# Checks a boolean variable for "true".
# Case insensitive: "1", "y", "yes", "t", "true", "o", and "on".
is-true() {
  [[ -n "$1" && "$1" == (1|[Yy]([Ee][Ss]|)|[Tt]([Rr][Uu][Ee]|)|[Oo]([Nn]|)) ]]
}