Number of Days in a Month

If you have to loop through everyday in a month to generate daily summary, you may want to know how many days in a particular month. It may not be as straightforward as you think because you need to take into account of leap year, short months, and so on. Here is a simple trick that can help you on this. Run the cal command to display the calendar and pipe it to awk to pick up the last number from the last non-empty line.

$ cal
    October 2009
Su Mo Tu We Th Fr Sa
             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

$ cal | awk 'NF>0{n=$NF}END{print n}'
31

$ cal 2 2012
    February 2012
Su Mo Tu We Th Fr Sa
          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

$ cal 2 2012 | awk 'NF>0{n=$NF}END{print n}'
29