I really like this; reminiscent of the Python Challenge, but for shell.

You should support bash's double-star wildcard, which works recursively. Several of the challenges that currently require

    find -name 'access.log*' -print0 | xargs -0 somecommand

could just use

    somecommand **/access.log*

instead.

You might also encourage people to look up manpages. For instance, someone who doesn't know about "grep -h" may want to look at the manpage for grep. You could make "man grep" link to http://man7.org/linux/man-pages/man1/grep.1.html .

Tab completion doesn't always include all files; for instance, it doesn't include split-me.txt

Have you considered logging all unique successful solutions (filtering out minor whitespace differences) to each challenge? That would let people who have completed each challenge see other potential approaches to the problem, and perhaps learn about new commands.