If you’ve ever used a spreadsheet, you’ve probably used or seen
\nfunctions for doing date math\u2014in other words, taking one date and adding some number
\nof days or months to it to get a new date, or taking two
\ndates and finding the number days between them.
\nThe same thing can be done from the command line using
\nthe lowly date command, possibly with a little
\nhelp from Bash’s arithmetic.<\/p>\n
The two most important features of the date command to
\nunderstand for doing date math are the +FORMAT option
\nand the –date option.
\nThe formatting option allows you to format the date in different ways
\nor get just certain parts of the date, such as the year,
\nthe month, the day and so on:<\/p>\n
$ date ‘+%Y’<\/b>
\n2018
\n$ date ‘+%m’<\/b>
\n09
\n$ date ‘+%d’<\/b>
\n16<\/p>\n
There are a few dozen formatting options; see the date(1)<\/a> The second option, the –date (or -d) option, allows you to specify $ date –date ‘March 1, 2015’ “+%Y”<\/b> The interesting part of the –date option is that it allows you to $ date –date ‘March 1, 2015 +7 days’<\/b> You also can use strings that refer to previous or upcoming days $ date –date ‘last Monday’<\/b> Note, however, that combining a date and things like “next Monday” does not work. So now that you’ve seen the basics of doing date math, let’s use it To not complicate the example, let’s ignore a couple breaks in $ cat date1.sh<\/b> $ bash date1.sh 284 285<\/b> Lines 5 and 6 calculate the number of issues One thing that can you trip you up when doing date math is that $ cat date2.sh<\/b> $ bash date2.sh 284 285<\/b> This incorrectly labels issue 285 as being in May rather $ date –date=”March 31, 2018 +1 month”<\/b> Since April has only 30 days, you end up skipping to the first of May.<\/p>\n The other common type of date math that I mentioned above is calculating $ cat date3.sh<\/b> sdate=$(date –date=”$start_date” ‘+%s’) $ bash date3.sh<\/b> In part two of this article, I plan to tackle how to get a $ date –date ‘March 1, 2015 previous Monday’ # won’t work<\/b> If you’ve ever used a spreadsheet, you’ve probably used or seen functions for doing date math\u2014in other words, taking one date and adding some number of days or months to it to get a new date, or taking two dates and finding the number days between them. The same thing can be done from the … <\/p>\n
\nman page for more.<\/p>\n
\nthe date to use, rather than the current date:<\/p>\n
\n2015
\n$ date –date=’March 1, 2015′ “+%m”<\/b>
\n03
\n$ date -d ‘March 1, 2015’ “+%d”<\/b>
\n01<\/p>\n
\nspecify date strings in a variety of ways. For example, given the
\nabove date of “March 1, 2015”, you can calculate dates relative to it:<\/p>\n
\nSun Mar 8 00:00:00 MST 2015
\n$ date –date ‘March 1, 2015 -1 year’<\/b>
\nSat Mar 1 00:00:00 MST 2014
\n$ date –date ‘March 1, 2015 +12 days +12 hours +15 minutes’<\/b>
\nFri Mar 13 12:15:00 MST 2015<\/p>\n
\nof the week (relative to the current date):<\/p>\n
\nMon Sep 10 00:00:00 MST 2018
\n$ date –date ‘next Monday’<\/b>
\nMon Sep 17 00:00:00 MST 2018<\/p>\n
\nThe date(1)<\/a> man page has a bit more information about the types
\nof date strings you can specify, however, the GNU Info date<\/a> page
\nhas much more complete documentation.<\/p>\n
\nto do something.
\nThis example comes from some recent scripts I was working on
\nwhile organizing Linux Journal’s online archives.
\nEach month we publish a new issue.
\nEach issue has an issue number that’s one more than the previous issue.
\nIssue numbers are easy to deal with when specifying arguments
\nto a script, but readers tend to think more in terms of months and years.
\nSo in a couple places I needed to generate months and years from issue numbers.<\/p>\n
\nLJ<\/em>‘s publishing history.
\nSo, let’s use issue #284 from March 2018 as the starting point
\nand make all calculations relative to it.
\nThe script takes one or more issue numbers and prints out the month
\nand year corresponding to that issue number:<\/p>\n
\n1 for inum in $*
\n2 do
\n3 [[ “$inum” =~ ^[0-9]$ ]] || { echo “Invalid issue number: $inum”; continue; }
\n4
\n5 month=$(date –date=”March 1, 2018 +$((inum – 284)) month” +’%B’)
\n6 year=$(date –date=”March 1, 2018 +$((inum – 284)) month” +’%Y’)
\n7 printf “Issue %d is from %s, %sn” $inum $month $year
\n8 done<\/p>\n
\nIssue 284 is from March, 2018
\nIssue 285 is from April, 2018<\/p>\n
\nthat have elapsed since issue 284 using Bash’s arithmetic
\nexpression syntax: $((inum – 284)).
\nThat result then is used to add that many months onto the
\ndate of the first issue, which gives the date
\nof the issue in question.
\nThen the date command’s format options provides the month and year
\n(the %B and %Y format specifiers, respectively).
\nThose values next are used to print out the month and year of the issue.
\nAnd just in case you were wondering, issue #1000 will be coming out
\nin November 2077.<\/p>\n
\nif you use dates near the end of the month when adding or
\nsubtracting months, you can end up with unexpected results.
\nThis is because the date command uses the same value for the
\nlength of each month when it adds months and ignores the fact
\nthat not all months are the same length.
\nSo, for example, look what happens if you change the reference
\ndate in the script above from the start of the month to the end of the month:<\/p>\n
\n5 month=$(date –date=”March 31, 2018 +$((inum – 284)) month” +’%B’)
\n6 year=$(date –date=”March 31, 2018 +$((inum – 284)) month” +’%Y’)<\/p>\n
\nIssue 284 is from March, 2018
\nIssue 285 is from May, 2018<\/p>\n
\nthan April.
\nYou can verify this if you run the effective date command
\nby itself:<\/p>\n
\nTue May 1 00:00:00 MST 2018<\/p>\n
\nthe difference between two dates.
\nUnfortunately, the date command doesn’t support this directly.
\nTo do that, you need to convert the dates in question into the number
\nof seconds since the epoch<\/a> (using the %s format specifier),
\nand then subtract the values and divide by the number of seconds
\nin a day (86,400) to get the days in between:<\/p>\n
\nstart_date=”March 1, 2018″
\nend_date=”March 31, 2018″<\/p>\n
\nedate=$(date –date=”$end_date” ‘+%s’)
\ndays=$(( (edate – sdate) \/ 86400 ))
\necho “$days days between $start_date and $end_date”<\/p>\n
\n30 days between March 1, 2018 and March 31, 2018<\/p>\n
\nspecific previous day of the week relative to a specific date
\n(which, as I noted above, is not directly doable with the date command).
\nIn other words, I want to be able to do something that effectively
\ndoes the following:<\/p>\n
\ndate: invalid date \u2018March 1 2015 previous Monday\u2019
\n
Source<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"