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