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Full Discussion: Safely parsing parameters
Top Forums Shell Programming and Scripting Safely parsing parameters Post 302405346 by drewk on Thursday 18th of March 2010 03:07:47 PM
Old 03-18-2010
If you you run your parameters through awk or perl, you can also break them apart if you do not want to use eval.

Code:
perl -nle 'BEGIN {map {print} @ARGV; exit;}' root=/dev/sda3 noacpi foo "Baz mumble" `echo muahahahaha >&2`
muahahahaha
root=/dev/sda3
noacpi
foo
Baz mumble

Note that the backticks you used in your example are being executed prior to being fed to the perl script. Whatever the user can execute using backticks on your command line, he would have the privileges to execute directly before he executed your script.

The security "hole" only exists if you elevate privileges in your script and then have a way to execute arbitrary code, no?

If you are still concerned, perl or awk can split arbitrary strings just like the shell inside of the interpreter, but this is not entirely trivial.

You would just need to decide which expansions you would want to support and which not:

Shell Expansions - Bash Reference Manual
 

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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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