[ILUG] test(1), [(1), and "X$str" [was- POTD reminder ]

[Brian Foster]

  | From: P at draigBrady.com
  | Date: Mon, 13 Dec 2004 17:31:45 +0000
  | 
  | kevin lyda wrote:
  | > On Mon, Dec 13, 2004 at 04:07:38PM +0000, P at draigBrady.com wrote:
  | >>Shell conditionals
  | >>   if [ "$str" = "1" ] || [ "$int" -eq "1" ]; then
  | >>        echo "str or int is 1"
  | >>   elif [ "$int" -lt "10" ] && [ -e "file" ]; then
  | >>        echo "int less than 10 and file exists"
  | >>   fi
  | > 
  | > it's better if you do the following:
  | >    if [ X"$str" = X"1" ] || [ "$int" -eq "1" ]; then
  | >        echo "str or int is 1"
  | >    elif [ "$int" -lt "10" ] && [ -e "file" ]; then
  | >        echo "int less than 10 and file exists"
  | >    fi
  | > that way if $str begins with a - it won't annoy test.

 good grief.
 I hereby award both of the above the
 Useless Use of Quotes Award.  multiple times.

 something like the following is sufficient (but, like the
 above, assumes that $int has an integer value (in which
 case the quotes bracketing $int are not strictly needed)):

        if [ "X$str" = X1 -o "$int" -eq 1 ]; then
             echo str or int is 1
        elif [ "$int" -lt 10 -a -e file ]; then
             echo int less than 10 and file exists
        fi

 on the "X$str" trick, however you write it, it _is_ needed
 on various (older) Bourne-ish shells.  current GNU bash(1)
 has heuristics that usually don't go wrong, but which apply
 only to its built-in test(1).  if you are (or may be) using
 some other Bourne-ish shell, or not using the built-in test,
 you are much much better off using the "X$str" trick.

  | Apart from that X"$str" trick looking horrible,
  | it's unecessary from my testing. test and the
  | builtin [ seem to be clever enough to parse options
  | only when appropriate.

 (IMHO, the only reason the above "X$str" trick looks horrible
 is because it was horribly written.)

 `test' and `[' are synonyms.  they are should be identical.
 both are builtin to most Bourne-ish shells, and both are
 also external commands (e.g., you probably have a /usr/bin/[
 linked to /usr/bin/test --- albeit having said that, I note
 that for some reason my SUSE 9.1 system has different binaries
 for /usr/bin/test and /usr/bin/[ --- they are not linked, and
 the files themselves are different, and they handle `--version'
 differently.  *sigh*)

cheers!
	-blf-
-- 
Experienced (20+ years) kernel engineer: |  Brian Foster      Montpellier,
 · Unix, ChorusOS, &tc; · documentation; |  blf at utvinternet.ie      FRANCE
 · IDL, automated testing, process, &tc. |    Stop E$$o (ExxonMobile)!
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