evaluation of env variables in DASH

evaluation of env variables in DASH

[Dima Sorkin]

Hi.
  The following DASH behaviour seems buggy to me

-----------------
$ export A='\n'
$ echo $A

$
-----------------

whereas using BASH would result in printing literally \n .

I.e. presumingly DASH does secondary interpolation of escaped
symols in values of environment variables. 

Please CC me with replies, I am not in the list.

Thanks, regards,
  Dima.

P.S.
  Wrom the other hand

$ export B=1
$ export A='$B'
$ echo $A
PRINTS $B
$

Re: evaluation of env variables in DASH

[Eric Blake]

On 10/19/2011 03:24 PM, Dima Sorkin wrote:
> Hi.
>    The following DASH behaviour seems buggy to me

The only bug here is your expectations.

>
> -----------------
> $ export A='\n'
> $ echo $A
>

Passing a literal backslash to echo is non-portable.  POSIX even says 
so.  And bash can match dash behavior:

$ (shopt -s xpg_echo; A='\n'; echo -$A-)
-

Re: evaluation of env variables in DASH

[Jilles Tjoelker]

On Wed, Oct 19, 2011 at 11:24:50PM +0200, Dima Sorkin wrote:
>   The following DASH behaviour seems buggy to me

> -----------------
> $ export A='\n'
> $ echo $A
> 
> $
> -----------------

> whereas using BASH would result in printing literally \n .

> I.e. presumingly DASH does secondary interpolation of escaped
> symols in values of environment variables. 

The echo builtin in dash differs from most other echo utilities on Linux
and *BSD in interpreting System V-like backslash escape sequences. This
is the "expansion" you are seeing and the same thing can be seen in
  echo '\n'
It is documented in dash's manual page.

This also happens in bash if you 'shopt -s xpg_echo'.

This behaviour is permitted by POSIX and required for the XSI option,
and is more commonly seen on Solaris or other System V derivatives.

The fix is to use printf(1) instead of echo(1) if there is a possibility
the string may start with '-' or contain '\'. In this case,
  printf '%s\n' "$A"

This has been asked/reported more frequently and the answer has been
that it will not be changed.

-- 
Jilles Tjoelker

Re: evaluation of env variables in DASH

[Dima Sorkin]

> ... The echo builtin in dash ...

What confused me is that I thought that /bin/echo is used, and wrong
argument is passed to it. I was not aware it is a builtin.

Thanks for replies, regards,
  Dima.