in reply to quick way to add a line to a file, if it's not already present (automating tweaking of /etc/ld.so.conf for installing Image::Magick)

on my system (CentOS), I do this: 
 echo "/usr/local/lib" > /etc/ld.so.conf.d/ImageMagick.conf
 ldconfig

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As that goes in its own file, there's no need for trickery.

Oh, and of course my /etc/ld.so.conf is just this: 

include ld.so.conf.d/*.conf

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