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Full Discussion: While Loop Syntax help needed
Top Forums Shell Programming and Scripting While Loop Syntax help needed Post 302335460 by kshji on Sunday 19th of July 2009 12:26:32 PM
Old 07-19-2009
Simple syntax:
Code:
while command_exit_code_is_zero
do
    somecmd1
    somecmd2
done

-lt, -gt, ... are options of test command, look shell builtin test (= also [ ] )

After while is some command line, shell will do it, check exit code and if it's 0 = okay, then do the command lines between do and done.

while true
while cp x y
while (( i < 100 ))
while [ "$some" -lt 100 ]
while test "$some -lt 100
while [ "$some" = "1" ]
while read line
while :
...
: is command, which is builtin and give always exit 0 status ( shell's nop command), do nothing. Today also true and false are builtin commands, so those are more "readable" as :

You can break the loop using command break.

You can jump to the while line using command continue.
ex.
Code:
while read line
do
      case "$line" in
           "") continue  ;;    # next line
           end*) break ;;     # end this while
      esac
      do_some_for "$line"
done

 

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LEARN ABOUT REDHAT

ppmtosixel

ppmtosixel(1)						      General Commands Manual						     ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.	The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
	      pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order	of  magni-
	      tude larger than a compressed file and prints much slower.

       -margin
	      If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
	      fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater	than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

								   26 April 1991						     ppmtosixel(1)
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