[gentoo-dev] Why don't they use -o in system scripts?

[gentoo-dev] Why don't they use -o in system scripts?

[C. Linus Hicks]

I'm confused about why they use [ expression1 ] || [ expression2 ]
rather than [ expression1 -o expression2 ] in system scripts like
/etc/profile. It doesn't do the same thing, and I suppose the results
are usually the same. However, my read of the script is that what they
really want is a logical OR which is what the -o gives you.

-- 
C. Linus Hicks <lhicks@nc.rr.com>


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Re: [gentoo-dev] Why don't they use -o in system scripts?

[Martin Schlemmer]

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On 31 Dec 2002 00:46:00 -0500
"C. Linus Hicks" <lhicks@nc.rr.com> wrote:

> I'm confused about why they use [ expression1 ] || [ expression2 ]
> rather than [ expression1 -o expression2 ] in system scripts like
> /etc/profile. It doesn't do the same thing, and I suppose the results
> are usually the same. However, my read of the script is that what they
> really want is a logical OR which is what the -o gives you.
> 

No offense, but I think you need to checkup on your bash again.

---------------------------
if [ -n $x ] && [ -n $y ]
then
	echo "yes"
fi
---------------------------

are equivalent to:

---------------------------
if [ -n $x -a -n $y ]
then
	echo "yes"
fi
---------------------------

Same goes for '-o'.  '-o' and '-a' are just the
simpler forms of [ ] || [ ] and [ ] && [ ].

Also, with '-o' and '-a' you cannot do more
advance things like:

----------------------------------------------
if [ -n $x ] && ([ -n $y ] || [ -n $z ]) ...
----------------------------------------------

Although above you could have done:

----------------------------------------------
if [ -n $x ] && [ -n $y -o -n $z ] ...
----------------------------------------------

But then you still would need the '&&' in this case,
or a '||' in maybe some other case ...

You might have been thinking of:

----------------------------------------------
if [[ -n $x && -n $y || &z ]] .....
----------------------------------------------

in which case the first equation that returned true, would
have satisfied the equation, and are NOT the same as above
examples  ....


Regards,

-- 

Martin Schlemmer
Gentoo Linux Developer, Desktop/System Team Developer
Cape Town, South Africa


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Re: [gentoo-dev] Why don't they use -o in system scripts?

[Phil Richards]

On Fri, Jan 03, 2003 at 11:59:32AM +0200, Martin Schlemmer wrote:
> On 31 Dec 2002 00:46:00 -0500
> "C. Linus Hicks" <lhicks@nc.rr.com> wrote:
> > I'm confused about why they use [ expression1 ] || [ expression2 ]
> > rather than [ expression1 -o expression2 ] in system scripts like
> > /etc/profile. [...]
> No offense, but I think you need to checkup on your bash again.
[...]
> Same goes for '-o'.  '-o' and '-a' are just the
> simpler forms of [ ] || [ ] and [ ] && [ ].
> 
> Also, with '-o' and '-a' you cannot do more
> advance things like:
> 
> ----------------------------------------------
> if [ -n $x ] && ([ -n $y ] || [ -n $z ]) ...
> ----------------------------------------------

Erm, yes you can:

  if [ -n $x -a \( -n $y -o -n $z \) ] ...

[...]
> You might have been thinking of:
> 
> ----------------------------------------------
> if [[ -n $x && -n $y || &z ]] .....
> ----------------------------------------------
> 
> in which case the first equation that returned true, would
> have satisfied the equation, and are NOT the same as above
> examples  ....

But,

  if [[ -n $x && ( -n $y || -n $z ) ]] .....

is.

My preference is for the "[[ ... ]]" form - it tends to be more readable
(especially for people from a C/C++ background) and doesn't require
\-protection for nested "(...)" expressions.

There is no practical reason for not using either [-a/-o] or [[&&/||]]
in preference to the current approach.

However, it is purely a stylistic thing.  There are no big wins
either way.

Anyway, the current /etc/profile uses both forms.

pihl

-- 
Phil Richards, <gentoo@derived-software.ltd.uk>
Derived Software Limited

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