On *nix systems, file descriptors are a number that are given to files that have a stream attached to them. C can directly read and write from file descriptors using the read() command that takes a file descriptor, a buffer and a length to be read.

There are different file descriptors for every process. To get the file descriptors for the current process, run ls -l /proc/$$/fd.

$$ is a substitute for the current process ID. Run ps and echo $$ to compare.

By standard, 0 is standard input, 1 is standard output, and 2 is standard error.