From "Operating Systems: Three Easy Pieces" chapter on "Process API" (section 5.4 "Why? Motivating The API") [1]: ... the separation of fork() and exec() i

From "Operating Systems: Three Easy Pieces" chapter on "Process API" (section 5.4 "Why? Motivating The API") [1]: ... the separation of fork() and exec() is essential in building a UNIX shell, because it lets the shell run code after the call to fork() but before the call to exec(); this code can alter the environment of the about-to-be-run program, and thus enables a variety of interesting features to be readily built. ... The separation of fork() and exec() allows the shell to do a whole bunch of useful things rather easily. For example: prompt> wc p3.c > newfile.txt In the example above, the output of the program wc is redirected into the output file newfile.txt (the greater-than sign is how said redirection is indicated). The way the shell accomplishes this task is quite simple: when the child is created, before calling exec(), the shell closes standard output and opens the file newfile.txt. By doing so, any output from the soon-to-be-running program wc are sent to the file instead of the screen. [1] https://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf https://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf