paste
TextPackage: coreutils
Joins files line by line, side by side. With -sd+ it turns a column of numbers into 1+2+3, which bc can add up.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
paste -d- The character used to join lines.
paste -s- Puts one file on a single line instead of merging several files into columns.
Lessons that teach it
No lesson names it among its useful commands; it appears inside other lessons' command lines.