1. File redirection ls > out
Shell opens out, closes stdout and dups(fd of out). Man dup. Dup duplicates file descriptor to lowest unused descriptor number
stdin
1) Open “out”
stdout
2) close stdout
out
stderr
3) dup(3)
out
4) close(3)

2. Exec. What we know so far..
As we know, the only way to run a new program is to use exec system call.
Exec will replace the current process with the new process
To keep the current process alive(running) we need to use a combination of fork and exec.
The process ID, and list of open files does not change on exec()
The system replaces the process spaces with the new program from disk.

3. Pipes pipes are created using the system call pipe()
Like named pipes, pipes are a FIFO byte stream between two processes
Unlike named pipe, pipes require a common process ancestor
Any process that writes to a pipe will hang until the data is read by another process.
Any process that reads from a pipe will hang if there is no data in the pipe AND if there is any process with an open file handle to the pipe

4. file descriptor table after pipe()
Pipes are the oldest form of UNIX interprocess communication.
The have the following limitations.
The are uni-directional
They are meant to be used between processes that have a common ancestor. ex: between parent and child, or between two children etc.
stdin
stdout
stderr
pd[0]
pd[1]
int pd[2];
pipe(pd)

5. pipe in one process
parent:
pipe()
pd[0]
pd[1]

6. pipe between two processes - after fork
parent:
pipe()
pd[0]
pd[1]
Child:
pd[0]
pd[1]
fork()

7. pipe between two processes - setup!
parent:
pipe()
pd[0]
pd[1]
fork()
Text
Child:
pd[0]
pd[1]
close (pd[1])
close (pd[0])

8. pipe between two commands
cat /etc/motd | grep upgrade
&& echo “New software installed”
Parent can only get exit status of child not grand children.
i.e. Parent has to fork both children.
parent:
pipe()
fork()
close..
child1:
setup pipe
child2:
setup pipe

9. pipe between two commands
cat /etc/motd | grep upgrade
&& echo “New software installed”
Parent can only get exit status of child not grand children.
i.e. Parent has to fork both children.
parent
stdin
stdout
sterr
pd[0]
pd[1]
child1:
stdin
stdout
stderr
pd[0]
pd[1]
child2:
stdin
stdout
stderr
pd[0]
pd[1]

10. pipe between two commands
cat /etc/motd | grep upgrade
&& echo “New software installed”
Parent can only get exit status of child not grand children.
i.e. Parent has to fork both children.
parent
stdin
stdout
sterr
pd[0]
pd[1]
child1:
stdin
stdout
stderr
pd[0]
pd[1]
child2:
stdin
stdout
stderr
pd[0]
pd[1]