Linux Script- Date Manipulations - linux

Question

I will set one date variable(Say '08-JUN-2011') and I want to do some calculations based on that date namely,
1. Have to get the first day of the given day's month.
2. Previous date of the given date's month.
3. Last day of the given date's month.
All I know is manipulating using the current system date and time but don't know how to implement with user defined date. I need this to be achieved using Linux shell script.
Any help will be appreciated.
Thanks,
Karthik

Answer 1

Here's how to perform the manipulations using GNU date:
#!/bin/sh
USER_DATE=JUN-08-2011
# first day of the month
FIRST_DAY_OF_MONTH=$(date -d "$USER_DATE" +%b-01-%Y)
PREVIOUS_DAY=$(date -d "$USER_DATE -1 days" +%b-%d-%Y)
# last day of the month
FIRST_DAY_NEXT_MONTH=$(date -d "$USER_DATE +1 month" +%b-01-%Y)
LAST_DAY_OF_MONTH=$(date -d "$FIRST_DAY_NEXT_MONTH -1 day" +%b-%d-%Y)
echo "User date: $USER_DATE"
echo "1. First day of the month: $FIRST_DAY_OF_MONTH"
echo "2. Previous day: $PREVIOUS_DAY"
echo "3. Last day of the month: $LAST_DAY_OF_MONTH"
The output is:
User date: JUN-08-2011
1. First day of the month: Jun-01-2011
2. Previous day: Jun-07-2011
3. Last day of the month: Jun-30-2011

Answer 2

This is going to be convoluted in a shell script. You are better off using Date::Manip in perl, or something similar in another full-featured language. However, I can think of some ways to do this with the date command. First of all, you can use a --date parameter to set a starting point for date, like so:
$ date --date='08-JUN-2011'
Wed Jun 8 00:00:00 EDT 2011
You can get the previous date like this:
$ date --date='08-JUN-2011 -1 days'
Tue Jun 7 00:00:00 EDT 2011
For the last day of the month, I would just walking back from 31 until date does not fail. You can check $? for that
$ date --date='31-JUN-2011';echo $?
date: invalid date `31-JUN-2011'
1
$ date --date='30-JUN-2011';echo $?
Thu Jun 30 00:00:00 EDT 2011
0
For the first day of the month...that is usually 01 :)

Answer 3

As you're using Linux, hhopefully you have the GNU date utility available. It can handle almost any description of a relative date that you think of.
Here are some examples
date --date="last month" +%Y-%m-%d
date --date="yesterday" +%Y-%m-%d
date --date="last month" +%b
x=$(date --date "10 days ago" +%Y/%m/%d)
To learn more about it see GNU Date examples
Once you use it some, you can shortcut your information gathering with date --help, which shows all the basic options (but is sometimes is hard to interpret.)
I hope this helps.

Answer 4

$ date +%m/01/%Y
I came here looking for a way to get the first day of the current month.

Answer 5

Using dateutils' dround tool:
Current month:
$ dround today -1
2014-02-01
Previous month
$ dround today -31
2014-01-31
Last day of current month:
$ dround today +31
2014-02-28
Of course you can use a custom date instead of today, the idea is to round down or up to the desired day-of-the month, e.g. the next first-of-the-month given 2010-10-04:
$ dround 2010-10-04 +1d
2010-11-01