What if we use MIT-licensed code in our propietary software? Is it considered stealing?
I saw some software that was packaged with MIT licensed code in the installation directory.
Some people think that I'm stealing. Is it true?
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For all software libraries, copyright law applies, and you must read and follow the license in order to use it legally. For a MIT licensed code library, if you distribute the resulting package, you must also distribute it with a copy of the license including the original copyright notices. Failure to do so is "stealing" or more technically a copyright violation, which you should address.
The MIT license has one major requirement you must follow to comply:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
To avoid confusion that your whole product is covered by the same license, it is good practice to include the whole original source of the MIT part and license in a separate folder, and provide this along with the rest of your package. Otherwise someone might think that your proprietary software may be freely distributed. However, MIT license is not copyleft in any way, so code you write yourself as part of a combined package may be distributed under any other license you see fit.
That includes closed source and even commercial use where a customer must pay to have a copy of your software installed. However, even though such a customer may be entirely uninterested in having a copy of the MIT-licensed source files and the license itself, you must still supply them bundled with whatever distribution mechanism you use, in order to be compliant.
The BSD license and Apache 2.0 license are similar (although the terms and license text you need to include are different). They are known as permissive licenses because they are compatible with many types of re-use including being part of non-open-source software, provided the terms are followed.