List modules that are missing from the filesystem using drush (Drupal 8)

d8-list-missing-modules.sh

drush updatedb-status 2>&1 > /dev/null | sed 's/\[warning\]//g' | tr '\n' ' ' | sed 's/bootstrap[^ ]* */\n/g' | grep 'module is missing' | sed -E 's/.*: *([^ ]+) */\1/g'

# annotated version:
drush updatedb-status 2>&1 > /dev/null |  # use updatedb-status because it will print an error message for each missing module but not make any changes to the drupal site; 2>&1 > /dev/null keeps only the error output
  sed 's/\[warning\]//g' |                # strip the string "[warning]" from the output
  tr '\n' ' ' |                           # convert line breaks to spaces, because the lines get broken in odd spots due to an 80-character line length limit
  sed 's/bootstrap[^ ]* */\n/g' |         # replace the pattern "bootstrap*" with a line break, because each separate warning ends with this string
  grep 'module is missing' |              # filter down to only lines containing the string "module is missing"
  sed -E 's/.*: *([^ ]+) */\1/g'          # strip everything but the word that appears after a colon, which is the module name