[bewuethr]

I use something like this:

  for dir in /path/to/dir1 /path/to/dir2; do
      case :${PATH:=$dir}: in
          *:"$dir":*) ;;
          *) PATH=$dir:$PATH ;;
      esac
  done

If PATH isn't set, ${PATH:=$dir} sets it to $dir.

  case :${PATH:=$dir}:

wraps PATH between colons to take care of the "it was empty" / "dir is at start/end" edge cases.

The first case is hit when the PATH contained the directory already (no-op); the second case prepends the new directory to the PATH.

[hvdijk]

Beware that PATH being unset or empty is a special case and does not necessarily mean that no directories are searched. Different shells handle it differently, and other utilities spawned by the shells handle it themselves in a way that may or may not match the shell. Whether setting PATH to $dir if it was previously unset or empty is the right thing to do is therefore a matter of opinion. Personally, I would stay out of that mess and either issue an error or leave PATH unmodified.