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Re: Equivalent of ksh, zsh {N}<[WORD] ?


From: R. Bernstein
Subject: Re: Equivalent of ksh, zsh {N}<[WORD] ?
Date: Tue, 26 Aug 2008 10:38:38 -0400

Pierre Gaston writes:
 > On Tue, Aug 26, 2008 at 4:49 PM, R. Bernstein <address@hidden> wrote:
 > > Pierre Gaston writes:
 > >  > On Tue, Aug 26, 2008 at 5:41 AM, R. Bernstein <address@hidden> wrote:
 > >  > > Both zsh and ksh have a way to open a file or duplicate a file
 > >  > > descriptor and let the interpreter pick the descriptor saving the
 > >  > > newly-allocated file descriptor number in a variable. In particular:
 > >  > >
 > >  > >   exec {fd}<&0
 > >  > >
 > >  > > will duplicate stdin and save the newly allocated file-descriptor
 > >  > > number to fd. Also:
 > >  > >
 > >  > >   exec {fd}<filename
 > >  > >
 > >  > > opens filename with a new file descriptor and saves the number
 > >  > > allocated in fd. Short of going outside of the language and using
 > >  > > lsof, /proc, or the processes table, I haven't been able to figure out
 > >  > > how to do the corresponding thing in bash. Is there a way?
 > >  > >
 > >  > > If not, it would be great if a future version had this extension that
 > >  > > zsh and ksh both seem to have.
 > >  > >
 > >  > > Thanks!
 > >  >
 > >  > This is a standard behaviour and you can do this in pretty much any
 > >  > shell out there, including bash.
 > >
 > > Really? It doesn't seem to be documented in bashref. And when I tried just
 > > a moment ago:
 > >
 > >  $ {fd}<&0
 > >  {fd}<&0
 > >  bash: {fd}: command not found
 > >  $ bash --version
 > >  bash --version
 > >  GNU bash, version 3.2.39(1)-release (i486-pc-linux-gnu)
 > >
 > > Perhaps you are thinking of the variation without braces?
 > >
 > 
 > well, I was thinking of the normal redirection syntax:
 > exec 3<&0
 > 
 > I doubt '{fd}<&0'  is meaningfull anywhere.....in ksh {fd} tries to
 > run the command {fd} like in bash
 > in zsh it tries to run the command "fd"
 > What are you talking about???
 > 

I suppose this is new enough stuff that you would need recent ksh
or zsh version to see it. From a recent ksh 93t manual:

       If one of the above, other than >&- and the ># and ># forms, is
       preceded by ing space, then a file descriptor number > 10 will
       be selected by the shell and stored in the variable varname.
       If >&- or the any of the ># and ># forms is preceded by
       {varname} the value of varname defines the file descriptor to
       close or position.  For example:

              ... 2>&1

       means  file  descriptor 2 is to be opened for writing as a duplicate of
       file descriptor 1 and

              exec {n}<file

      means open file named file for reading and store the file descriptor
      number in variable n.

zsh has something similar.




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