NAME paste -- merge corresponding or subsequent lines of files SYNOPSIS paste [-s] [-d list] file ... DESCRIPTION The paste utility concatenates the corresponding lines of the given input files, replacing all but the last file's newline characters with a single tab character, and writes the resulting lines to standard output. If end-of-file is reached on an input file while other input files still contain data, the file is treated as if it were an endless source of empty lines.
man paste on macOS
r.txt are lists of IDs you want compare and to use in a SQL
WHERE in (4,5,6) clause:
$ cat l.txt 1 2 3 4 5 6 $ cat r.txt 1 2 3 $ paste -sd ',' <(diff l.txt r.txt | grep '<' | sed 's/< //') 4,5,6