Calendaring from the command line

On Schedule

Article from Issue 169/2014
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We take a spin through several personal calendar apps that you can manage from the command line.

If you have done any administration on a Linux system, you may have used Cron [1] or Anacron [2] for scheduling system commands. What about scheduling personal information from the command line, however? Although you could just about manage to schedule events using Cron or Anacron, free software has several more versatile applications for personal calendaring, most notably Cal or NCal [3], Calendar [4], Remind [5], and When [6].

Most of these applications are in the grand Unix tradition, focusing on a limited set of functions and are configurable in a text editor. Often, the main differences between them is the format for entering events and the syntax for a particular function, although these applications appear to borrow heavily from one another. All of these alternatives are standard in the repositories of most distributions, although only Cal is generally installed by default. Simple and quick, you may find them a welcome change after wrestling with KDE's Akonadi personal information manager or the unresponsiveness in the Google Calendar interface.

Cal and NCal

Cal and NCal provide the command-line equivalent of the date and time applet on a desktop panel that displays calendars but lacks any ability to add personal events. The main difference between Cal and NCal is that Cal organizes each month by columns, and NCal organizes by rows (Figure 1).

Figure 1: Formatting is the main difference between Cal (top) and NCal (bottom).

By itself, either command displays a calendar for the current month. However, you can display the calendar for an entire year by specifying the year as a four-digit number immediately after the basic command.

Using -A [NUMBER] or -B[NUMBER], you can set the number of months to display before and after the current month. Similarly -3 displays the previous, current, and next month. By default, each week starts on Monday, but you can change the start of the week to Sunday with -S, or back to Monday with -M.

Calendar

Calendar looks for files called ~/calendar.[EXTENSION]. The default calendar it looks for is calendar, but you can use the -f [FILE] option to specify another file, such as calendar.birthday if you are organizing events by files.

Within a calendar file, Calendar looks for lines with the following format:

[DATE] [TAB] [EVENT DESCRIPTION]

The date is expressed in two digits for the day, month, and year, separated by a space, with a day or month assumed to come first. If the calendar file starts with a Language=[LANGUAGE] field, then the calendar command looks specifically for date formats that fit that locale's default format. Otherwise, almost any date format can be used, although in the case of confusion, a format is assumed to be MM DD rather than DD MM (Figure 2).

Figure 2: Calendar's entries start with either the day or the month, defaulting to the month if any doubt exists.

By default, Calendar reprints all lines for the current and next day. On Fridays, it takes into account the weekend by printing entries from Friday to Monday.

Appointments can also be defined in others ways. For example, an asterisk (*) indicates that an event takes place every month, and a day of the week means that the event occurs whenever that day occurs.

The default display of Calendar can also be modified with options, such as -A [NUMBER] to change the number of future days that displays and -B [NUMBER] to change the number of previous days.

Calendar is a serviceable choice, but the expectations of the date format may be a nuisance to those who prefer to start dates with a year.

Remind

Of all the applications mentioned here, Remind has by far the most options and variations. Because it does not include a preference file, many of these options have to be added to the command line.

Remind uses an event file of any name. The format for an event entry is:

REM [DAY] [DATE] +[ADVANCED WARNING] MSG [MESSAGE]

In this format, REM signals that the line should be included when Remind prints out events, whereas the advanced warning is an optional entry for the number of days before the event that reminders should start.

Dates in an entry are varied. A two-digit year means a year between 1990 and 2075, whereas a month or a day of the week is case insensitive and can be either the full name or the first three letters as an abbreviation. A range of weekdays can be specified, or, if only a date is given, the event is mentioned every month on that date. About halfway down, Remind's man pages give 16 examples that can help you learn how dates are read.

The simplest form of command is:

remind -n [FILENAME]

The option -n prints events in calendar format (Figure 3). Alternatives include specifying the number of months in the calendar with -c =[NUMBERS] and displaying all events with -t.

Figure 3: Remind lists events in the order it finds them, so the | sort pipe can make the list of events chronological.

If anything, Remind offers more options than many users want. However, remember Remind if you are having trouble finding the exact behavior you need in a calendar application. Odds are, if any of the applications mentioned here have the options you need, Remind will have them.

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