How can I detect BSD vs. GNU version of date in shell script

Question

I am writing a shell script that needs to do some date string manipulation. The script should work across as many *nix variants as possible, so I need to handle situations where the machine might have the BSD or the GNU version of date.

What would be the most elegant way to test for the OS type, so I can send the correct date flags?

EDIT: To clarify, my goal is to use date's relative date calculation tools which seem distinct in BSD and GNU.

BSD example

date -v -1d

GNU example

date --date="1 day ago"

Answer 1

Use portable flags. The standard is available here

For the particular problem of printing a relative date, it is probably easier to use perl than date:

perl -E 'say scalar localtime( time - 86400 )'

(note that this solution utterly fails on 23 or 25 hour days, but many perl solutions are available to address that problem. See the perl faq.)

but you could certainly use a variation of Keith's idea and do:

if date -v -1d > /dev/null 2>&1; then
  DATE='date -v 1d'
else
  DATE='date --date="1 day ago"'
fi
eval $DATE

or just

DATE=$(date -v -1d 2> /dev/null) || DATE=$(date --date="1 day ago")

Another idea is to define a function to use:

if date -v -1d > /dev/null 2>&1; then
    date1d() { date -v -1d; }
else
    date1d() { date --date='1 day ago'; }
fi

Answer 2

You want to detect what version of the date command you're using, not necessarily the OS version.

The GNU Coreutils date command accepts the --version option; other versions do not:

if date --version >/dev/null 2>&1 ; then
    echo Using GNU date
else
    echo Not using GNU date
fi

But as William Pursell suggests, if at all possible you should just use functionality common to both.

(I think the options available for GNU date are pretty much a superset of those available for the BSD version; if that's the case, then code that assumes the BSD version should work with the GNU version.)

Answer 3

To just answer your question, probably "uname -o" is what you want. In my case it's "GNU/Linux". You can decide for yourself if detecting the os type is worthless or not.