std io still confusing..

[kimes]

hi.. I'm newbie linux programmer. so I'm still confused in std IO.
I made simple demo programming which just gets every input from stdin and print out to stdout.. I think I dont need to post it. You can imagine it right? (gets/puts in loop)

Anyway let the program name be 'stdiodemo'

when I executed like this
echo 123 | ./stdiodemo
It printed out 123 that's what I wanted..
'echo 123's stdout is redirected to my demo program.

But my question.. How can I redirect stderr to stdin without redirecting stdout to stdin?

Is all my questions clear?
Thanks in advance..

[intangible]

So you want the program to only get STDERR huh?

Try this:
ls -e 2>&1 1>/dev/null|less
As you can see, only the error message for the invalid argument gets piped to less.

If you try this:
ls -l 2>&1 1>/dev/null|less
You see nothing because -l is a valid argument so there is no STDERR and you are redirecting STDOUT to /dev/null

2>&1 redirects STDERR to STDOUT
1>/dev/null redirects STDOUT to /dev/null

1>&2 would redirect STDOUT to STDERR

Hope this helps.

[kimes]

So you want the program to only get STDERR huh?

Try this:
ls -e 2>&1 1>/dev/null|less
As you can see, only the error message for the invalid argument gets piped to less.

If you try this:
ls -l 2>&1 1>/dev/null|less
You see nothing because -l is a valid argument so there is no STDERR and you are redirecting STDOUT to /dev/null

2>&1 redirects STDERR to STDOUT
1>/dev/null redirects STDOUT to /dev/null

1>&2 would redirect STDOUT to STDERR

Hope this helps.
Thanks this helps a lot..
But.. I'm still not sure the syntax like above '2>&1'..
Can I get the full references..?

[desdinova]

http://www.linuxcommand.org/lts0060.php

[LordHunter317]

Read the bash manpage for the full details on the redirects it supports.

For details on maniuplating them programatically, get a good UNIX programming book and look at the dup2 system call.

Also, using gets()/puts() is extremely bad practice and you shouldn't do it.