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Full Discussion: difference in the two test operators
Top Forums Shell Programming and Scripting difference in the two test operators Post 302587794 by zulfi123786 on Friday 6th of January 2012 02:56:57 AM
Old 01-06-2012
difference in the two test operators
could any one please help me understand the difference between double square bracket test operator and single square bracket test operator.

I have seen one difference with the below code but not sure which one has to be used and when:

Code:
 
unix79:z54402:/data/ds/scripts $ cat > T
one,,two
unix79:z54402:/data/ds/scripts $ z=`cat T|cut -d, -f2,2`
unix79:z54402:/data/ds/scripts $ if [[ -z $z ]]
> then echo success
> else echo fail
> fi
success
unix79:z54402:/data/ds/scripts $ if [ -z $z ]
> then echo success
> else echo fail
>  fi
ksh: test: 0403-004 Specify a parameter with this command.
fail
unix79:z54402:/data/ds/scripts $

 

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LEARN ABOUT DEBIAN

pnmpsnr

pnmpsnr(1)						      General Commands Manual							pnmpsnr(1)

NAME

       pnmpsnr - compute the difference between two portable anymaps

SYNOPSIS

       pnmpsnr [pnmfile1] [pnmfile2]

DESCRIPTION

       Reads  two  PBM,  PGM, or PPM files, or PAM equivalents, as input.  Prints the peak signal-to-noise ratio (PSNR) difference between the two
       images.	This metric is typically used in image compression papers to rate the distortion between original and decoded image.

       If the inputs are PBM or PGM, pnmpsnr prints the PSNR of the luminance only.  Otherwise, it prints the separate PSNRs of the luminance, and
       chrominance (Cb and Cr) components of the colors.

       The  PSNR  of  a  given component is the ratio of the mean square difference of the component for the two images to the maximum mean square
       difference that can exist betwee any two images.  It is expressed as a decibel value.

       The mean square difference of a component for two images is the mean square difference of the component value, comparing  each  pixel  with
       the pixel in the same position of the other image.  For the purposes of this computation, components are normalized to the scale [0..1].

       The maximum mean square difference is identically 1.

       So the higher the PSNR, the closer the images are.  A luminance PSNR of 20 means the mean square difference of the luminances of the pixels
       is 100 times less than the maximum possible difference, i.e. 0.01.

SEE ALSO

       pnm(5)

								   04 March 2001							pnmpsnr(1)
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