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On Thu, Jun 09, 2016 at 10:41:27PM -0400, Gene Heskett wrote:
> Greetings;
>
> A bash script that has worked most of a decade now refuses.
>
> For instance, assume that var InMail is = "gene", and can be echoed from
> the command line using $InMail like this.
> gene@coyote:~$ echo $InMail
> gene
> But I'll be switched if I can get a result from a line of code resembling
> this from the command line while attempting to troubleshoot a 110 line
> bash script: which asks "if test [${InMail} = "gene"]
> then
> -----
> elif (another name)
> yadda yadda
>
> gene@coyote:~$ echo `test [${InMail} = "gene"]`
Most has been answered in this thread here and there, but I think some
things bear repeating:
test and []
Basically, 'test' and '[' are the same -- well, more or less. There's
even an executable /usr/bin/[ (no kidding) which in former times was
just a hardlink to /usr/bin/test (I lost track of why it ain't these
days). Those days test and/or [ are mostly shell builtins.
So in
if test [ ... ] ; then
either the test or the [] is redundant. Better spell as
if test ...
or
if [ ... ]
depending on your taste.
exit value vs stdout vs command line args
echo just echoes its command line args. So
echo foo
will output foo to stdout
echo $InMail
will output 'gene", because the shell will get its first pass through
the line, will replace $InMail with gene and echo will see 'gene' as
its first command line arg and do its thing. In
echo `test [${InMail} = "gene"]`
the shell again will have a first go. The `` will tell it to interpret
the thing inside (i.e. test ...) and literally replace its *standard
output* in place. Test doesn't output anything, thus the whole
construct `...` gets replaced by nothing, echo sees an empty arg
and outputs nothing (followed by a newline, which echo does by
default).
This is the empty line you are seeing.
But, you will ask, where's the result of the test gone? Test is
called for its *exit value*, a number (which by convention is
0 when everything went OK and different from 0 for some error
condition). You can use this exit value in the "if" construct
(also in "while" and so on), and you can extract it from the
special variabl $?. Also, you can use it in chained logical
constructs as in "foo && bar || baz" (e.g.:
rm -f bumble || echo "can't remove this thing"
would be a more concise way to spell
if ! rm -f bumble ; then echo "darn" ; fi
Happy hacking!
regards
- -- t
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