Measure disk space of certain file types in aggregate

find folder1 folder2 -iname '*.txt' -print0 | du --files0-from - -c -s | tail -1

This will report disk space usage in bytes by extension:

find . -type f -printf "%f %s\n" |
  awk '{
      PARTSCOUNT=split( $1, FILEPARTS, "." );
      EXTENSION=PARTSCOUNT == 1 ? "NULL" : FILEPARTS[PARTSCOUNT];
      FILETYPE_MAP[EXTENSION]+=$2
    }
   END {
     for( FILETYPE in FILETYPE_MAP ) {
       print FILETYPE_MAP[FILETYPE], FILETYPE;
      }
   }' | sort -n

Output:

3250 png
30334451 mov
57725092729 m4a
69460813270 3gp
79456825676 mp3
131208301755 mp4

Simple:

du -ch *.txt

If you just want the total space taken to show up, then:

du -ch *.txt | tail -1

Here's a way to do it (in Linux, using GNU coreutils du and Bash syntax), avoiding bad practice:

total=0
while read -r line
do
    size=($line)
    (( total+=size ))
done < <( find . -iname "*.txt" -exec du -b {} + )
echo "$total"

If you want to exclude the current directory, use -mindepth 2 with find.

Another version that doesn't require Bash syntax:

find . -iname "*.txt" -exec du -b {} + | awk '{total += $1} END {print total}'

Note that these won't work properly with file names which include newlines (but those with spaces will work).