dn = "/dev/null"
dn = "nul" if (/mswin|mingw|bccwin/ =~ RUBY_PLATFORM)
APT_   = ( system( "apt-get --version 2> #{dn} > #{dn}" ) ? true : false )
CVS_   = ( system( "cvs --version 2> #{dn} > #{dn}" ) ? true : false )
GIT_   = ( system( "git --version 2> #{dn} > #{dn}" ) ? true : false )
GREP_  = ( system( "grep --version 2> #{dn} > #{dn}" ) ? true : false )
SVN_   = ( system( "svn --version 2> #{dn} > #{dn}" ) ? true : false )
ZIP_   = ( system( "zip --version 2> #{dn} > #{dn}" ) ? true : false )
UNZIP_ = ( system( "unzip -v 2> #{dn} > #{dn}" ) ? true : false )
GZIP_  = ( system( "gzip --version 2> #{dn} > #{dn}" ) ? true : false )
BZIP2_ = ( system( "bzip2 --help 2> #{dn} > #{dn}" ) ? true : false )
TAR_   = ( system( "tar --version 2> #{dn} > #{dn}" ) ? true : false )
ARC_ = ( ZIP_ && UNZIP_ && GZIP_ && BZIP2_ && TAR_ )