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In Unix-like operating systems, every process except process 0 (the swapper) is created when another process executes the fork() system call. The process that invoked fork is the parent process and the newly created process is the child process. Every process (except process 0) has one parent process, but can have many child processes.
The other process, the parent, receives from fork the process identifier of the child, which is always a positive number. The parent process passes this identifier to the waitpid system call to suspend execution until the child has exited. When this has happened, the parent resumes execution and exits by means of the return statement.
A child process inherits most of its attributes, such as file descriptors, from its parent. In Unix, a child process is typically created as a copy of the parent, using the fork system call. The child process can then overlay itself with a different program (using exec) as required. [1]
When a process forks, a complete copy of the executing program is made into the new process. This new process is a child of the parent process, and has a new process identifier (PID). The fork() function returns the child's PID to the parent process. The fork() function returns 0 to the child process. This enables the two otherwise identical ...
In a strictly technical sense, a Unix-like system process is a daemon when its parent process terminates and the daemon is assigned the init process (process number 1) as its parent process and has no controlling terminal. However, more generally, a daemon may be any background process, whether a child of the init process or not.
In all Unix and Unix-like systems, as well as on Windows, each process has its own separate set of environment variables.By default, when a process is created, it inherits a duplicate run-time environment of its parent process, except for explicit changes made by the parent when it creates the child.
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When the shell fork s a new child process for a command pipeline, both the parent shell process and the child process immediately make the child process the leader of the process group for the command pipeline. This ensures that the child is the leader of the process group before either the parent or child relies on this being the case.