If you want to put a FAT32 partition called WINDOWS on an APFS volume this is what I created to do it in our environment.
You might need to find some analogous commands for none APFS or other formats than FAT32.
It will default to around 50% otherwise you can pass it a preferred MacOS size as a percent.
Diskutil is annoying in that you pass it a % and it says I can't do percentages, so someone took the time to create an error message specifically about percentages instead of just using that time to create the calculations for the actual partition size.
#!/bin/bash
MacOS_Preferred_Size=$4
if [[ ! "$MacOS_Preferred_Size" ]];then
MacOS_Preferred_Size=50
fi
Boot_File_Type=$(diskutil info / | awk '/File System Personality:/ {print $NF}')
Boot_Drive_Used=$(diskutil info / | awk '/Volume Used Space:/ {print $NF}' | sed -e 's/^(//g' -e 's/[.].*//g')
Windows_Partition=$(diskutil list | awk '/Microsoft Basic Data/ || /WINDOWS/ {print $NF}')
if [[ "$Boot_File_Type" == "APFS" ]] && [[ "$Boot_Drive_Used" -lt 80 ]] && [[ "$Windows_Partition" == "" ]]; then
echo "Do"
macOS_Size=$((10 + Boot_Drive_Used))
if [[ "$macOS_Size" -lt $MacOS_Preferred_Size ]]; then
macOS_Size=$MacOS_Preferred_Size
fi
Container_ID=$(diskutil info / | awk '/Part of Whole:/{print $NF}')
Old_Container_Size=$(diskutil info / | awk '/Volume Total Space:/' | sed -e 's/ Bytes.*//g' -e 's/^.*(//g')
New_Container_Size=$((Old_Container_Size / 100 * macOS_Size))
echo "The APFS volume will be shrunk to $New_Container_Size Bytes and the remainder will be made into WINDOWS"
if [[ "$New_Container_Size" -gt 0 ]]; then
diskutil APFS resizecontainer $Container_ID $New_Container_Size FAT32 WINDOWS 0
fi
else
echo "Do not"
fi
