From: dom@happygiraffe.net (Dominic Mitchell)
To: "S. Cowles" <scowles@earthlink.net>
Cc: zsh-users@sunsite.dk
Subject: Re: problem testing file descriptor 3
Date: Mon, 11 Apr 2005 07:58:27 +0100 [thread overview]
Message-ID: <20050411065827.GA1819@ppe.happygiraffe.net> (raw)
In-Reply-To: <200504102333.36134.scowles@earthlink.net>
On Sun, Apr 10, 2005 at 11:33:32PM -0700, S. Cowles wrote:
>
> I am having trouble testing if file descriptor 3 is already opened in the
> calling environment. If the file descriptor is not open, I would like to
> open it and redirect it to stderr. The test script I am using is:
>
> #!/bin/zsh
> [[ -t 3 ]] &&
> { echo "fd3 is open."; } ||
> { echo "fd3 is not open."; }
> echo no1 >&1
> echo no2 >&2
> echo no3 >&3
> exit 0
The -t flag tests to see whether an fd is open *and attached to a
terminal*. So it's probably no good for what you need.
> If the script is invoked in this manner:
> cp /dev/null fd3.out
> exec 3>&-
> ./test 3> fd3.out
> the script reports that fd3 is not open even though fd3 output does correctly
> redirect to the file fd3.out.
>
> If the script is invoked like so:
> exec 3>&-
> exec 3> /dev/tty
> ./test
> the script reports that fd3 is open and all output correctly appears on the
> terminal session.
>
> If I use a test like this:
> echo -n "" >&3 1> /dev/null 2> /dev/null || {
> echo "fd3 is not open."
> exec 3>&2
> }
> error messages are not hidden.
>
> So, as a recap, if fd3 is already opened when the script is called, I want
> output to redirect as coded in the calling environment. If fd3 is not
> already opened, I want to redirect fd3 output to an existing, open fd and
> I want any error messages hidden.
>
> What is a good test to determine if file descriptor 3 is open in the calling
> environment? Thanks.
I think that the only way you could do this would be using the stat
plugin.
% zmodload -i zsh/stat
% stat -f 3
stat: 3: bad file descriptor
% stat -f 3 3</etc/hosts
device 365
inode 8285
mode 33188
nlink 1
uid 0
gid 0
rdev 32807
size 177
atime 1113201458
mtime 1100873702
ctime 1100873702
blksize 4096
blocks 4
link
You should be able to make that do what you need.
-Dom
next prev parent reply other threads:[~2005-04-11 6:58 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-04-11 6:33 S. Cowles
2005-04-11 6:58 ` Dominic Mitchell [this message]
2005-04-11 7:32 ` S. Cowles
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