hello from shellcheck

shellcheck has some hints what could be improved and some of the are worth a look:

$ shellcheck cargo.sh 

In cargo.sh line 9:
SCRIPT_PATH=`realpath $0`
^-- SC2034: SCRIPT_PATH appears unused. Verify use (or export if used externally).
            ^-- SC2006: Use $(..) instead of legacy `..`.
                      ^-- SC2086: Double quote to prevent globbing and word splitting.


In cargo.sh line 10:
SOURCE_DIR=`dirname $0`
           ^-- SC2006: Use $(..) instead of legacy `..`.
                    ^-- SC2086: Double quote to prevent globbing and word splitting.


In cargo.sh line 12:
export CARGO_TARGET_DIR=`pwd`
       ^-- SC2155: Declare and assign separately to avoid masking return values.
                        ^-- SC2006: Use $(..) instead of legacy `..`.


In cargo.sh line 13:
if [ ! -z ${1} ]; then
          ^-- SC2086: Double quote to prevent globbing and word splitting.


In cargo.sh line 14:
    OUT_PATH=`realpath "${1}"`
             ^-- SC2006: Use $(..) instead of legacy `..`.


In cargo.sh line 17:
cd $SOURCE_DIR
   ^-- SC2086: Double quote to prevent globbing and word splitting.


In cargo.sh line 19:
cargo $BUILD_ARG $@
      ^-- SC2086: Double quote to prevent globbing and word splitting.
                 ^-- SC2068: Double quote array expansions to avoid re-splitting elements.


In cargo.sh line 21:
if [ ! -z ${OUT_PATH} ]; then
          ^-- SC2086: Double quote to prevent globbing and word splitting.