Portable Shell Script - Determine Which Version of Binary is Installed?

Shell Programming and Scripting
1

I currently have a shell script that utilizes the "Date" binary - this application is slightly different on OS X (BSD General Commmand) and Linux systems (gnu date). In particular, the version on OS X requires the following to get a date 14 days in the future "date -v+14d -u +%Y-%m-%d" where gnu date requires "date -d "+ 14 days" -u +%Y-%m-%d".

I desire to make my script portable - so I believe that I'll need to utilize a test to determine which version of the date application is installed and thus, what command to run. I've come up with the following two methods, but am looking for advice:

Use "File" tool:

date_binary_full_path=`which date`
date_binary=`file -b $date_binary_full_path`
if [[ $date_binary ~= "Mach-O 64-bit executable x86_64" ]]
    then date_command="date -v+14d -u +%Y-%m-%d"
elif  [[ $date_binary == "ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped" ]]
	date_command="date -d \"+ 14 days\" -u +%Y-%m-%d"
else
	date_command="date -d \"+ 14 days\" -u +%Y-%m-%d"
fi
echo $date_command

Use "date" --version switches:

if [[ `date --version 2>&1 | grep -c "date (GNU coreutils)"` > 1 ]]
	then date_command="date -d \"+ 14 days\" -u +%Y-%m-%d"
else
	$="date -v+14d -u +%Y-%m-%d"
fi
echo $date_command
2

An alternative to find the date 14 days in future, without checking which variant of date is installed on system, is to insert this perl one-liner in your script:

perl -e 'use POSIX qw/strftime/; print strftime("%Y-%m-%d",localtime(time+(14*86400)))'