Re: Error message garbage when parameter expansion used inside (()) and variable unset

[PRussell]

Chet, is the output on opensuse running bash 4.4.19, correct?

The specific output:

./t.sh: line 9: ���#V: var1 ==  : syntax error: operand expected (error 
token is "==  ")

archlinux has the same version of bash and I got the same results as on 
opensuse.

Below are the details of running ./t.sh &>t.log on multiple versions of 
bash.

When I run this script (t.sh),

......................
#!/bin/bash

# system info
lsb_release -d
printf -- 'Bash Version: %s\n\n' "${BASH_VERSION}"

# test example
echo 3B
  ( set -x; var=0;var1=var; (( var1 == $var2 )) && echo yes || echo no )
......................

I get,

......................
Description:  openSUSE Tumbleweed
Bash Version: 4.4.19(1)-release

3B
+ t.sh line 9 : var=0
+ t.sh line 9 : var1=var
+ t.sh line 9 : ((  var1 ==   ))
./t.sh: line 9: ���#V: var1 ==  : syntax error: operand expected (error 
token is "==  ")
+ t.sh line 9 : echo no
no
......................

......................
Description:    Linux Mint 17.3 Rosa
Bash Version: 4.3.11(1)-release

3B
+ t.sh line 9 : var=0
+ t.sh line 9 : var1=var
+ t.sh line 9 : ((  var1 ==   ))
./t.sh: line 9:  var1 ==  : var1 ==  : syntax error: operand expected 
(error token is "==  ")
+ t.sh line 9 : echo no
no
......................

If I remove the "set -x" I get,

......................
Description:  openSUSE Tumbleweed
Bash Version: 4.4.19(1)-release

3B
./t.sh: line 9: ((: var1 ==  : syntax error: operand expected (error 
token is "==  ")
no
......................

......................
Description:    Linux Mint 17.3 Rosa
Bash Version: 4.3.11(1)-release

3B
./t.sh: line 9: ((: var1 ==  : syntax error: operand expected (error 
token is "==  ")
no
......................

--
Peggy Russell

On 04/03/2018 08:49 AM, Chet Ramey wrote:
> On 4/2/18 5:16 PM, PRussell wrote:
> > Section 6.5 Shell Arithmetic says,
> >
> > "Within an expression, shell variables may also be referenced by name without
> > using the parameter expansion syntax. A shell variable that is null or unset
> > evaluates to 0 when referenced by name without using the parameter expansion
> > syntax." - http://www.gnu.org/software/bash/manual/bash.html#Shell-Arithmetic
> >
> > The above tells us what happens to an unset variable if not using parameter
> > expansion.
> >
> > But if a shell variable uses parameter expansion and is null or unset, what
> > does it evaluate to inside (()) syntax? 
>
> Since ((...)) is equivalent to let "...", as noted in the description of
> `((' in the man page and info doc, it expands to the same thing that it
> would when you perform a double-quoted word expansion. If you expand a
> shell variable that's null or unset, you get the usual: it disappears.
>
>
> > The problem is what is happening with 3B and 4B.  I tested on bash 4.3.11 and
> > bash 4.4.19 and got a slightly different error message.
> >
> > Bash version 4.3.11:
> >
> > ./tt: line 18: var1: var1 ==  : syntax error: operand expected (error token is "==  
> > ")
> >
> > Bash version 4.4.19 (???? was garabage):
> >
> > ./tt: line 18: ????: var1 ==  : syntax error: operand expected (error token is "==  
> > ")
>
> I don't get this. I get `((' as the command name in the error message for
> both bash-4.3.46 and bash-4.4.19:
>
> ./x18: line 1: ((: var1 ==  : syntax error: operand expected (error token
> is "==  ")