I don't think anyone is going to be able to solve your problem on Stackoverflow. Your choice of tools, methods, and process are much more affected by social constraints, e.g. what your boss wants and what you want, then technical merits. That's not necessarily bad.
The short answer is "Use what is going to be most pleasing to the developers". If everyone likes Python more than Perl, for whatever reason, they are probably going to get more done in Python. If they like Ruby more than Python, it's the same thing.
Some things to evaluate as part of your selection:
- What do the developers already know?
- What are they most willing to learn?
- How much weekly time can your team spend learning new things (e.g. lunch seminars, formal classes, etc)?
- What do most people in the community use to work with the tool you need to support? For instance, Fisheye has a Java API, and some REST examples for Perl and Python. If you're writing Fisheye extensions, Java seems to be the win there. If you're merely accessing Fisheye data, any language can use the REST stuff.
- What is most of your code base in already? What can you replace and what do you have to continue to support? I find that many companies can't answer this question because every developer seems to add two new technologies they don't tell anyone about. :)
- Which platforms do you need to support? Some languages have platform specific issues, and I don't mean just Windows vs. Unix. Do you have legacy hardware you have to support? Does your tool work on that stuff?
- How much of the stuff you produce can benefit other parts of the company? What are other teams using?
- Do the people advocating one tool know it well enough to be its champion? I ask What are five things he hate about your favorite language? If people can't name five valid things that are wrong with their language or tool, they don't have enough experience with it.
The Longer Answer
People tend to try to reduce this to a technical argument because they are afraid to admit their biases or examine why they think what they think. Your boss might favor bash and Perl because that's what he did a lot of work in when he was getting started. You might like Python because you have a personal affinity for the way Python does things. I like Perl because I like its flexibility and DWIMmery. Like any social situation, different people are going to be attracted to different parts of different things. Just because you like chocolate doesn't make vanilla evil. I could give you lots of good arguments why Perl can be useful, but that doesn't mean that something else can't give you the same value.
What problems do we solve with scripts?
That one you have to answer for yourself. :)
Would we benefit from a library of common functions by our team, or are most of our projects more isolated?
This is most likely a good thing in Python, Perl, Ruby, Java, and almost any other language that you might choose. I think this part of your requirement is language agnostic. No matter which one you choose, you'll probably want to do this.
What is it reasonable to expect my co-workers to learn?
A good developer should be able to work with several different languages at least to an apprentice level. Those languages should include ones that have vastly different assumptions about how people express problems, say, for example, the set { Smalltalk Perl C Lisp Java }.
The best developers I've hired and worked with have always wanted to use the right tool for the job instead of making the job right for the tool. They might have their favorite language, but they didn't grouse (too much) about using a different tool when it made more sense.
Many "developers", however, seem to think that they are getting paid to play with their favorite tool. You need to convert them into people who think they have a toolbox to solve problems that create business value.
And remember, you never stop learning. As a developer you don't have to choose one language then defend it with your life, forsaking all others, in sickness and in health, and so on. Good developers are going to continue to track new technologies and evaluate them for usefulness for their tasks. Just because you choose one tool over another doesn't mean you stop paying attention.
No matter what you choose, someone is going to complain. Don't look for the solution that makes everyone happy. There isn't one, short of getting rid of the developers who aren't happy.
What languages give us the most ease of development and ease of modification?
A skilled practitioner in just about any language will think that his chosen language is the easiest to develop, modify, and maintain. Unskilled practitioners tend to blame the language and the tools for their problems. Some languages have steeper learning curves, and some have bigger payouts. A person's tolerance with immediate gratification is a big factor here.
That being said, different languages have developed different cultures and different toolsets. Perl people tend to like vi or emacs, Ruby people tend to like TextMate, Java people tend to like Eclipse or IntelliJ. That's not always true, but the culture that evolves around the tools are often more important than the technical details of the tool. If your developers like a particular type of tool, they are probably going to like the language that has a culture built around that sort of tool.
Some processes and tools take more time to get used to or require more education, but they can have larger advantages when used properly. Other tools get you started sooner but might not give you a path to bigger and greater things, such as cross-team development. The trick, however, is to not code to the tools so you aren't stuck in any particular toolset.