On Sat, 2021-03-20 at 07:09 -0400, Scott Andrews wrote:
> On Fri, 19 Mar 2021 23:26:01 +0100
> Pierre Labastie <pierre.labas...@neuf.fr> wrote:
>
> > On Fri, 2021-03-19 at 07:24 -0400, Scott Andrews wrote:
> > > On Fri, 19 Mar 2021 07:09:06 +0100
> > > Pierre Labastie <pierre.labas...@neuf.fr> wrote:
> > >
> > > >
> > > > Using brackets depends on "something". If something is a
> > > > predicate, you need brackets. If something is a (compound)
> > > > command, no brackets. I've not looked in details at the "if" in
> > > > the boot scripts, but they must be correct in this respect,
> > > > since they seem to work...
> > > >
> > >
> > > By brackets are you talking about [ ] or [[ ]]?
> > >
> > > if [ something ]; then this; fi,
> > >
> > > Really means
> > >
> > > if test something; then this; fi
> > >
> > > [ is actually test, and the [ must be followed by ]
> > >
> > > [[ ]] is an expression
> > >
> > > See the gnu bash reference manual for clarification.
> > >
> >
> > Here is an experiment:
> > ---
> > pierre [ ~ ]$ toto () {
> > > return 1
> > > }
> > pierre [ ~ ]$ if toto; then echo true; else echo false; fi
> > false
> > pierre [ ~ ]$ if [ toto ]; then echo true; else echo false; fi
> > true
> > ---
> >
> > Pierre
> >
> Here is a more complete test:
>
>
> toto () {
> return 1
> }
> gogo () {
> return 0
> }
>
> echo "toto test"
>
> if toto; then echo true; else echo false; fi
>
> if [ toto ]; then echo true; else echo false; fi
>
> if test toto; then echo true; else echo false; fi
>
> if [[ toto ]]; then echo true; else echo false; fi
>
> echo "gogo test"
>
> if gogo; then echo true; else echo false; fi
>
> if [ gogo ]; then echo true; else echo false; fi
>
> if test gogo; then echo true; else echo false; fi
>
> if [[ gogo ]]; then echo true; else echo false; fi
>
> Results:
> toto test
> false
> true
> true
> true
> gogo test
> true
> true
> true
> true
>
> Here is what the gnu bash manual states:
>
> test
> [
> test expr
> Evaluate a conditional expression expr and return a status of 0
> (true) or 1 (false). Each operator and operand must be a separate
> argument. Expressions are composed of the primaries described below
> in Section 6.4 [Bash Conditional Expressions], page 92. test does not
> accept any options, nor does it accept and ignore an argument of --
> as signifying the end of options. When the [ form is used, the last
> argument to the command must be a ].
Conclusion: test/[ must be used with a conditional expression (what I called a
"predicate", which is not the right word, sorry about that). To add to the test,
try with an undefined variable/function:
if [ undefined ]; then echo true; else echo false; fi
(returns true: reason, a non empty string is always true as a conditional
expression)
if undefined; then echo true; else echo false; fi
(returns an error "bash: undefined: command not found". Could be anticipated)
and with an empty variable:
empty=
if [ $empty ]; then echo true; else echo false; fi
(returns false: an empty string is false as a conditional)
if $empty; then echo true; else echo false; fi
(returns true! the empty command (typing just enter) has exit status equal to 0)
but:
if ; then echo true; else echo false; fi
(returns an error "bash: syntax error near unexpected token `;')
and just typing:
""<enter>
returns "bash: : command not found"
Anyway, I hope you are now convinced that the right command for testing script
status is:
if ! check_script_status; then continue; fi
Pierre
--
http://lists.linuxfromscratch.org/listinfo/lfs-support
FAQ: http://www.linuxfromscratch.org/blfs/faq.html
Unsubscribe: See the above information page
Do not top post on this list.
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
http://en.wikipedia.org/wiki/Posting_style
