Methods for calling other programs in Perl

There are three types: system, exec, ''(unquoted ).
1. System: After the command is executed, run the Perl script and output it to stdout.
2. Exec: Once you execute this command, Perl will exit and output it to stdout.
3. (My $ output =) '': After the command is executed, run the Perl script. The command output is returned to $ output. Even if it is written like this: '', Perl tries its best to return the returned value even though no variable receives the returned value. We do not recommend that you do not receive the return value.