Re: Lonely spacecowboy

["Brian K. White" <brian@xxxxxxxxx>]

----- Original Message ----- 
From: "Stephane Chazelas" <Stephane_Chazelas@xxxxxxxx>
To: "Brian K. White" <brian@xxxxxxxxx>
Cc: <zsh-users@xxxxxxxxxx>
Sent: Tuesday, March 27, 2007 3:31 PM
Subject: Re: Lonely spacecowboy


> On Tue, Mar 27, 2007 at 03:01:42PM -0400, Brian K. White wrote:
> > You did not say in what way it fails, nor what <cmd> might be.
> > But I can say 2 things:
> >
> > 1) pipeing into read only "works" in ksh, the real korn ksh not pdksh or
> > any of the others like bash and zsh that have a "ksh compatibility" mode.
> > But you can still use a similar construct with file redirection that will
> > work.
> > The difference is "|read" creates a child process that read runs in, and
> > any env variables set in that child process are not visible from the
> > parent, and the while-do-done command (and all commands in that loop) ar
> > running in the parent.
>
> While this is true for bash and pdksh it's not true for zsh:
>
> ~$ echo foo | read
> ~$ echo $REPLY
> foo

Color me updated.

It doesn't work in one enviroment I use zsh a lot in.

$P$Gecho this is a test |read aa bb cc
$P$Gecho $aa $bb $cc

$P$G


This is latest freestanding (static binary not needing cygwin dll) version 
of zsh for windows that I have been able to locate, which is quite old,
$P$Gecho $VERSION
3.0.5.001
$P$G

The way cygwin only works by having the cygwin dll installed in the os and 
can't be delivered in a self-contained package with the file makes cygwin 
unsuitable for me.


> [...]
> > You might be able to redirect stdin just for the cmd but I never tried 
> > that
> > in a loop like this so I don't know if it actually works.
>
> Generally done as:
>
> while IFS= read -r <&3; do
>  cmd ... "$REPLY"
> done 3< file.txt
>
> (read without -r or with the default value of IFS is very
> special, you generally don't want that in scripts).

Ah of course.


> > # test for no tty to allow for cron jobs, cgi script, print filter, 
> > etc...
> > tty -s && MYTTY=`tty` || MYTTY=/dev/null
> > while read
> > do
> >      vi "$REPLY" <$MYTTY
> > done < files.txt
>
> Even if there's a tty, there's no guarantee that stdin is
> guaranteed. There's no need for that trick except if you're
> using csh type shells that hardly have any fd handling
> capability.
>
>
> > You could also just use awk and not worry about stdin and tty and read
> > etc...
> > no redirection or piping, no read command, no tty issues, etc...
> >
> > awk '{system("somecommand \""$0"\"")}' files.txt
>
> Then you'd run into more problems (possibly serious) if the
> lines in files.txt contain any ", \, $, `...

Well I quoted the string, we do already know it contains spaces.
I suppose it wouldn't be that much harder to get it single-quoted instead of 
double-quoted and then it's just up to command to continue handling the 
string safely.
Or you could add as much sanity checking to the awk script as seems 
necessary.

But I forgot all about xargs, that was nicer than this anyways.

> > It's a little less efficient because system() spawns a new sh process to
> > run command in, where the while-read loop will run command right in the
> > same top level shell process where the while loop itself is running.
> [...]
>
> If "<cmd>" is not builtin, there's be as many processes (unless
> is not optimised to exec the last command is run). The
> difference is on the loading and initialising of sh.
>
> --
> Stéphane

Nice. Thanks for all the pointers. I appreciate it.

Brian K. White    brian@xxxxxxxxx    http://www.myspace.com/KEYofR
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