It's a well known fact that GNU tools have plenty of extra features which you have to be careful about using if you want portibility AND that many of the legacy commercial Unix implementations have positively ancient implementations and feature sets. I wouldn't really say it is so very difficult though.

Every script you write isn't going to be portable, but it's not that much of a stretch to endeavor to keep your script simple, not make assumptions, and be mindful of the potentially missing features of some implementations.

I take a special objection to [5], `python -V` isn't difficult at all to run, hoping and guessing are not necessary.

There's a good guide here: http://www.gnu.org/software/autoconf/manual/autoconf.html#Po...

Portability is a red herring anyway. If you pursue it, you'll always end up chasing the lowest common denominator.

YOU can control where the app is deployed (this is largely true even if you're selling your app just by having installation requirements or by selling appliances instead of installable apps).

> I take a special objection to [5], `python -V` isn't difficult at all to run, hoping and guessing are not necessary.

I mostly meant that in a simple statement of:

  python -c "code"

…you're probably forced to assume that it's Python 2 (or write a 2/3 code) and hope that your assumption is right. You can't run `python -V`: you're a script! The point is that it is automated, or we wouldn't be having this discussion.

Of course, you can inspect the output of python -V (or just import sys and look at sys.version_info.major) and figure it out, but now you need to do that, which requires more code, more thought, testing…

I'd argue that you should probably stick to one subset of things in your bootstrap script -- and I'd say grep, awk, sed and (ba)sh go together, anything "higher level" like python/ruby/perl/tcl does not fit within that. You might want to check for python with a combination of "python -V" and the dance described above -- and, as part of bootstrapping, make a symlink (or copy, if you need to support windows and/or a filesystem without symlink support) to eg: python2. Save that tidbit as "assert-python2.sh" and then first "assert-python2.sh" then "check-bootstrap-deps.sh" and finally "bootsrap.sh" :-)

Interesting, for 1 and 4 -- I immediately assumed that might break (as for tar, I'd generally prefer something like zcat (or for scripts gzip -dc) | tar -x... makes it easier to change format (both gzip to lzma and tar to cpio). For 2,3 I'd be wary of sed for anything that needs to be portable in general. For 3, it seems prudent to use a suffix with -i anyway; explicit being better than implicit most of the time.

As for 5; How many system does have python 2 installed, but no python2 binary/sym-link? (I've never had to consider this use-case for production).

Note a slight benefit of splitting tar to zcat and replacing python with python2, is that you'll get a nice "command not found" error. You could of course do a dance in the top of your script trying to check for dependencies with "command -v"[1]. If nothing else such a section will serve as documentation of dependencies.

Something like:

    # NOT TESTED IN PRODUCTION ;-)
    checkdeps() {
      depsmissing=0
      shift
      for d in "${@}"
      do
          if ! command -v "${d}" > /dev/null
          then
            depsmissing=$(( depsmissing + 1 ))
            if [ ${depsmissing} -gt 126 ]
            then
              depmissing=126 # error values > 126 may be special
            fi
            echo missing dependency: "${d}"
          #debug outpt
          #else
            #echo "${d}" found
          fi
      done
      return ${depsmissing}
    }

    deps="echo zcat foobarz python2"
    checkdeps ${deps}
    missing=${?}

    if [ "${missing}" -gt 0 ]
    then
      echo "${missing} or more missing deps"
      exit 1
    else
      echo "Deps ok."
    fi

    # And you could go nuts checking for alts, along the lines of
    # pythons="python2 python python3"
    # and at some point have a partial implemntation of half of
    # autotools ;-)

[1] https://stackoverflow.com/questions/762631/find-out-if-a-com...