Redirection. What is "<>", "<&" and ">&-"?

Question

<> – This operator I saw few months ago on the one site, but I don't remember what does it mean. Maybe I'm wrong and ksh has no <>.

a<&b – I know that this operator merge input stream a with output stream b. Am I right? But I don't know where I can use it. Can you get me examples?

>&- – I know nothing about it. For example, what does 2>&- mean?

• michas: What does your man page say?

Answer 1

From http://www.manpagez.com/man/1/ksh/:

 <>word Open file word for reading and writing as standard out-
 put.
 <&digit The standard input is duplicated from file descriptor
 digit (see dup(2)). Similarly for the standard output
 using >&digit.
 <&- The standard input is closed. Similarly for the standard
 output using >&-.

You will find all those details by typing man ksh.

Especially 2>&- means: close the standard error stream, i.e. the command is no longer able to write to STDERR, which will break the standard which requires it to be writable.

---

To understand the concept of file descriptors, (if on a Linux system) you may have a look at /proc/*/fd (and/or /dev/fd/*):

$ ls -l /proc/self/fd
insgesamt 0
lrwx------ 1 michas users 1 18. Jan 16:52 0 -> /dev/pts/0
lrwx------ 1 michas users 1 18. Jan 16:52 1 -> /dev/pts/0
lrwx------ 1 michas users 1 18. Jan 16:52 2 -> /dev/pts/0
lr-x------ 1 michas users 1 18. Jan 16:52 3 -> /proc/2903/fd

File descriptor 0 (aka STDIN) is used per default for reading, fd 1 (aka STDOUT) is default for writing, and fd 2 (aka STDERR) is default for error messages. (fd 3 is in this case used by ls to actually read that directory.)

If you redirect stuff it might look like this:

$ ls -l /proc/self/fd 2>/dev/null </dev/zero 99<>/dev/random |cat
insgesamt 0
lr-x------ 1 michas users 1 18. Jan 16:57 0 -> /dev/zero
l-wx------ 1 michas users 1 18. Jan 16:57 1 -> pipe:[28468]
l-wx------ 1 michas users 1 18. Jan 16:57 2 -> /dev/null
lr-x------ 1 michas users 1 18. Jan 16:57 3 -> /proc/3000/fd
lrwx------ 1 michas users 1 18. Jan 16:57 99 -> /dev/random

Now the default descriptors do no longer point to your terminal but to the corresponding redirects. (As you see, you can also create new fds.)

---

One more example for <>:

echo -e 'line 1\nline 2\nline 3' > foo # create a new file with three lines
( # with that file redirected to fd 5
 read <&5 # read the first line
 echo "xxxxxx">&5 # override the second line
 cat <&5 # output the remaining line
) 5<>foo # this is the actual redirection

You can do such things, but you very seldom have to do so.

• Nikolay Bildeyko: <>word - Will program read and after that append input to word? <&digit - So, will stderr will regirection to stdin if I write "<&2"?
• Costas: @NikolayBildeyko < is input rather > is output. So stderr is 2> and to redirrect it use 2>&1
• Nikolay Bildeyko: @Costas Could give me an example when I can use <&digit?
• Costas: @NikolayBildeyko See above answer
• michas: @mikeserv In theory you are right. In praxis I never saw such an error. Can you give an example how to provoke such an error?