Fork (operating System) - Importance of Forking in Unix

Importance of Forking in Unix

Forking is an important part of Unix, critical to the support of its design philosophy, which encourages the development of filters. In Unix, a filter is a (usually small) program that reads its input from stdin, and writes its output to stdout. A pipeline of these commands can be strung together by a shell to create new commands. For example, one can string together the output of the find(1) command and the input of the wc(1) command to create a new command that will print a count of files ending in ".cpp" found in the current directory and any subdirectories, as follows:

In order to accomplish this, the shell forks itself, and uses pipes, a form of interprocess communication, to tie the output of the find command to the input of the wc command. First, a pipe is created at two fresh file descriptors, and then two child processes are created, one for each command (find and wc). Each child process first dup2(2)s the appropriate pipe file descriptor to either its standard input or output. Then these child processes overlay themselves with the code associated with the programs they are intended to execute, using the exec(3) family of system calls.

More generally, forking is also performed by the shell each time a user issues a command. A child process is created by forking the shell, and the child process is overlaid, once again by exec, with the code associated with the program to be executed.