Finding yesterday’s date in Linux / Unix

I recently wrote a set of scripts to backup a production database on a nightly basis. The ultimate goal is to setup a DR site which automatically restores day-old backups. In order to accomplish this, I need to be able to figure out the date for ‘yesterday’, and I didn’t want to resort to using any other code (perl, PHP, python, etc). It turns out that reasonably modern versions of the ‘date’ command can do this for you:

Today’s Date:

# date +'%Y-%m-%d'
2011-04-02

Yesterday’s Date:

# date +'%Y-%m-%d' --date='1 day ago'
2011-04-01

A month ago:

# date +'%Y-%m-%d' --date='1 month ago'
2011-03-02

You can also convert between timestamps:

# date +'%s'
1301786252
# date --date='@1301786252' +'%Y-%m-%d'
2011-04-02

Doing math with timestamps could be useful too:

# TS=$(date +'%s'); let TS="${TS} - 604800"; date --date="@${TS}" +'%Y-%m-%d'
2011-03-26