calculate the date of yesterday

[Kelvin Yu]

Hi,

I read some old postings and found the following, but need clarification:

Case #1
-------
export TZ=
date

Case #2
-------
Yesterday=`TZ=aaa24 date +%Y%m%d`

If I put this in a shell script that runs by crontab, which is preferable?

Thanks,
Kelvin

[Geoff Wild]

I like case 2 - straight to the point - less processing...etc.

Rgds...Geoff

[A. Clay Stephenson]

The problem with this approach is that it is not portable and does not work with all TZ settings. The behavior of the date functions is undefined when the offset is out of range. A much better approach is to convert the current date to a Julian Day and then subtract 1 from it and then redisplay the Julian Day as a calendar date.

YESTERDAY=$(caljd.sh -y -s $(caljd.sh -p 1))
echo "Yesterday = ${YESTERDAY}"

This will output in your desired format.


The caljd.sh will work for any number of days in the past by changing -p n (or days in the future by changing -p n to -n n). Invoke as caljd.sh -u for full usage.