Module 5: Interacting with Processes

Question 1

redirection

Answer 1

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

Question 2

dup2 system call

Answer 2

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

Question 3

standard input

Answer 3

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

Question 4

standard output

Answer 4

the location where programs stream their outputs; for programs initiated in CL, this defaults to your terminal

standard out can be redirected and child processes inherit standard out from their parent process

the standard in file descriptor is 1

Question 5

pipes

Answer 5

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

Question 6

SIGPIPE

Answer 6

a signal generated when a process tries to write to a pipe whose read end has been closed

Question 7

process pipelineing

Answer 7

running multiple processes at the same time where the output of one of the process is used as input to another process

Question 8

half-duplex

Answer 8

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

Question 9

FIFOs

Answer 9

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

Question 10

process group

Answer 10

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

Question 11

PGID

Answer 11

process group ID

Question 12

SSH

Answer 12

a secure network protocol that allows a remote user to be authenticated to login to a machine

Question 13

session id

Answer 13

one instance of your login to your shell is a session; and all processes launched in this session have the same session ID

Question 14

foreground process

Answer 14

there is always exactly one foreground process; this process 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

Question 15

background processes

Answer 15

there can be any number of background processes; these processes allow the user to execute other programs and interact with the shell without waiting

Question 16

job/terminal control

Answer 16

ability to manipulate multiple jobs from a single terminal and also to allow us to toggle jobs between background and foreground

Question 17

synchronous waiting

Answer 17

a process waits and finds out outcome of another process by implementing a loop that constantly checks for a signal “busy wait”

Question 18

asynchronous waiting

Answer 18

a process continues executing normally and when another process has a signal to send, a generic SIGCHLD is sent to the executing process, who handles the signal with a SIGCHLD signal handler