Shell Programming

Every unix program gets 3 open file descriptors at startup

• 0: stdin, 1:stdout, 2:stderr

• Most unix tools send output to stdout and provide no way to send to a file

• If you want to send to a file, use

  • cmd > filename

  and the shell redirects. Tool is not aware of redirection operator

Every process has an array of open files

A file descriptor is an index of said array

• When you open a file you get the lowest index

close…open

• Start with 0 1 2 descriptor

• close(0)

• fd = open(‘file’, 0)

• Closes the 0 file and opens a new one in the 0 position

Open…close…dup…close

• fd = open(“data”)

• close(0)

• dupe(fd)

• close(fd)

• Create file in 3 position, close 0, dupe 3 into 0, close 3