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Full Discussion: if/else comparison with wildcard
Top Forums Shell Programming and Scripting if/else comparison with wildcard Post 302447105 by agama on Saturday 21st of August 2010 02:08:45 AM
Old 08-21-2010
Quote:
Originally Posted by cola
What's the problem if single bracket is used?
The open single bracket is a symlink to the test command. Shells (Ksh, bash) must treat it as an external command, and thus all of the command line is treated as an external command. Therefore, wildcards, unquoted, are expanded in the same manner as would be on any other command: as matching files in the filesystem. Quoted wildcard characters are treated as literals and passed to test which treats them as literals. So executing the following code takes the false branch:

Code:
if [ foo = "*o" ]
then
    echo "seemingly broken because this doesn't echo"
else
    echo "this will echo because * is interpreted literally by test"
fi

While this if statement takes the expected true branch:
Code:
if [[ foo == *"o" ]]

Beyond those differences, the double square bracketed expressions are interpreted by the shell, and no fork/exec overhead is needed, so they are much more efficient than using a single bracket.

Once in a long time does it make sense to use a single bracket expression (except when needing to write a pure bourn shell script), and even then I'd question its use.
 

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LEARN ABOUT HPUX

pextrem

PEXTREM(1)						      General Commands Manual							PEXTREM(1)

NAME

       pextrem - find minimum and maximum values in RADIANCE picture

SYNOPSIS

       pextrem [ -o ] [ picture ]

DESCRIPTION

       Pextrem	locates  the  minimum and maximum values for the input picture, and prints their pixel locations and color values.  The first line
       printed contains the x and y pixel location (x measured from the left margin, y measured from the bottom), followed by the red,	green  and
       blue values.  The second line printed contains the same information for the maximum value.

       The -o option prints the original (radiance) values, undoing any exposure or color correction done on the picture.

       If no input picture is given, the standard input is read.

AUTHOR

       Greg Ward

BUGS

       The luminance value is used for comparison of pixels, although in certain anomolous cases (ie. highly saturated colors) it is possible that
       pextrem will not pick the absolute minimum or maximum luminance value.  This is because a fast integer-space comparison is  used.   A  more
       reliable floating-point comparison would be slower by an order of magnitude.

SEE ALSO

       falsecolor(1), getinfo(1), pcomb(1), pcompos(1), pextrem(1), pfilt(1), pflip(1), protate(1), psign(1), rpict(1), ximage(1)

RADIANCE
							     11/15/93								PEXTREM(1)
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