In /usr/bin/libnet-config line 28:
    -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;
                 ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
    -*=*) optarg=$(echo "$1" | sed 's/[-_a-zA-Z0-9]*=//') ;;


In /usr/bin/libnet-config line 29:
    *) optarg= ;;
       ^----^ SC2034: optarg appears unused. Verify use (or export if used externally).


In /usr/bin/libnet-config line 51:
    echo $libnet_libs
         ^----------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    echo "$libnet_libs"


In /usr/bin/libnet-config line 55:
    echo $libnet_cflags
         ^------------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    echo "$libnet_cflags"


In /usr/bin/libnet-config line 59:
    echo $libnet_defines
         ^-------------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    echo "$libnet_defines"

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- optarg appears unused. Verify use...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...