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can't close stdin/stdout in shell

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Top Forums Shell Programming and Scripting can't close stdin/stdout in shell
# 8  
Old 02-18-2010
Yes, I meant redirect, not close. In boiling down the problem to its minimum I found that they didn't even need to be closed to cause this. I noticed I'd titled the thread wrong a minute after I posted but it wouldn't let me fix it.

Why would you redirect stdin/stdout to /dev/null? Daemons do it all the time. That way, stdout and stdin never hang but never go anywhere either, and the fd's can't get recycled when you open a new file.

I've finally realized what's going on, and why my question is silly. I'm not sure what I was thinking, and can't even blame this one on lack of caffiene Smilie Sorry.
# 9  
Old 02-18-2010
Code:
 That way, stdout and stdin never hang but never go anywhere either

Cool! Still learning this stuff Smilie
But I meant why would you redirect /dev/null into stdin? The reverse makes sense..

Sometimes caffeine makes you overthink stuff. Glad you got it worked out.
# 10  
Old 02-18-2010
Quote:
Originally Posted by mandelbrot333
But I meant why would you redirect /dev/null into stdin? The reverse makes sense..
You can read from /dev/null too. Or rather, it does about the same thing writing to it does -- nothing.
Code:
$ strace echo > /dev/null
execve("/bin/echo", ["echo"], [/* 26 vars */]) = 0

...

write(1, "\n"..., 1)                    = 1
close(1)                                = 0
munmap(0xb7fde000, 4096)                = 0
close(2)                                = 0
exit_group(0)                           = ?

$ strace cat /dev/null
execve("/bin/cat", ["cat", "/dev/null"], [/* 26 vars */]) = 0

...

open("/dev/null", O_RDONLY|O_LARGEFILE) = 3

...

read(3, ""..., 4096)                    = 0
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
$

Writing to /dev/null always returns success, and reading from /dev/null always returns EOF. So redirecting /dev/null into stdin means nothing will ever hang trying to read from stdin, reads instantly fail. This is better than just closing stdin since it's more graceful than a screenful of "bad file descriptor" errors and prevents the kernel from recycling FD 0 as the next file you open.
Last edited by Corona688; 02-18-2010 at 03:02 PM..
# 11  
Old 02-18-2010
Quote:
Originally Posted by Corona688
Code:
#!/bin/sh
exec 0</dev/null
exec 1>/dev/null

ls -l /proc/self/fd >&2

produces
Code:
total 0
lr-x------ 1 tyler users 64 Feb 18 10:38 0 -> /proc/7886/fd
lrwx------ 1 tyler users 64 Feb 18 10:38 1 -> /dev/pts/4
lrwx------ 1 tyler users 64 Feb 18 10:38 2 -> /dev/pts/4

I've verified the shell is actually redirecting stdout as expected, but when I run ls it is not inheriting this change. If I leave out the >&2, it produces no output, meaning it's only re-redirecting stdout to my console when I don't redirect stdout into stderr -- in other words, it only redirects stdout back to the console when I tell it not to use stdout at all! Why is it restoring stdout at all?
As far as I can see, the shell is doing everything exactly right. You have stdout directed to /dev/null and stderr directed to the console (by default). When you do:
Code:
ls -l /proc/self/fd >&2

you redirect stdout away from /dev/null and to where stderr is pointing (the console).

Please tell us what exactly you are trying to acomplish.
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