But, when you use those functions, it will be executed by the shell, and the value will be returned.

Really?

q:~ [10:07:26]$ ps U blazar
  PID TTY      STAT   TIME COMMAND
11831 ?        S      0:00 sshd: blazar@pts/0
11838 pts/0    Ss     0:00 -bash
11857 pts/0    R+     0:00 ps U blazar
q:~ [10:07:32]$ perl -e 'system qw/sleep 600/'

[1]+  Stopped                 perl -e 'system qw/sleep 600/'
q:~ [10:08:09]$ ps U blazar
  PID TTY      STAT   TIME COMMAND
11831 ?        S      0:00 sshd: blazar@pts/0
11838 pts/0    Ss     0:00 -bash
11908 pts/0    T      0:00 perl -e system qw/sleep 600/
11909 pts/0    T      0:00 sleep 600
11922 pts/0    R+     0:00 ps U blazar
[download]

I don't see additional shells...

(To the benefit of the OP) this "detail" apart, indeed system calls are expensive.