set -u

[petef4]

I would add set -u into your advice for set -eo pipefail, i.e. set -euo pipefail

An enhancement to that is to fail early in a script with useful feedback.

: ${FOO:?You have not set the FOO variable}

[ernstki]

I have mixed feelings about set -e in a library script with a lot of functions that do something like

is_integer() {
    [[ $1 =~ ^[-0-9][1-9]*$ ]]
}

…but totally agree that -u needs to be in there 100% of the time.