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...