A shell script to join PDFs using macOS API ( https://doratex.hatenablog.jp/entry/20220330/1648629672 )

join.sh

#!/bin/bash
SCRIPTNAME=$(basename "$0")

function realpath () {
  f=$@;
  if [ -d "$f" ]; then
    base="";
    dir="$f";
  else
    base="/$(basename "$f")";
    dir=$(dirname "$f");
  fi;
  dir=$(cd "$dir" && /bin/pwd);
  echo "$dir$base"
}

function joinPDF () {
    /usr/bin/osascript \
      -e 'use framework "Quartz"' \
      -e "global CA, NSURL, PDFDocument" \
      -e "set CA to current application" \
      -e "set NSURL to CA's NSURL" \
      -e "set PDFDocument to CA's PDFDocument" \
      -e "on pdfPageCount(pdfPath)" \
      -e "set url to NSURL's fileURLWithPath:pdfPath" \
      -e "set doc to PDFDocument's alloc's initWithURL:url" \
      -e "doc's pageCount()" \
      -e "end pdfPageCount" \
      -e "on joinPDF(pdfPaths, outputPath)" \
      -e "set newPDF to PDFDocument's alloc's init" \
      -e "set pageCount to 0" \
      -e "repeat with pdfPath in pdfPaths" \
      -e "set url to (NSURL's fileURLWithPath:pdfPath)" \
      -e "set doc to (PDFDocument's alloc's initWithURL:url)" \
      -e "set lastIndex to ((my pdfPageCount(pdfPath)) - 1)" \
      -e "repeat with i from 0 to lastIndex" \
      -e "(newPDF's insertPage:(doc's pageAtIndex:i) atIndex:pageCount)" \
      -e "set pageCount to (pageCount + 1)" \
      -e "end repeat" \
      -e "end repeat" \
      -e "newPDF's writeToFile:outputPath" \
      -e "end joinPDF" \
      -e "my joinPDF({$1}, \"$2\")" \
      > /dev/null
}

function usage() {
    echo "Usage: $SCRIPTNAME -o <output> <input1> <input2> ..."
    echo
    echo "options:"
    echo "  -h, --help                       Show help"
    echo "  -o <output>, --output <output>   Specify output file"
    echo
}

# parse arguments
declare -a args=("$@")
declare -a params=()

OUTPUT=""

I=0
while [ $I -lt ${#args[@]} ]; do
    OPT="${args[$I]}"
    case $OPT in
        -h | --help )
            usage
            exit 0
            ;;
        -o | --output )
            if [[ -z "${args[$(($I+1))]}" ]]; then
                echo "$SCRIPTNAME: option requires an argument -- $OPT" 1>&2
                exit 1
            fi
            OUTPUT="${args[$(($I+1))]}"
            I=$(($I+1))
            ;;
        -- | -)
            I=$(($I+1))
            while [ $I -lt ${#args[@]} ]; do
                params+=("${args[$I]}")
                I=$(($I+1))
            done
            break
            ;;
        -*)
            echo "$SCRIPTNAME: illegal option -- '$(echo $OPT | sed 's/^-*//')'" 1>&2
            exit 1
            ;;
        *)
            if [[ ! -z "$OPT" ]] && [[ ! "$OPT" =~ ^-+ ]]; then
                params+=( "$OPT" )
            fi
            ;;
    esac
    I=$(($I+1))
done

# handle invalid arguments
if [ ${#params[@]} -eq 0 ]; then
    echo "$SCRIPTNAME: too few arguments" 1>&2
    echo "Try '$SCRIPTNAME --help' for more information." 1>&2
    exit 1
fi

if [ "$OUTPUT" = "" ]; then
    echo "Option -o cannot be omitted."
    echo "Try '$SCRIPTNAME --help' for more information." 1>&2
    exit 1
fi

# change input path list into AppleScript style
declare -a list=()
for FILE in "${params[@]}"; do
    list+=("\"$(realpath $FILE)\"")
done
INPUTS="$(echo "$(IFS=","; echo "${list[*]}")")"

# join PDFs
joinPDF "$INPUTS" "$OUTPUT"