John,
I fully understand your example. And I think it's you who is confused.
The example I provided was intended to illustrate that even though the
environment variable AAA *is* set to "aaa", the echo command *never sees*
that new value, as the sequence '$AAA' is expanded by the *originating
shell*, rather than the echo command itself. In fact, the echo command is
not even aware it's echoing the value of some environment variable -- all
it sees is the value. The parts of the example with the single quotes and
invoking another shell illustrated that once you protect the '$' from the
originating shell, the new shell *does* pick up the new value of the
environment variable.
In other words, if you invoke a shell script that internally uses the
value of AAA, it will see the new value. However, all the echo command
sees is the *value* of AAA, expanded for it by the originating shell,
which expansion happened *before* the assignment was in effect.
A careful reading of the bash man page, particularly the section on
variable substitution, might be helpful here.
Igor
On Thu, 24 Oct 2002, John Vincent wrote:
> Hi,
>
> I think you are confusing shell variables with environment
> variables. This is easy to do if you are not careful, since
> shell variables can be put into the environment using the
> "export" built-in command.
>
> Still, they are different things, and in this case the assignment
> is to an environment variable, not to a shell variable. It's
> done to keep backwards-compatibility with old shell scripts.
>
> The command:
>
> AAA=aaa echo $AAA
>
> does NOT change the shell variable AAA it changes the environment
> variable AAA for the echo command only. Maybe this example will
> make it clearer:
>
> Administrator@JPV ~
> $ AAA=bbb
>
> Administrator@JPV ~
> $ AAA=aaa echo $AAA
> bbb
>
> Administrator@JPV ~
> $
>
> In the above example the echo command is run with an environment
> variable AAA with a value of "aaa" but the shell variable AAA is
> passed with a value of "bbb" set on the previous line.
>
> I hope this helps.
>
> /John Vincent.
>
>
> >From: Igor Pechtchanski <[EMAIL PROTECTED]>
> >Reply-To: [EMAIL PROTECTED]
> >To: Andrew DeFaria <[EMAIL PROTECTED]>
> >CC: [EMAIL PROTECTED]
> >Subject: Re: About ENV?
> >Date: Fri, 18 Oct 2002 11:36:49 -0400 (EDT)
> >
> >On Fri, 18 Oct 2002, Andrew DeFaria wrote:
> >
> > > John Vincent wrote:
> > >
> > > > Hi,
> > > >
> > > > Not quite right there ... You can preceed a command with an
> > > > assignment, and the assigned variable is then put into the environment
> > > > of the command that is run. Thus the command
> > > > "echo" is run with an environment variable AAAA with the value aaaa.
> > > > However, this does not change the AAAA shell variable (which is
> > > > currently empty) so the echo prints a blank (it's empty argument) and
> > > > ignores the AAAA in it's environment.
> > > >
> > > > This syntax is useful for setting environment variables for one
> > > > command invocation only. They are not saved.
> > > >
> > > > I hope this fully explains what is going on.
> > >
> > > It doesn't. If "This syntax is useful for setting environment variables
> > > for one command invocation only" then the echo command should have
> > > echoed "aaa". Otherwise the syntax is not useful for setting environment
> > > variables for one command invocation. Perhaps what is meant is that this
> > > syntax is useful for overriding environment variables for one command
> > > invocation? However this leaves a glaring inconsistancy prone to error
> > > if the variable was not set already then no override takes place.
> >
> >Andrew,
> >
> >[pechtcha:~] AAAA=aaa && echo $AAAA
> >aaa
> >[pechtcha:~] AAAA=aaa echo $AAAA
> >
> >[pechtcha:~] AAAA=aaa eval 'echo $AAAA'
> >aaa
> >[pechtcha:~] AAAA=aaa sh -c 'echo $AAAA'
> >aaa
> >[pechtcha:~]
> >
> >Does this help?
> > Igor
--
http://cs.nyu.edu/~pechtcha/
|\ _,,,---,,_ [EMAIL PROTECTED]
ZZZzz /,`.-'`' -. ;-;;,_ [EMAIL PROTECTED]
|,4- ) )-,_. ,\ ( `'-' Igor Pechtchanski
'---''(_/--' `-'\_) fL a.k.a JaguaR-R-R-r-r-r-.-.-. Meow!
"Water molecules expand as they grow warmer" (C) Popular Science, Oct'02, p.51
--
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
Bug reporting: http://cygwin.com/bugs.html
Documentation: http://cygwin.com/docs.html
FAQ: http://cygwin.com/faq/
