Confounded by conditional expressions
in [Shell]
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From: Wayne on 12 Jul 2010 15:24 On 7/12/2010 2:00 PM, Robert Latest wrote: > I've written a script that contains this line: > > if [ ($last_seen -gt 0) -a ($last_seen -lt $toolate) ] ; then > # whatever > fi > > It throws this syntax error: > > test.sh: 23: Syntax error: word unexpected (expecting ")") > > Why? > > > robert The round parenthesis need quoting: if [ \($last_seen -gt 0\) -a \($last_seen -lt $toolate\) ] ; then -- Wayne
From: Thomas 'PointedEars' Lahn on 12 Jul 2010 16:12 Chris F.A. Johnson wrote: > Kenny McCormack wrote: >> In article <8a15fuFhq1U1(a)mid.individual.net>, >> Chris F.A. Johnson <cfajohnson(a)gmail.com> wrote: >> ... >>> Avoid -a and -o; use && and ||: >>> >>>if [ $last_seen -gt 0 ] && [ $last_seen -lt $toolate ] ; then >> >> Why? >> >> (Not that I have any strong opinion on the matter, and I even agree with >> you, but you usually have some obscure reason for your pronouncements) > > I can't cite chapter and verse OTTOMH, but I recall seeing an Open > Group recommendation about not using -a and -o. See the Single UNIX® Specification, Version 3 (SUSv3) at <http://www.opengroup.org/onlinepubs/000095399/utilities/test.html>, under "Application Usage". Note, however, that this non-normative recommendation refers to user-supplied input. So ISTM that in all other cases the options should be preferred as they do not require another invocation of test(1). On a side node, I have newly discovered link to a Firefox Search Plugin there; it leads to <http://mycroft.mozdev.org/download.html?name=unix.org&submitform=Search> which works quite well in Iceweasel 3.6.3. -- PointedEars
From: Sven Mascheck on 12 Jul 2010 17:15 Chris F.A. Johnson wrote: > On 2010-07-12, Kenny McCormack wrote: >> Chris F.A. Johnson <cfajohnson(a)gmail.com> wrote: >> ... >>> Avoid -a and -o; use && and || [...] >>> >> Why? >> > I can't cite chapter and verse OTTOMH, but I recall seeing an Open > Group recommendation about not using -a and -o. In the posix description of the "test" command these features are marked as "obsolescent". Strictly conforming applications shall not use them, and they might be removed in a future version of posix. The informative section "application usage" explains that some expressions are ambiguous depending on the specific expressions being used. A drastic example, especially if the input is not well known: test -n "$a" -a -n "$b" || echo at-least-one-is-empty with a==; b=x This wrongly echoes or even yields a syntax error in current bash, ksh93 and dash, and also in ksh88 and most bourne shells. BTW, noteworthy differences are: - && and || have equal precedence (order matters) - most shells always evaluate both sides of -a or -o, even if they could stop after the left expression. -- http://www.in-ulm.de/~mascheck/various/test/
From: Kenny McCormack on 12 Jul 2010 17:34 In article <i1g0mbUf5mL1(a)news.in-ulm.de>, Sven Mascheck <mascheck(a)email.invalid> wrote: >Chris F.A. Johnson wrote: >> On 2010-07-12, Kenny McCormack wrote: >>> Chris F.A. Johnson <cfajohnson(a)gmail.com> wrote: >>> ... >>>> Avoid -a and -o; use && and || [...] >>>> >>> Why? >>> >> I can't cite chapter and verse OTTOMH, but I recall seeing an Open >> Group recommendation about not using -a and -o. > >In the posix description of the "test" command these features are >marked as "obsolescent". Strictly conforming applications shall not >use them, and they might be removed in a future version of posix. >The informative section "application usage" explains that some >expressions are ambiguous depending on the specific expressions >being used. > > >A drastic example, >especially if the input is not well known: > > test -n "$a" -a -n "$b" || echo at-least-one-is-empty > >with > a==; b=x > >This wrongly echoes or even yields a syntax error in current >bash, ksh93 and dash, and also in ksh88 and most bourne shells. > > >BTW, noteworthy differences are: >- && and || have equal precedence (order matters) >- most shells always evaluate both sides of -a or -o, > even if they could stop after the left expression. These are convincing arguments. I never liked -a and -o anyway. -- "We should always be disposed to believe that which appears to us to be white is really black, if the hierarchy of the church so decides." - Saint Ignatius Loyola (1491-1556) Founder of the Jesuit Order -
From: realto on 12 Jul 2010 22:48
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