Question on the <> Linux Redirection Operator

Question

I'm studying for my LPIC-1 and I'm confused by the <> redirection operator. I know the official definition for this is that it "causes the specified file to be used for both standard input and standard output"... but I'm still confused.

Could someone please provide an example of how/when I would use the <> redirection operator in Linux?

Answer 1

An example would be when you need to supply a program input data, and also send output data somewhere (eg. a file):

$ sort < data.txt > result.txt

this runs the data in data.txt through the sort command, and sends the output to result.txt. It is identical to doing this:

$ sort data.txt > result.txt

Think of it like this:

(2 <- 1) -> 3

The middle file is fed to the program at number 2, and the result is finally sent to 3.

Answer 2

If John T's answer was what you wanted, then you mis-stated the question :)

He's succinctly described the < and > operators, but you asked about <>. This is a rarely used operator which I'm not sure I'd expect to see in LPIC-1, but just for the record (given that you've probably done with that exam by now):

When you run interactively a command without redirection its standard input, output and error will normally be attached to the tty; it will read from stdin and write to stdout/stderr. Though you might not expect it, those file descriptors are typically opened read-write. Try running a script like

#!/bin/bash
echo "I'm writing to stdin!" >&0

Running this bare will work, at least in current bash versions. However, when you use the < and > redirectors, the stdin and stdout file descriptors are only opened for read and write respectively, so if you invoke this with input redirected from a file -

./write-to-stdin < /tmp/some-input

it won't work (reasonably enough). If you really want a program you invoke with redirection to be able to write and write to a single file descriptor, <> will do the job.

./read-and-write-stdin   <> /tmp/some-input-and-output
./read-and-write-stdout 1<> /tmp/some-input-and-output

As most programs only expect stdin to be a read descriptor and stdout to be a write descriptor, you can probably see why this is rather rarely useful, but if you're writing subordinate scripts (or using functions) you can take advantage of the behaviour yourself.