ordinary file
A file on the system that contains data, text, or program instructions.

output redirection
In UNIX the process of capturing the output of a command and storing it in a file is called output redirection, because it redirects the output of a command into a file instead of the screen.

parent process
Process that controls another often referred to as the child process or subprocess. See process.

parent process identifier
Shown in the heading of the ps command as PPID. The process identifier of the parent process. See also parent process.

parent shell
Shell (typically the login shell) that controls another, often referred to as the child shell or subshell. See shell.

piping
The process used to redirect the output of one command into the input of another command. Piping is accomplished with the pipe character, |.

process
A discrete running program under UNIX. The user’s interactive session is a process. A process can invoke (run) and control another program that is then referred to as a subprocess. Ultimately, everything a user does is a subprocess of the operating system.

process identifier
Shown in the heading of the ps command as pid. The unique number assigned to every process running in the system.

prompt
When you see a prompt, you can type the name of a command and press Enter. In this book, we will use the $ character to indicate the prompt.