Re: if [[ $Something ]] – in what situations does this work?

[Robert P. J. Day]

On Sat, 18 Feb 2012, J wrote:

> On Sat, Feb 18, 2012 at 10:02, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
> > On Sat, 18 Feb 2012, franz.reitinger wrote:
> >
> >> First of all I suppose you're using the bash.
> >> Amoung others the shell treats several parameters specially; these
> >> parameters may only be referenced and assignement to them is not
> >> allowed.
> >>
> >> One of them is $?, which expands to the exit status of the most
> >> recently executed command. However, the exit status 0 means that the
> >> command has been executed without errors; any other return value has to
> >> be interpreted as error code.
> >> e.g.
> >> # any command ...
> >> if [ $? -ne 0 ]; then
> >>     # error handling
> >> fi
> >
> >  i'd have to check but i think the constuct
> >
> >  if [[ $VAR ]]
> >
> > does nothing but check if either that variable is defined, or if it
> > has a non-zero string length.  so no matter what the value of $? is,
>
> Correct:
>
> bladernr at klaatu:~$ A=foo
> bladernr at klaatu:~$ if [[ $A ]]; then echo YES; fi
> YES
> bladernr at klaatu:~$ if [[ $B ]]; then echo YES; fi
>
> AFAIK, that syntax will only work on user defined variables...

  a simpler sequence of statements demonstrating that:

$ cat /etc/passwd
...
$ [[ $? ]] && echo yup
yup
$ cat /etc/paswd
No such file
$ [[ $? ]] && echo yup
yup
$

  obviously, the value of $? is different in those two cases, but
testing it as above evaluates to "true" each time.

  again, if you're testing $?, you should do an actual comparison.

rday

p.s.  not to brag or anything but if you have a copy of o'reilly's
"classic shell scripting", yes, that's my name in the
acknowledgements. :-)

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
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