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

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 dateelse    echo Not using GNU datefi

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.)

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"'fieval $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

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.