As described by ss-3-1415926535897932384626433 there is no flag, but you have to get a list of files first and then check if your local files match. I wrote a little script for it:
#!/bin/sh
rfolder=/sdcard/DCIM/Camera
lfolder=Camera
adb shell ls "$rfolder" > android.files
ls -1 "$lfolder" > local.files
rm -f update.files
touch update.files
while IFS= read -r q; do
# Remove non-printable characters (are not visible on console)
l=$(echo ${q} | sed 's/[^[:print:]]//')
# Populate files to update
if ! grep -q "$l" local.files; then
echo "$l" >> update.files
fi
done < android.files
script_dir=$(pwd)
cd $lfolder
while IFS= read -r q; do
# Remove non-printable characters (are not visible on console)
l=$(echo ${q} | sed 's/[^[:print:]]//')
echo "Get file: $l"
adb pull "$rfolder/$l"
done < "${script_dir}"/update.files
Adjust the remote folder rfolder
and the local folder lfolder
to locations of your own choice.