test [EXPR]
Options: !, =, !=, -eq, -ne, -ge, -gt, -le, -lt, -o, -a, -z, -n, -d, -e, -f, -L; see ‘man test’ on your PC for more information.
3.6.11. source - execute shell script in current shell environment
3.6.13. true - do nothing, successfully