by shigemk2


mac date 昨日


date -v-1d "+%Y-%m-%d"

man dateすると以下説明があるけど、なんていうか日跨ぎ月跨ぎ年跨ぎサマータイムあたりで注意しないといけない

     -v      Adjust (i.e., take the current date and display the result of the adjustment; not actually set the date) the second, minute, hour, month day, week day, month or year
             according to val.  If val is preceded with a plus or minus sign, the date is adjusted forwards or backwards according to the remaining string, otherwise the relevant part
             of the date is set.  The date can be adjusted as many times as required using these flags.  Flags are processed in the order given.

             When setting values (rather than adjusting them), seconds are in the range 0-59, minutes are in the range 0-59, hours are in the range 0-23, month days are in the range
             1-31, week days are in the range 0-6 (Sun-Sat), months are in the range 1-12 (Jan-Dec) and years are in the range 80-38 or 1980-2038.

             If val is numeric, one of either y, m, w, d, H, M or S must be used to specify which part of the date is to be adjusted.

             The week day or month may be specified using a name rather than a number.  If a name is used with the plus (or minus) sign, the date will be put forwards (or backwards)
             to the next (previous) date that matches the given week day or month.  This will not adjust the date, if the given week day or month is the same as the current one.

             When a date is adjusted to a specific value or in units greater than hours, daylight savings time considerations are ignored.  Adjustments in units of hours or less honor
             daylight saving time.  So, assuming the current date is March 26, 0:30 and that the DST adjustment means that the clock goes forward at 01:00 to 02:00, using -v +1H will
             adjust the date to March 26, 2:30.  Likewise, if the date is October 29, 0:30 and the DST adjustment means that the clock goes back at 02:00 to 01:00, using -v +3H will
             be necessary to reach October 29, 2:30.

             When the date is adjusted to a specific value that does not actually exist (for example March 26, 1:30 BST 2000 in the Europe/London timezone), the date will be silently
             adjusted forwards in units of one hour until it reaches a valid time.  When the date is adjusted to a specific value that occurs twice (for example October 29, 1:30
             2000), the resulting timezone will be set so that the date matches the earlier of the two times.

             It is not possible to adjust a date to an invalid absolute day, so using the switches -v 31d -v 12m will simply fail five months of the year.  It is therefore usual to
             set the month before setting the day; using -v 12m -v 31d always works.

             Adjusting the date by months is inherently ambiguous because a month is a unit of variable length depending on the current date.  This kind of date adjustment is applied
             in the most intuitive way.  First of all, date tries to preserve the day of the month.  If it is impossible because the target month is shorter than the present one, the
             last day of the target month will be the result.  For example, using -v +1m on May 31 will adjust the date to June 30, while using the same option on January 30 will
             result in the date adjusted to the last day of February.  This approach is also believed to make the most sense for shell scripting.  Nevertheless, be aware that going
             forth and back by the same number of months may take you to a different date.