01-27-2004
Short File Descriptors Tutorial
Here's a nice little (basic) tutorial:
http://www.livefirelabs.com/unix_tip...3/06092003.htm.
This explains what "info" about a file they hold ... as for the more technical side, you'll have to dig deeper.
Quote:
The Standard UNIX File Descriptors - Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)
If you are familiar with UNIX I/O redirection, syntax similar to the following should not be new to you:
Briefly, when command runs it sends "normal" output to file, and any error messages generated by command are also written to file. "2>&1" handles the latter.
Have you ever wondered where the numbers 2 and 1 come from? This may be common knowledge to our more experienced readers, buy may need some explaining to those who are relatively new to the UNIX operating system environment. The 2 and 1 are file descriptors. Okay, so what's a file descriptor?
When a UNIX program wants to use a file, it must first open that file. When it does so, UNIX will associate a number with the file. This number, which is used by the program when reading from and writing to the file, is the file descriptor.
A typical UNIX program will open three files when it starts. These files are:
- standard input (also known as stdin)
- standard output (also known as stdout)
- standard error (also known as stderr)
Standard input has a file descriptor of 0, standard output uses 1, and the number 2 is used by standard error. Are you starting to see where this is headed?
Looking at our command again,
you should now recognize that 2>&1 instructs the shell to send messages headed to stderr (2) to the same place messages to stdout (1) are sent. In our example, that place is file.
If you are wondering, > is equivalent to 1>, and < is short for <0.
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PIPE(2) System Calls Manual PIPE(2)
NAME
pipe - create an interprocess communication channel
SYNOPSIS
pipe(fildes)
int fildes[2];
DESCRIPTION
The pipe system call creates an I/O mechanism called a pipe. The file descriptors returned can be used in read and write operations. When
the pipe is written using the descriptor fildes[1] up to 4096 bytes of data are buffered before the writing process is suspended. A read
using the descriptor fildes[0] will pick up the data.
It is assumed that after the pipe has been set up, two (or more) cooperating processes (created by subsequent fork calls) will pass data
through the pipe with read and write calls.
The shell has a syntax to set up a linear array of processes connected by pipes.
Read calls on an empty pipe (no buffered data) with only one end (all write file descriptors closed) returns an end-of-file.
Pipes are really a special case of the socketpair(2) call and, in fact, are implemented as such in the system.
A signal is generated if a write on a pipe with only one end is attempted.
RETURN VALUE
The function value zero is returned if the pipe was created; -1 if an error occurred.
ERRORS
The pipe call will fail if:
[EMFILE] Too many descriptors are active.
[ENFILE] The system file table is full.
[EFAULT] The fildes buffer is in an invalid area of the process's address space.
SEE ALSO
sh(1), read(2), write(2), fork(2), socketpair(2)
BUGS
Should more than 4096 bytes be necessary in any pipe among a loop of processes, deadlock will occur.
4th Berkeley Distribution August 26, 1985 PIPE(2)