Re: Bash Question
Am Donnerstag, 6. Juli 2017, 11:50:44 CEST schrieb tomas@tuxteam.de:
> On Thu, Jul 06, 2017 at 09:57:50AM +0100, Darac Marjal wrote:
> > On Thu, Jul 06, 2017 at 12:22:29AM +0200, Javier Barroso wrote:
> > >Hi,
> > >
> > >On Wed, Jul 5, 2017 at 11:12 PM, Rainer Dorsch <ml@bokomoko.de> wrote:
> > >>Hi,
> > >>
> > >>
> > >>
> > >>can anybody help to explain what is going on here ?
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>rd@mohot:~$ echo $SHELL
> > >>/bin/bash
> > >>rd@mohot:~$ if [ "abc" > "dec" ]; then echo bad; fi
> > >>bad
> > >>rd@mohot:~$ if [ "abc" < "dec" ]; then echo good; fi
> > >>good
> > >>rd@mohot:~$
> > >>
> > >>How can abc sort before and after dec at the same time?
> > >
> > >You need to scape "<" and ">":
> > >
> > >if [ "abc" \> "dec" ] ; then ... ;fi
> > >if [ "abc" '>' "dec" ]; then ... ; fi
> > >if [ "abc" ">" "dec" ]; then ... ; fi
> > >
> > >
> > >Delete "dec" file, ( ">" is redirection in bash, so you was creating that
> > >file>
> > That's a very good point. ">" and "<" are NOT the greater-than/less-than
> > operators in bash.
>
> They are (in bash: see below!) -- just check bash's manual page under
> "conditional expressions".
>
> For strings, they list '==' and '=', '!=', '<' and '>'.
>
> Now *if* you use '[' (single square bracket) as above, you are not using
> bash's built in [this is a little white lie[1], see below], but the external
> command 'test' or some cousin of that (just try "ls -l /usr/bin/[" to see
> what I mean. This one doesn't understand < and >.
>
> If you want bash's builtin, use '[[' (double square bracket). It's faster
> (one less fork/exec), but also more convenient, because it "knows" that
> there's special syntax inside, so you don't need to escape < and > (as
> someone pointed out in this thread, see (again) below).
>
> This works:
>
> if [[ "abc" < "bcd" ]] ; then echo "yo" ; else echo "no" ; fi
>
> Or this
>
> if [[ -n "abc" && ( "abc" < "bcd" ) ]] ; then echo "yo" ; else echo "no"
> ; fi
>
> (note I didn't have to escape the parens, which would wreak havoc
> elsewhere in the command line, because they have a special meaning
> for the shell, and the shell does expansion before calling commands.
> Only a builtin can achieve that).
>
> Now try this:
>
> if [ "abc" < "bcd" ] ; then echo "yo" ; else echo "no" ; fi
>
> (note I chose '<' -- and hope there's no file 'bcd' lying around).
> Then I get:
>
> bash: bcd: No such file or directory
>
> Now this all was a lie :-)
>
> Or half. The '[' can be a builtin, after all (in bash). You can
> switch that on or off with the enable -- uh -- builtin.
>
> Just type "help enable" at your favourite bash instance to learn
> about that.
>
> But the '[' builtin has to behave as far as possible like the
> /usr/bin/[ of yore (aka POSIX 'test'). Including the way the
> shell "expands its innards" before handing things to the command.
>
> That's why there is [[. Much easier to write and much easier on
> the eye.
>
> BUT: this all won't work on dash (Debian's preferred non-interactive
> shell: much smaller and faster). There you have to spell things like
> so:
>
> if [ "abc" \< "bcd" ] ; then echo "yo" ; else echo "no" ; fi
>
> So dash's '[' is a builtin too (remember: /usr/bin/[ doesn't grok
> '<' at all). But dash hasn't the '[[' with its syntactic convenience.
>
> Hope that helps, somewhat :-)
>
Many thanks for all the answers (and scaring that kmail just omitted some
stuff in the Text mail and just included it in the HTML part...this time w/o
html). They helped a lot to solve my problem...
Thanks again
Rainer
--
Rainer Dorsch
http://bokomoko.de/
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