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The exec() builtin function is the standard mechanism for executing system commands; it is a wrapper around the POSIX exec family of system calls. The system() builtin function is similar to exec, but it takes a single string whereas exec()takes a list. The qx() function, often represented by encasing a command in backquotes (``), can also be used to execute an arbitrary command. Finally the open() command function can also execute commands in a subprocess and either send data to them or fetch data from them (but not both).
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