Best current version, without need to deal with numeric search within NSString is to define macros (See original answer: Check iPhone iOS Version)

Those macros do exist in github, see: https://github.com/carlj/CJAMacros/blob/master/CJAMacros/CJAMacros.h

Like this:

#define SYSTEM_VERSION_EQUAL_TO(v)                  ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedSame)
#define SYSTEM_VERSION_GREATER_THAN(v)              ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedDescending)
#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)  ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] != NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN(v)                 ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN_OR_EQUAL_TO(v)     ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] != NSOrderedDescending)

and use them like this:

if (SYSTEM_VERSION_LESS_THAN(@"5.0")) {
    // code here
}

if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"6.0")) {
    // code here
}

Outdated version below

to get OS version:

[[UIDevice currentDevice] systemVersion]

returns string, which can be turned into int/float via

-[NSString floatValue]
-[NSString intValue]

like this

Both values (floatValue, intValue) will be stripped due to its type, 5.0.1 will become 5.0 or 5 (float or int), for comparing precisely, you will have to separate it to array of INTs check accepted answer here: Check iPhone iOS Version

NSString *ver = [[UIDevice currentDevice] systemVersion];
int ver_int = [ver intValue];
float ver_float = [ver floatValue];

and compare like this

NSLog(@"System Version is %@",[[UIDevice currentDevice] systemVersion]);
NSString *ver = [[UIDevice currentDevice] systemVersion];
float ver_float = [ver floatValue];
if (ver_float < 5.0) return false;

For Swift 4.0 syntax

below example is just checking if the device is of iOS11 or greater version.

let systemVersion = UIDevice.current.systemVersion
if systemVersion.cgFloatValue >= 11.0 {
    //"for ios 11"
  }
else{
   //"ios below 11")
  }