Curious bash evaluation

[NoSpaze]

Hello. See this:

# A="echo 'hi'"

# echo $($A)
'hi'

# echo 'hi'
hi

Does anyone understand why does the first command 
evals the echo but echoes the simple quotes? 

Is there any dark shopt option which makes it run correctly?

Thanks...
----------------------------------------------
Rodolfo Alcazar Portillo - nospaze(a)gmail.com
otbits.blogspot.com / counter.li.org: #367962
----------------------------------------------
# unzip; strip; touch; finger; grep; mount; fsck; more; yes; fsck; fsck;
fsck; umount; sleep

[inode0]

On Mon, Dec 27, 2010 at 10:51 PM, Rodolfo Alcazar Portillo
<nospaze(a)gmail.com&gt; wrote:
>  On Mon, 2010-12-27 at 13:31 -0500, Chris Tyler wrote:
> > On Mon, 2010-12-27 at 17:05 +0100, Rodolfo Alcazar Portillo wrote:
> > > Hello. See this:
> > >
> > > # A="echo 'hi'"
> > >
> > > # echo $($A)
> > > 'hi'
> > >
> > > # echo 'hi'
> > > hi
> > >
> > > Does anyone understand why does the first command
> > > evals the echo but echoes the simple quotes?
> >
> > $() provides a type of quoting, so it's evaluated similar to:
> >
> >    "echo" "'hi'"
> >
> > which produces the observed result.
>
>  Hmmm. Somehow obvious. Is that "type of quoting" documented
>  somewhere? Why does it eval just the first word? 
While I don't agree with the above characterization the process is
documented in the EXPANSION section of the bash manpage.

John

[NoSpaze]

On Mon, 2010-12-27 at 13:31 -0500, Chris Tyler wrote:
>  On Mon, 2010-12-27 at 17:05 +0100, Rodolfo Alcazar Portillo wrote:
>  > Hello. See this:
>  > 
>  > # A="echo 'hi'"
>  > 
>  > # echo $($A)
>  > 'hi'
>  > 
>  > # echo 'hi'
>  > hi
>  > 
>  > Does anyone understand why does the first command 
>  > evals the echo but echoes the simple quotes? 
>  
>  $() provides a type of quoting, so it's evaluated similar to:
>  
>     "echo" "'hi'"
>  
>  which produces the observed result. 
Hmmm. Somehow obvious. Is that "type of quoting" documented 
somewhere? Why does it eval just the first word?

:)
----------------------------------------------
Rodolfo Alcazar Portillo - nospaze(a)gmail.com
otbits.blogspot.com / counter.li.org: #367962
----------------------------------------------
Q. How do you solve bus problems?
A. Shoot the driver.

[Suvayu Ali]

On Monday 27 December 2010 08:05 AM, Rodolfo Alcazar Portillo wrote:
>  Hello. See this:
>
>  # A="echo 'hi'" 
I think this assignment makes the single quotes part of the string to be 
"echo"ed. Its is equivalent to saying, echo \'hi\'.

If you don't want the single quotes echoed, don't use them inside the 
double quotes either.

-- 
Suvayu

Open source is the future. It sets us free.

[Chris Tyler]

On Mon, 2010-12-27 at 17:05 +0100, Rodolfo Alcazar Portillo wrote:
>  Hello. See this:
>  
>  # A="echo 'hi'"
>  
>  # echo $($A)
>  'hi'
>  
>  # echo 'hi'
>  hi
>  
>  Does anyone understand why does the first command 
>  evals the echo but echoes the simple quotes?  
$() provides a type of quoting, so it's evaluated similar to:

   "echo" "'hi'"

which produces the observed result.

-Chris

[inode0]

On Mon, Dec 27, 2010 at 11:30 AM, Rodolfo Alcazar Portillo
<nospaze(a)gmail.com&gt; wrote:
>  On Mon, 2010-12-27 at 10:46 -0600, inode0 wrote:
> > On Mon, Dec 27, 2010 at 10:05 AM, Rodolfo Alcazar Portillo
> > <nospaze(a)gmail.com&gt; wrote:
> > > Hello. See this:
> > >
> > > # A="echo 'hi'"
> > >
> > > # echo $($A)
> > > 'hi'
> >
> > That seems rather convoluted. Are you sure you don't just want to
> > "eval $A" anyway?
>
>  Very sure. Not "bash", nor "eval". Just simple command substitution.
>  Another reason: it's shorter. Another reason: It must be understood, not
>  just avoided in a micro$oft software fashion. 
Ok. Quote removal is done at the very end of the expansion process and
only removes unquoted occurrences of \, ', and " that did not result
from an earlier expansion. Since the quotes in this case did occur in
an earlier expansion of the variable A they are left unchanged.

