I have a script that's supposed to get the list of files of two directories, get differences and execute some code for certain files.

These are the commands to get the file lists:

list_in=$(find input/ -maxdepth 1 - type f | sed 's/input\///' | sort -u);
list_out=$(find output/ -maxdepth 1 - type f | sed 's/output\///' | sort -u);

I execute the script in the correct directory, so this shouldn't fail. The unprocessed files are determined by

list_todo=$(comm -23 <(echo "$list_in") <(echo "$list_out"));

since the option -23 for comm only prints lines of the first argument, that don't appear in both arguments and doesn't print lines that uniquely appear in the second argument.

However, only occasionally I get an error saying

command substitution: line 3: syntax error near unexpected token `('
command substitution: line 3: `comm -23 <(echo "$list_in") <(echo "$list_out")'

This really puzzles me, since the exact same script worked fine for the last 3 weeks. I'm using this on a cluster, so several processes might execute the script simultaneously. May the error be caused by this?

Update. The script is called with ./script and I've obviously set chmod +x script before.

(Disclaimer: Even though I'm working on a cluster and these first three lines of my script don't include any locking mechanisms: Of course, no file is ever processed twice)