On Sun, Aug 03, 2003 at 06:57:44AM -0400, Shawn Lamson wrote:
> On Sat, August 02 at 12:08 PM EDT
> Dave Carrigan <[EMAIL PROTECTED]> wrote:
> >The "MY_ENV=abc printf" syntax sets the environment variable for the
> >printf subprocess. And, in fact, when printf runs, MY_ENV is truly set
> >to abc. However, the "${MY_ENV}\n" is expanded *before* printf is
> >executed, and since MY_ENV is not set in the existing shell, the
> >expansion results in an empty string. The printf command doesn't even
> >see MY_ENV in its arguments, all it sees is a single argument that
> >looks like ``\n''.
>
> Can you elaborate a little more on this? I am curious, too. After
> reading your email I tried this:
> [EMAIL PROTECTED]:~$ unset COMMAND
> [EMAIL PROTECTED]:~$ COMMAND="printf" $COMMAND "x${COMMAND}\n"
> bash: x\n: command not found
> [EMAIL PROTECTED]:~$ COMMAND="printf" && $COMMAND "x${COMMAND}\n"
> xprintf
> [EMAIL PROTECTED]:~$
Your second command is not a good example because it is actually a
complex command consisting of two simple commands and a conjunctive. An
equivalent syntax would be
COMMAND="printf"
if [ $? = 0 ]; then
$COMMAND "x${COMMAND}\n"
fi
It should be clear that the above is very different than
COMMAND="printf" $COMMAND "x${COMMAND}\n"
It will also set COMMAND in the current shell, which the original poster
seems not to want.
> So it seems that the variable is not assigned even for the subprocess.
See my other message that goes into this in more detail.
> Does the shell see programs to execute before it looks to do variable
> substitution? I know the first things it sees are pipes and redirects
> but I don't know more.
Actually, the first thing it does is variable expansion, before it sets
up the redirects.
> I must admit I had never thought of just running a command after an
> assignment so it has never come up. "Bug hunting" is great, but if the
> OP needs a solution for something, what about this:
> [EMAIL PROTECTED]:~$ unset MY_ENV
> [EMAIL PROTECTED]:~$ (MY_ENV=abc && printf "x${MY_ENV}\n") xabc
> [EMAIL PROTECTED]:~$ echo x$MY_ENV
> x
> [EMAIL PROTECTED]:~$
This is pretty close, but is not actually exporting MY_ENV to the
subshell. The reason it looks like it worked is because MY_ENV was
expanded before printf even saw it.
This would export a variable to the subshell but still let it be
expanded on the current command without setting it in the current shell:
$ (export MY_ENV=abc && printf "${MY_ENV}\n")
However, I have no idea why this should be considered useful.
--
Dave Carrigan
Seattle, WA, USA
[EMAIL PROTECTED] | http://www.rudedog.org/ | ICQ:161669680
UNIX-Apache-Perl-Linux-Firewalls-LDAP-C-C++-DNS-PalmOS-PostgreSQL-MySQL
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