$ ./calendula
March 2021
Mo Tu We Th Fr Sa Su
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31
$ ./calendula --version
calendula 0.2.0
$ ./calendula --help
Usage:
./calendula [options]
Options:
-s, --sunday Sunday as first day of week
-m, --monday Monday as first day of week
--tuesday Tuesday as first day of week
--wednesday Wednesday as first day of week
--thursday Thursday as first day of week
--friday Friday as first day of week
--saturday Saturday as first day of week
-V, --version display version information and exit
-h, --help display this help text and exit
The source code of cal is here https://github.com/karelzak/util-linux/blob/31862cde0a0551b42e6b84015da7719a784007d9/misc-utils/cal.c