assets.external now supports specifying target subdir

I did this to lessen risk of filename clashes in the thesis/assets/data/
folder. This way, I can easily have a separate subdir for each manuscript.

chertex.sh

@@ -134,10 +134,33 @@ if [ $# -eq 1 ]; then
       # find all files named "assets.external" in the assets/ tree
       assetsexternalfiles=$(find "$path_wd/assets/" -type f -name "assets.external")
       for externalfilepath in $assetsexternalfiles; do
-         # this weird-looking while-loop reads the assets.external file line-by-line and copies each asset into the target path inside the thesis' assets/ tree
          # https://stackoverflow.com/questions/10929453/read-a-file-line-by-line-assigning-the-value-to-a-variable
          while IFS='' read -r asset || [[ -n "$asset" ]]; do
+            # $asset contains one line from the current external.assets
+            # assetpathtarget is the directory of the current external.assets file
             assetpathtarget=$(dirname "$externalfilepath")
+            # if $asset contains a space, the second element should be considered a local target folder for the copy operation
+            # https://www.tutorialkart.com/bash-shell-scripting/bash-split-string/
+            # https://stackoverflow.com/a/30212526
+            IFS=' ' # reset IFS
+            # asset is read into an array as tokens separated by IFS
+            read -ra asset_array <<< "$asset"
+            # sanity check for array length
+            if [ ${#asset_array[@]} -gt 2 ]; then
+               echo "<thesis> Cannot handle more than one space per line"
+               echo "<cheRTeX> Terminating..."
+               simpledelay.sh 2
+               exit 1
+            fi
+            if [ ${#asset_array[@]} -gt 1 ]; then
+               # echo "Placing this asset into subdirectory ${asset_array[1]}"
+               # redefine assetpathtarget to include the local subdir
+               assetpathtarget="$assetpathtarget/${asset_array[1]}"
+               # also redefine $asset so we keep only the asset path
+               asset="${asset_array[0]}"
+               # create the local subdir inside assets/<current/ 
+               mkdir -p "$assetpathtarget" # -p suppresses error if dir already exists
+            fi            
             echo "<thesis> Copying $asset to $assetpathtarget"
             # cp but don't overwrite existing files
             cp --preserve=timestamps --no-clobber $asset $assetpathtarget