Feature Detection Is Better than Version Detection

Summary: Version Detection like checking $BASH_VERSION or User-Agent == "Mozilla" is bad for portability.

TODO: Fill out this page more.

• Comment on Hacker News Thread: Bash Completion in JSON Fields
  • An unnecessary $BASH_VERSION check -- eval 'complete ...' would suffice.
• All browsers pretend they're Mozilla because web servers detect it
• LLVM Has to Pretend It's GCC to compile code in the wild

NOTE: OSH won't pretend it's bash! It is largely compatible with bash, but not identical.

How Do I Use Feature Detection?

In shell (or JavaScript), an easy and effective way is to use eval. (This is perhaps the best reason to use eval!)

For example, you can test if a shell has declare with something like

if eval 'declare myvar'; then
  have_declare=yes
else
  have_declare=''
fi

# usage:

if test -n "$have_declare"; then
  ...
fi