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Top Forums UNIX for Dummies Questions & Answers Understanding the output command Post 302386816 by Corona688 on Wednesday 13th of January 2010 04:54:30 PM
Old 01-13-2010
Quote:
Originally Posted by Ariean
I am missing something what do u mean by standard error? i understand standard output is the output printed on screen when i execute the script. But i didn't understand "2>&1" part, whats happening over there?

Thanks,
Ariean
By tradition, a UNIX process has three default streams: standard input(stdin), standard output(stdout), and standard error(stderr). stdin is represents the terminal keyboard as file descriptor 0, stdout represents the terminal screen as file descriptor 1, and stderr is also directed to the terminal screen as file descriptor 2.

The idea with having two files going to the terminal is to keep data and error messages separate. You can redirect the data output of a process into a file and still see human-readable error messages on your terminal. It also helps keep error messages OUT of data files since other programs probably won't need or understand them.

Here they're redirecting both stdout and stderr into the data file, for better or for worse. The "2>&1" bit tells it to redirect FD 2, stderr, to the same destination as FD 1, stdout.
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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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