If you want them removed I don't think there is any way to remove them
without doing something like eval to trigger a second round of quote
removal.

John

[NoSpaze]

On Mon, 2010-12-27 at 10:46 -0600, inode0 wrote:
>  On Mon, Dec 27, 2010 at 10:05 AM, Rodolfo Alcazar Portillo
>  <nospaze(a)gmail.com&gt; wrote:
>  > Hello. See this:
>  >
>  > # A="echo 'hi'"
>  >
>  > # echo $($A)
>  > 'hi'
>  
>  That seems rather convoluted. Are you sure you don't just want to
>  "eval $A" anyway? 
Very sure. Not "bash", nor "eval". Just simple command substitution.
Another reason: it's shorter. Another reason: It must be understood, not
just avoided in a micro$oft software fashion.

>  I realize your example may be a simple abstraction of something more
> complex. 
Of course. I'm building a short script from some string. The rest is
boring.

Another failing example:
>  cat script  
echo "hello"

>  $(<script) 
"hello"

The $(<file) form is documented in the manual:
http://www.faqs.org/docs/bashman/bashref_30.html

Thanks!
-- 
----------------------------------------------
Rodolfo Alcazar Portillo - nospaze(a)gmail.com
otbits.blogspot.com / counter.li.org: #367962
----------------------------------------------
Q. What's the difference between Batman and Bill Gates?
A. When Batman fought the Penguin, he won.

[inode0]

On Mon, Dec 27, 2010 at 10:05 AM, Rodolfo Alcazar Portillo
<nospaze(a)gmail.com&gt; wrote:
>  Hello. See this:
>
>  # A="echo 'hi'"
>
>  # echo $($A)
>  'hi' 
That seems rather convoluted. Are you sure you don't just want to
"eval $A" anyway?

I realize your example may be a simple abstraction of something more complex.

John

[NoSpaze]

On Mon, 2010-12-27 at 08:13 -0800, Marvin Kosmal wrote:
>  I will do the short explanation
>  This expression does what I call Double expansion
>   echo $($A)
>  First it expands    echo HI
>  Then it does the echo which yields   HI 
Please read the former mail, it echoes the quotes.

>  In your second command
>   echo 'hi'
>  It just echos   hi .. like it should
>  Sorry, if this doesn't sound real elegant..    Someone will give a
>  better description 
Nor elegant neither understandable, but thanks for trying.

:)
----------------------------------------------
Rodolfo Alcazar Portillo - nospaze(a)gmail.com
otbits.blogspot.com / counter.li.org: #367962
----------------------------------------------
"Die dümmsten Programmierer haben die längsten Programme."
- Unbekannter Autor

[Marvin Kosmal]

On 12/27/10, Rodolfo Alcazar Portillo <nospaze(a)gmail.com&gt; wrote:
>  Hello. See this:
>
>  # A="echo 'hi'"
>
>  # echo $($A)
>  'hi'
>
>  # echo 'hi'
>  hi
>
>  Does anyone understand why does the first command
>  evals the echo but echoes the simple quotes?
>
>  Is there any dark shopt option which makes it run correctly?
>
>  Thanks...
>  ----------------------------------------------
>  Rodolfo Alcazar Portillo - nospaze(a)gmail.com
>  otbits.blogspot.com / counter.li.org: #367962
>  ----------------------------------------------
>  # unzip; strip; touch; finger; grep; mount; fsck; more; yes; fsck; fsck;
>  fsck; umount; sleep
>
>  
I will do the short explanation

This expression does what I call Double expansion
 echo $($A)

First it expands    echo HI

Then it does the echo which yields   HI

In your second command

 echo 'hi'

It just echos   hi .. like it should

Sorry, if this doesn't sound real elegant..    Someone will give a
better description

HTH
Marvin J. Kosmal

Disclaimer: This email is not intended to provide medical advice,
diagnosis or treatment. Views expressed here do not necessarily
reflect those of Farmer Marvin or any of the hens.