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Home > Archive > Unix Shell > December 2007 > if true ???
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| Hi folks,
the ksh knows the following exported aliases (amongst others):
true=':'
false='let 0'
So I could do a Boolean test in the following way:
if $BOOLTEST
then
# some code
fi
(provided I made sure that $BOOLTEST was set to true or false before)
Is this correct ksh-usage (syntax was checked by me, so this would be
o.k.) or is there something contradicting this usage?
I just ask since, although quite handy (avoiding the square braces
stuff), I have not seen that kind of code in a programmers script.
Cheers
Bernd
| |
| Lew Pitcher 2007-12-19, 7:27 pm |
| On Dec 19, 2:14 pm, bernd <bew...@gmx.net> wrote:
> Hi folks,
>
> the ksh knows the following exported aliases (amongst others):
>
> true=':'
> false='let 0'
>
> So I could do a Boolean test in the following way:
>
> if $BOOLTEST
> then
>
> # some code
> fi
>
> (provided I made sure that $BOOLTEST was set to true or false before)
>
> Is this correct ksh-usage (syntax was checked by me, so this would be
> o.k.) or is there something contradicting this usage?
I'm no ksh expert, but this usage looks OK to me; Mostly useless, but
OK
> I just ask since, although quite handy (avoiding the square braces
> stuff), I have not seen that kind of code in a programmers script.
What's wrong with
if /bin/true
then
# some code that is always performed
fi
and
if /bin/false
then
# some code that is never performed
fi
and (given your definitions of true and false above) why would you
need squarebracketted expressions?
| |
| Claudio 2007-12-19, 7:27 pm |
| On 2007-12-19, bernd <bew_ba@gmx.net> wrote:
> Hi folks,
>
> the ksh knows the following exported aliases (amongst others):
>
> true=':'
> false='let 0'
>
> So I could do a Boolean test in the following way:
>
> if $BOOLTEST
> then
>
> # some code
> fi
>
> (provided I made sure that $BOOLTEST was set to true or false before)
>
> Is this correct ksh-usage (syntax was checked by me, so this would be
> o.k.) or is there something contradicting this usage?
If you are not sure, execute the code in debug mode:
sh -x script
> I just ask since, although quite handy (avoiding the square braces
> stuff), I have not seen that kind of code in a programmers script.
I haven't too. You can't check the return value from a command/script in
this way. Is it a possible cause ?
| |
| Janis Papanagnou 2007-12-19, 7:27 pm |
| bernd wrote:
> Hi folks,
>
> the ksh knows the following exported aliases (amongst others):
>
> true=':'
> false='let 0'
>
> So I could do a Boolean test in the following way:
>
> if $BOOLTEST
> then
>
> # some code
> fi
>
> (provided I made sure that $BOOLTEST was set to true or false before)
>
> Is this correct ksh-usage (syntax was checked by me, so this would be
> o.k.) or is there something contradicting this usage?
"Correct" if the variable expands to the name of a command.
> I just ask since, although quite handy (avoiding the square braces
> stuff), I have not seen that kind of code in a programmers script.
And I think you've not yet seen it in existing code for good reasons.
The if expects a "compound list", so any command is appropriate. Now
assume you have branches in your code where variable BOOLTEST will
not be set (by accident or oversight or ignorance or a bug or future
extensions by other people, whatever). Then you can feed commands
from the environment. Assuming your script is named ifvar...
BOOLTEST="/bin/rm -rf" ksh ifvar
Choose arbitrary commands at your delight. It may not even be that
apparent as in my example if an export BOOLTEST="..." is defined
somewhere hidden in another script which calls your script.
Janis
>
> Cheers
>
>
> Bernd
| |
| Stephane Chazelas 2007-12-20, 1:40 am |
| On Wed, 19 Dec 2007 11:14:35 -0800 (PST), bernd wrote:
> Hi folks,
>
> the ksh knows the following exported aliases (amongst others):
>
> true=':'
> false='let 0'
>
> So I could do a Boolean test in the following way:
>
> if $BOOLTEST
> then
>
> # some code
> fi
>
> (provided I made sure that $BOOLTEST was set to true or false before)
>
> Is this correct ksh-usage (syntax was checked by me, so this would be
> o.k.) or is there something contradicting this usage?
[...]
It will only work as long as the IFS special parameter does not
contain any of the t, r, u, e, f, a, l, s characters, because by
leaving $BOOLTEST unquoted, you *explicitely request* it to
split it according to $IFS (and perform filename generation).
The correct syntax would be:
if "$BOOLTEST"
then
...
fi
Or you could use a function:
booltest() true
booltest() false
if booltest
then
...
fi
--
Stephane
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