Doesn't MY_ENV=abc printf "${MY_ENV}\n" suppose to print abc?

[Shaul Karl]

Package: bash
Version: 2.05b-8.1

According to my understanding of the manual page,

$ MY_ENV=abc printf "${MY_ENV}\n"

Should have print abc. But it does not:

$ MY_ENV=abc printf "${MY_ENV}\n"

$

What am I missing?


-- System Information:
Debian Release: testing/unstable
Architecture: i386
Kernel: Linux rakefet 2.4.21-3.custom586.1 #1 Fri Aug 1 21:48:39 IDT 200
Locale: LANG=C, LC_CTYPE=C

Versions of packages bash depends on:
ii base-files 3.0.8 Debian base system miscella
ii libc6 2.3.1-16 GNU C Library: Shared libra
ii libncurses5 5.3.20030510-2 Shared libraries for termin

-- no debconf information

--

Shaul Karl, shaul @ actcom . net . il


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[iain d broadfoot]

* Shaul Karl (shaul@actcom.net.il) wrote:

> Package: bash
> Version: 2.05b-8.1
>
> According to my understanding of the manual page,
>
> $ MY_ENV=abc printf "${MY_ENV}\n"
>
> Should have print abc. But it does not:
>
> $ MY_ENV=abc printf "${MY_ENV}\n"
>
> $
>
> What am I missing?

an 'export' and a ';', like so:

$ export MY_ENV=abc ; printf "${MY_ENV}\n"
abc

iain

--
wh33, y1p33 3tc.

"If sharing a thing in no way diminishes it, it is not rightly owned if it is
not shared." -St. Augustine


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[iain d broadfoot]

* Shaul Karl (shaul@actcom.net.il) wrote:

> On Sat, Aug 02, 2003 at 10:22:19PM +0100, iain d broadfoot wrote:

> > * Shaul Karl (shaul@actcom.net.il) wrote:

> > > Package: bash
> > > Version: 2.05b-8.1
> > >
> > > According to my understanding of the manual page,
> > >
> > > $ MY_ENV=abc printf "${MY_ENV}\n"
> > >
> > > Should have print abc. But it does not:
> > >
> > > $ MY_ENV=abc printf "${MY_ENV}\n"
> > >
> > > $
> > >
> > > What am I missing?

> >
> > an 'export' and a ';', like so:
> >
> > $ export MY_ENV=abc ; printf "${MY_ENV}\n"
> > abc
> >

>
>
> Your proposal has some side effects which might be undesirable. More
> specifically, with your proposal MY_ENV will be set until you
> explicitly unset it or exit the shell. With what I tried to do, MY_ENV
> will only be set for the following command. There will be no need to
> explicitly unset it or exit the shell in order for it to disappear
> afterwards. In addition, the construct I am trying to use is a well
> known construct and the man page says it should work. Is it a bug?

i can't see in the man page an example like the one you show, what
section is it in?

> Can you try on your machine the line that I have asked about and see
> if that is working for you? What version of bash are you using?

i get the same results as you.

the other respondent explained the reason for the (apparent) problem, i
think.

iain

--
wh33, y1p33 3tc.

"If sharing a thing in no way diminishes it, it is not rightly owned if it is
not shared." -St. Augustine


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[Shaul Karl]

On Sat, Aug 02, 2003 at 10:22:19PM +0100, iain d broadfoot wrote:

> * Shaul Karl (shaul@actcom.net.il) wrote:

> > Package: bash
> > Version: 2.05b-8.1
> >
> > According to my understanding of the manual page,
> >
> > $ MY_ENV=abc printf "${MY_ENV}\n"
> >
> > Should have print abc. But it does not:
> >
> > $ MY_ENV=abc printf "${MY_ENV}\n"
> >
> > $
> >
> > What am I missing?

>
> an 'export' and a ';', like so:
>
> $ export MY_ENV=abc ; printf "${MY_ENV}\n"
> abc
>

Your proposal has some side effects which might be undesirable. More
specifically, with your proposal MY_ENV will be set until you
explicitly unset it or exit the shell. With what I tried to do, MY_ENV
will only be set for the following command. There will be no need to
explicitly unset it or exit the shell in order for it to disappear
afterwards. In addition, the construct I am trying to use is a well
known construct and the man page says it should work. Is it a bug?

Can you try on your machine the line that I have asked about and see
if that is working for you? What version of bash are you using?
--

Shaul Karl, shaul @ actcom . net . il


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[Shaul Karl]

On Sat, Aug 02, 2003 at 11:55:03PM +0100, iain d broadfoot wrote:

>
> i can't see in the man page an example like the one you show, what
> section is it in?
>

Quoting the ENVIRONMENT section:

The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described
above in PARAMETERS. These assignment statements affect only the envi-
ronment seen by that command.
--

Shaul Karl, shaul @ actcom . net . il


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[Shawn Lamson]

On Sat, August 02 at 12:08 PM EDT
Dave Carrigan <dave@rudedog.org> wrote:

>On Sat, Aug 02, 2003 at 08:49:36PM +0300, Shaul Karl wrote:
>

>> According to my understanding of the manual page,
>>
>> $ MY_ENV=abc printf "${MY_ENV}\n"
>>
>> Should have print abc. But it does not:
>>
>> $ MY_ENV=abc printf "${MY_ENV}\n"
>>
>> What am I missing?

>
>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:
slamson@callerio:~$ unset COMMAND
slamson@callerio:~$ COMMAND="printf" $COMMAND "x${COMMAND}\n"
bash: x\n: command not found
slamson@callerio:~$ COMMAND="printf" && $COMMAND "x${COMMAND}\n"
xprintf
slamson@callerio:~$

So it seems that the variable is not assigned even for the subprocess.
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.

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:
slamson@callerio:~$ unset MY_ENV slamson@callerio:~$ (MY_ENV=abc &&
printf "x${MY_ENV}\n") xabc
slamson@callerio:~$ echo x$MY_ENV
x
slamson@callerio:~$

The parens make a subshell which is the "new environment" I assume the
the man page is talking about, right?

Thank you,

Shawn Lamson
[email]shawn.lamson@verizon.net[/email]


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