Editing C shell (section)

==== Piping ====

Commands can be connected using a pipe, which causes the output of one command to be fed into the input of the next.  Both commands run [[Concurrency (computer science)|concurrently]].

*<code>|</code> means connect stdout to stdin of the next command.  Errors still come to the shell window.
*<code>|&</code> means connect both stdout and stderr to stdin of the next command.

Running concurrently means "in parallel".  In a [[Multi-core processor|multi-core]] (multiple processor) system, the piped commands may literally be executing at the same time, otherwise the [[Scheduling (computing)|scheduler]] in the operating system [[Time slice|time-slices]] between them.

Given a command, e.g., "<code>a | b</code>", the shell creates a [[pipeline (computing)|pipe]], then starts both <code>a</code> and <code>b</code> with stdio for the two commands redirected so that <code>a</code> writes its stdout into the input of the pipe while <code>b</code> reads stdin from the output of the pipe.  Pipes are implemented by the operating system with a certain amount of buffering so that <code>a</code> can write for a while before the pipe fills but once the pipe fills any new write will block inside the OS until <code>b</code> reads enough to unblock new writes.  If <code>b</code> tries to read more data than is available, it will block until <code>a</code> has written more data or until the pipe closes, e.g., if <code>a</code> exits.