What is file descriptors ? In most contexts, a file descriptor is simply an integer used to indicate to the operating system which open “file” or “file like thing” we would like an operation to be performed over. For instance, we might open a new file, be told by the operating system that this corresponds to file descriptor 3, and be able to read from that file again by telling the operating system to read from file descriptor 3. It is called a “descriptor” because historically this integer was an index into a per-process array used by the operating system to store a struct with information about each open file.
What are file descriptors 0, 1, 2 ? 0 = stdin 1 = stdout 2 = stderror