This script can be used to add a list of words on the command line to the user dictionary of Aspell, e.g. ~/.aspell.en.pws.

for word in "$@"
do
    echo -e "*$word\n#" | aspell -a
    echo "$word is added!"
done

The option -a lets aspell enter a pipe mode, which supports the following commands. In our script, combining the commands *word and #, the output of echo is redirected to the standard input of aspell.

*word Add a word to the personal dictionary
&word Insert the all-lowercase version of the word in the personal dictionary
@word Accept the word, but leave it out of the dictionary
# Save the current personal dictionary
~ Ignored for Ispell compatibility.
+ Enter TeX mode.
+mode Enter the mode specified by mode.
- Enter the default mode.
! Enter terse mode
% Exit terse mode
^ Spell-check the rest of the line