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Pro Access to really powerful terminals
Pro Most software is open source
Pro Most likely also your deployment target
Makes testing while developing easier. According to a September 2014 study by W3 Techs, *nix based servers are used on over 2/3 of websites.
Pro Lots of development tools available
Pro Package managers
You can install any library or package that you need (gcc, php, node) with just a couple of commands in the terminal.
Pro Most Linux distributions are free
Pro Large percentage of Linux users are developers
Pro Follows the UNIX philosophy
The UNIX philosophy: 'Write programs that do one thing and do it well'. Since Linux itself follows this philosophy then it's very easy to start creating scripts and programs.
Pro A wide variety of distributions available
With a lot of variety, one can use the distribution that fits the type of work best because of the many choices that are given, instead of just one.
Pro Familiarity with Linux is often required from a developer
Nice, developer-friendly environment.
GNU/Linux handles desktop sessions differently than Windows. Users may customize their own sessions; in fact, a single user may use different desktop environments for different login sessions.
Pro Choose any type of desktop environment (or none)
Most Linux distributions support a range of desktop environments, be it plain old X, a tiling window manager or a fully fledged mammoth desktop like GNOME or KDE.
Pro Hardly ever crashes
Pro Works great on older hardware.
I've got some 7-10 year-old Dell laptops that Unix runs very well on where Windows would grind/drag/vomit.
Pro No telemetry, unlike Windows
Con Issues with drivers if your hardware is not officially supported
Con Steep learning curve
Con Maintenance is time-consuming
Con Low user base to develop to
Con Too much customization
To get features on par with OS X, you need to research packages, install them and configure them. Even then, it may not be as good as OS X.
Con A wide variety of distributions available
With a lot of variety, one cannot deploy to a single system and has to prepare for a bundle of distributions, instead of just one.
Con Less and worse professional software is developed, due to the low user base
Depending on what type of work you are doing, you may find Linux software lacking compared to their Win/Mac counterparts.
For example in game development, tools, like Unreal Engine or Unity, usually lack in quality or novelty compared with Windows. Having crashes or bugs that aren't fixed for a while.