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Shell Programming and Scripting
Variable File Descriptor
Post 302586545 by rpaskudniak on Monday 2nd of January 2012 11:11:30 AM
01-02-2012
Variable File Descriptor: Solved
Greetings again.
Very fortunately, I was able to bring my question to DGK himself and obtained permission to quote him. Here is his response, which answers several questions and even addresses my intended follow-up.
The really great thing about this is that it relieves me of the responsibility of finding an available file descriptor to use for my ancillary I/O; in obtaining the next available file descriptor from a pool, it behaves very much like the OPEN calls in C and Perl.
With gratitude to Dr. K, who has thus spared y'all from that lengthy followup, 
-- Rasputin Paskudniak
Very fortunately, I was able to bring my question to DGK himself and obtained permission to quote him. Here is his response, which answers several questions and even addresses my intended follow-up.
Quote:
The answer is that the posix standard only recognizes a digit in front of > to be taken as the file descriptor.
The reason for this is that when Steve Bourne added this feature, there were many scripts that used > without leaving a space. By restricting it to just digit, fewer scripts would break.
However, in 2005, in version ksh93r, I added the ability to put
{var}
in front of a redirection in which case, the shell would choose a file descriptor (> 10) and store the result in var.
Thus,
exec {fd}> file
print -u $fd ...
exec {fd}<& -
could be used to write to file and close it.
The } must precede the redirection operator without a space.
The reason for this is that when Steve Bourne added this feature, there were many scripts that used > without leaving a space. By restricting it to just digit, fewer scripts would break.
However, in 2005, in version ksh93r, I added the ability to put
{var}
in front of a redirection in which case, the shell would choose a file descriptor (> 10) and store the result in var.
Thus,
exec {fd}> file
print -u $fd ...
exec {fd}<& -
could be used to write to file and close it.
The } must precede the redirection operator without a space.
With gratitude to Dr. K, who has thus spared y'all from that lengthy followup, -- Rasputin Paskudniak
These 3 Users Gave Thanks to rpaskudniak For This Post:
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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)