Navigating and manipulating files/directories

If you don't know these commands already, now would be a good time to learn.

• Show the current directory: pwd
• Change the current directory: cd dirname
• Create a directory: mkdir dirname
• Remove a directory: rmdir dirname
• Remove a file/link: rm filename
• Remove an entire subdirectory tree (dangerous!): rm -rf dirname
• Make a hard link to a file: ln filename linkname
• Make a symbolic link to a file: ln -s filename linkname
• Rename ("move") a file: mv oldname newname (conceptually equivalent to ln oldname newname; rm oldname, including maybe not working across filesystem boundaries)
• Copy a file (duplicates contents, instead of making a link alias): cp filename newname
• Change permissions on a file/directory: chmod perm filename
  • perm can be an octal code (755) or a symbolic representation (+x, ug-w, etc.)
• Change ownership of a file: chown username filename
• Change group owner of a file: chgrp groupname filename
• Both at once: chown username:groupname filename

The man command can be used to get detailed documentation on any of these, for example, man chmod.