cross-platform scripting for windows, Linux, MacOS X [closed]
I'm a huge fan of Lua:
Syntax is vaguely Pascal-like and works well in scripts.
Superb power-to-weight ratio. Superb engineering. Very good design.
Extremely portable to any platform with an ANSI C compiler.
GUI support through wxLua and other bindings
Some support for hiding OS differences in common tasks, e.g., the Lua File System add-on
The core system and libraries are simple enough that you can understand all of what you're using, but still have excellent leverage compared to bash/bat. Expressive power is comparable to Python or Ruby.
You're not overwhelmed with libraries and frameworks, which can be a plus or a minus.
There is an excellent book: Roberto Ierusalimschy's Programming in Lua; you can get the previous edition free online.
Performance beats tcl, perl, python, ruby
For even faster performance on x86 hardware, there is LuaJIT.
Finally, and this is the ace in the hole: if you run into any kind of platform-specific problem, it is easy to write platform-specific C code and load it into a Lua script dynamically. Lua was designed with this task in mind and does it extremely well. You can also easily dip into C for performance (e.g., compute MD5 checksum).
Over the last 3 to 5 years, I have been gradually migrating scripts from bash/ksh/awk/sed/grep/perl into Lua. I have been very happy with the results.
You could try Batsh
Batsh is a simple programming language that compiles to Bash and Windows Batch. It enables you to write your script once runs on all platforms without any additional dependency.