Improve compatibility with ImageMagick 7

[alessandroberna]

The convert command has been deprecated starting from ImageMagick 7.
Attempting to use the script results in something like this:

This patch checks for the ImageMagick version and uses the new command if the major version is greater or equal to 7.
After applying it, the warnings are no more:

Fixes #71
Fixes #77

[ibehnam]

This solves the problem I'm having on Mac.

[thomasmerz]

Is this is a duplicate of PR #74 but without any shellchecking? 🤔

[alessandroberna]

Is this is a duplicate of PR #74 but without any shellchecking? 🤔

PR #74 doesn't check for the installed imagemagick version and just replaces convert with magick.
I haven't had the time to test it out but it should break compatibility with earlier imagemagick versions.

[thomasmerz]

Well, there are now three PRs dealing with issue #71 plus shellcheck (has been fixed in PR #74 but not in #73 and there's my PR #75 that has only the shellcheck-fix plus a GitHub Action for automatic shellcheck'ing).

@hackerb9 should decide which one of #73 and #74 suits better and then I will fix my #75 with shellcheck stuff… 🤔 Does everybody involved agree? @sammcj @alessandroberna

[alessandroberna]

I believe that each issue should be addressed in a different PR to make it easier for the maintainer(s) to review the changes.
Of course I have no objections to the shellcheck issue or the improved comparison logic.
It goes without saying that the repo owner always has the final say on every issue; I'm okay with your proposal, but there isn't much to agree to on my end.

[hackerb9]

I like that this patch fixes only one thing at a time, but I'm wondering if montage is also going to be deprecated by ImageMagick 7. It may make more sense to do something like:

magick=$(which magick)
...
$magick convert ...  sixel:-
$magick montage ... sixel:-

That way the error message on non-sixel terminals can give the correct command for people to test their terminals:

echo "$magick convert   foo.jpg  --geometry 800x480  sixel:-"

[hackerb9]

I like that you were thorough, but parsing version numbers like this is notoriously brittle. Also, while I make pipelines like that at the command line all the time, in a shell script I'd prefer to avoid forking quite so many processes. For example: $(identify -version | awk 'NR==1 { print $3 >= 7 }'.

Another problem is that for many years ImageMagick 7 had all the same executables as IM6 before deprecating convert. Will identify be deprecated someday? All we know for sure is that IM6 does not have magick and IM7 does.

[hackerb9]

I believe the latest release fixes this issue, but please re-open if I am mistaken. Thanks!