The Unix family includes several incredibly popular, open-source operating systems (including Ubuntu, Debian, and Redhat). Here are #a few resources on how to use Unix systems and operate the command line.
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Linux Journey: A comprehensive Linux shell concepts tutorial with interactive command line lessons creating fun and free learning for beginners to advanced users.
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EDX: Introduction to Linux: great intro to upper-intermediate course on what the hell is Linux
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Basic Unix Commands: a guide from Oxford University on Unix commands
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About Unix: Key Unix terms and ideas; website created by Indiana University.
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Linux tutorials: Linux tutorials at the beginner, intermediate, and advanced levels, from the Linux Foundation.
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The Unix Specification: The Open Group's description of the Unix specification. The website also includes links to some helpful white papers, accessible via the top menu.
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UNIX Resources Page: North Carolina School of Science and Mathematics's list of useful Unix resources