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Re: [Help-bash] Redirecting both stdout and stdin to a file

From: Peng Yu
Subject: Re: [Help-bash] Redirecting both stdout and stdin to a file
Date: Sat, 28 Jan 2012 18:25:08 -0600

On Sat, Jan 28, 2012 at 6:09 PM, Bob Proulx <address@hidden> wrote:
> Peng Yu wrote:
>> There must be some rule that I missed. Could anybody let me know how
>> to understand the difference between the two commands?
> The shell holds the file descriptors in program variables.  These
> variables are assigned in the order seen on the command line from left
> to right.  The following pseudo code isn't from the actual shell code
> but perhaps thinking of it this way will help model it for you.
>  ">FILE 2>&1"  # Redirects both stdout to FILE and then redirects
>                # stderr to the current stdout.
>    1>FILE      <--  fd[1] = open_for_reading(FILE)
>    2>&1        <--  fd[2] = fd[1]
>  "2>&1 >FILE"  # Redirects stderr to the current stdout and then
>                # redirects stdout to file.
>    2>&1        <--  fd[2] = fd[1]
>    1>FILE      <--  fd[1] = open_for_reading(FILE)
> In the second case it should now be apparent that the stderr isn't
> associated with FILE because of the ordering.

I didn't understand that this is like file handle assignment under the
surface. Now I understand how this works.

BTW, it seems that the word "redirection" is little bit misleading.
For example, I original thought that the original stdout is redirected
to FILE and the orignal stderr is redirected to the new stdout. That
is why I thought FILE should got the original stdout and what was in
stderr get print on the screen. A better term should be something
"file handle assignment".

">FILE 2>&1"

I reread the Redirection section in the man page. I don't think that
the description there help me understand the two cases that I
mentioned in the first email. Please let me know which lines explain
them, if anybody think that the redirection section explains the two
cases. Otherwise, I suggest to put Bob's explanation in the manual to
clarify how redirection works (at least as a mental model, even it is
not exact what bash does internally).


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