Can do system functions, kernel driver development, GUI interface development, automated test script development, all can be ah, very widely used. The development language can be C language, C language, Shell script, Expect script, Perl script, Python script, etc. Therefore, your question covers a wide range.
Nowadays, many communication, medical, military and household appliances use embedded Linux system, which can be cut and run on mobile devices. In the final analysis, both Android and IOS can be counted as Linux platforms. Of course, it can also be developed independently based on the free Linux platform, and there are many third-party libraries that can be used directly.
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