmdir command directly

system("rmdir foo > out.txt"); # forks off a shell because of ">"