M5: Interacting with Processes: Managing Process Input and Output Flashcards
Redirection
modifying the location to which an input is taken from or an output is sent for a given command; redirection operators are commutative, meaning they can appear in swapped orders and produce the same result
“dup2” system call :
implement redirection by passing in two file
descriptors (oldfd and newfd) and causing newfd to point to the file or data structure that was pointed to by oldfd
Standard input :
: the location where programs look for their inputs; for
programs initiated in command-line, this defaults to your terminal; Standard in can be redirected, and child processes inherit standard in from their parent
process; the Standard in file descriptor is “0”
Standard output:
the location where programs stream their outputs; for
programs initiated in command-line, this defaults to your terminal; Standard out can be redirected, and child processes inherit standard out from their
parent process; the Standard out file descriptor is “1”
Pipes
a form of interprocess communication, providing a way to stream data
from one process to the other; pipes have a “write end” to which data gets
written by one process to be sent to another process, and a “read end” from
which the receiving process reads the incoming data
SIGPIPE
a signal generated when a process tries to write to a pipe whose read end has been closed
Process pipelining:
running multiple processes at the same time where the output
of one process is used as input to another process
Half-duplex:
data flows in one direction; pipes between two processes are in the half
duplex communication paradigm as data flows in one direction between the two
processes
FIFOs
these are similar to pipes; while pipes are temporary and unnamed connections, FIFOs are files with names that allow two processes to communicate by writing to the file
Process group:
collection of processes that are established in a way such that you can send (or block) a signal to the entire group at the same time; the processes in a pipeline are in a single process group
PGID
process group ID
Session ID:
one instance of your login to your shell is a session, and all processes
launched in this session have the same session ID
SSH
a secure network protocol that allows a remote user to be authenticated to
login to a machine
Foreground process:
there is always exactly one foreground process group; this process group has access to interactive shell output (in the absence of explicit redirection) and input, and receives all the terminal generated signals; the shell must wait on this process before executing another process
Background processes:
there can be any number of background processes; these processes will allow the user to execute other programs and interact with the shell
without waiting
