#!/bin/sh
SELF=$(basename $0)                                                 #uses the same name as the file
UPDATE_BASE=https://gitlab.com/swe_toast/self-update/raw/master/    #dont link directly to file link to the folder
UPDATE_SELF=${UPDATE_SELF:-1}

self_updater () {
        echo "Performing self-update... Please wait..."
if      ! wget --quiet --output-document="$0.tmp" $UPDATE_BASE/$SELF ;
then    echo "Failed downloading new version!"
        echo "File requested: $UPDATE_BASE/$SELF"
        exit 1
fi

OCTAL_MODE=$(stat -c '%a' $0)
    case ${OCTAL_MODE:--1} in
        -[1] )          echo "Failed: Error : OCTAL_MODE was empty"; exit 1 ;;
        777|775|755 )   echo "OCTAL mode set:${OCTAL_MODE}" ;;
        * )             echo "Failed: Error in OCTAL_MODEs, found value=${OCTAL_MODE}" exit 1 ;;
    esac

if      ! chmod $OCTAL_MODE $0.tmp ;
then    echo "Error on chmod $OCTAL_MODE %0.tmp from $UPDATE_BASE/$SELF, can't continue"
        exit 1
fi

cat > /tmp/updatescript.sh << EOF
#!/bin/sh
if mv "$0.tmp" "$0";
then    echo "Update complete.."
        exec env UPDATE_SELF=0 /bin/sh "$0"
        rm \$0
else    echo "Update failed.."
fi
EOF
        echo "Inserting update process..."
            exec /bin/sh /tmp/updatescript.sh; rm /tmp/updatescript.sh
}

check_update () { if [ ${UPDATE_SELF} -ne 0 ]; then self_updater; fi }

check_update

Simple script that updates itself upon launch then lauches whatever else thats included afterwards works well in almost any linux system tested on busyboxes and Debian systems

Selfupdating Script

Bash