I accidentally came across a very interesting UNIX utility, calendar. Despite
its name, it does not actually show you a calendar (there’s cal for that),
but rather, it shows you past events that occurred on any given day. The
calendar program is pre-installed on BSD systems (including macOS). If you’re
on Linux, you might need to install it:
- Ubuntu:
sudo apt install bsdmainutils - Fedora:
sudo dnf install calendar
You can then create a calendar file like so:
mkdir -p ~/.calendar
echo "#include <calendar.world>" > ~/.calendar/calendar
Then, just run calendar. Here’s what today’s entries look like:
Sep 10 National Day in Belize
Sep 10 Moon Festival in Taiwan
Sep 10 Korean Thanksgiving Day (Chusuk) in South Korea
Sep 10 Gandalf escapes from Orthanc
Sep 10 Mountain Meadows Massacre. Mormons kill Gentile wagon train, 1857
Sep 11 National Holiday in Chile
Sep 11 Ethiopian New Year in Ethiopia
Sep 11 Anniversary of military coup in Chile
Sep 11 Terrorists destroy World Trade Center in New York, 2001
Sep 11 CIA-sponsored terrorists overthrow Chilean government, murder President Allende, 1973
You can #include additional calendars - the full list can be seen via ls
/usr/share/calendar - or add your own events in the same format.
To get daily reminders of your calendar, run sudo crontab -e and add the
following:
0 12 * * * calendar -a
This will mail you everyday at 12 PM with a notification in your terminal,
which you can read by running mail and hitting the return key, followed by
q and return to quit.
History
The original version of calendar was released with V7 UNIX, which was the
last UNIX to come out of Bell Labs. It had a very short man page, with the
following description:
Calendar consults the file ‘calendar’ in the current directory and prints out lines that contain today’s or tomorrow’s date anywhere in the line. Most reasonable month-day dates such as ‘Dec. 7,’ ‘december 7,’ ‘12/7,’ etc., are recognized, but not ‘7 December’ or ‘7/12’. On weekends ‘tomorrow’ extends through Monday.
When an argument is present, calendar does its job for every user who has a file ‘calendar’ in his login directory and sends him any positive results by mail(1). Normally this is done daily in the wee hours under control of cron(8).
It essentially reads a file like this:
Jan. 1 New Year
Jan. 2 Break New Year's resolutions
and prints out the days that match today and tomorrow.
I searched for the original implementation out of curiosity, and found it on the helpful UNIX history repo:
/* /usr/lib/calendar produces an egrep -f file
that will select today's and tomorrow's
calendar entries, with special weekend provisions
used by calendar command
*/
#include <time.h>
#define DAY (3600*24L)
char *month[] = {
"[Jj]an",
"[Ff]eb",
"[Mm]ar",
"[Aa]pr",
"[Mm]ay",
"[Jj]un",
"[Jj]ul",
"[Aa]ug",
"[Ss]ep",
"[Oo]ct",
"[Nn]ov",
"[Dd]ec"
};
struct tm *localtime();
tprint(t)
long t;
{
struct tm *tm;
tm = localtime(&t);
printf("(^|[ (,;])((%s[^ ]* *|%d/)0*%d)([^0123456789]|$)\n",
month[tm->tm_mon], tm->tm_mon + 1, tm->tm_mday);
}
main()
{
long t;
time(&t);
tprint(t);
switch(localtime(&t)->tm_wday) {
case 5:
t += DAY;
tprint(t);
case 6:
t += DAY;
tprint(t);
default:
t += DAY;
tprint(t);
}
}
A surprisingly short file. At first glance, it doesn’t seem to be reading anything and just prints a few lines. C has unbreakable backwards compatibility, so we can try to compile and run it:
cc -o calendar calendar.c && ./calendar
The output:
(^|[ (,;])(([Ss]ep[^ ]* *|9/)0*10)([^0123456789]|$)
(^|[ (,;])(([Ss]ep[^ ]* *|9/)0*11)([^0123456789]|$)
This is when the comment at the top of the file comes in handy:
/* /usr/lib/calendar produces an egrep -f file
that will select today's and tomorrow's
calendar entries, with special weekend provisions
used by calendar command
*/
It prints a fancy regex to be used by egrep for selecting today’s and
tomorrow’s date. Where’s egrep getting called? In /usr/bin/calendar:
PATH=/bin:/usr/bin
tmp=/tmp/cal$$
trap "rm $tmp; exit" 0 1 2 13 15
/usr/lib/calendar >$tmp
case $1 in
-)
sed '
s/\([^:]*\):.*:\(.*\):[^:]*$/y=\2 z=\1/
' /etc/passwd \
| while read x
do
eval $x
if test -r $y/calendar; then
egrep -f $tmp $y/calendar 2>/dev/null | mail $z
fi
done;;
*)
egrep -f $tmp calendar
esac
The shell script does a couple of interesting things. First, it uses a trap
command to run cleanup actions on exit, which in this case is to remove the
temporary file that will hold the regexes. The file is created on the next line
via /usr/lib/calendar which is the short C program we just compiled. Then, it
checks if an argument is given, and if so, runs sed on /etc/passwd with yet
another regex which returns each line as y=/home/dir z=username. This is then
piped into a while loop and evald to get them as separate variables.
Finally, a check for the calendar file is done in each user’s home directory,
and if it exists, egrep is run on it via the regex file and the resulting
events of the day are mailed to the user.
The implementation is interesting because it’s an excellent example of
composition and the UNIX philosophy. It uses C for the core logic, egrep for
filtering, sed for querying, mail for notifications and finally cron for
running it all.
At some
point
during the development of 4.1BSD, someone came up with the clever idea of
running the calendar file through cpp, the C preprocessor, taking the
composition approach even further. This two-line change would allow “a global
data base of dates to be included in users’ calendar files” as the commit
mentions. Users could now use existing calendars, such as ones for holidays and
world events, with a simple #include.