Oops, I hit the wrong button ...

    cat > /tmp/x <<!
    b
    a
    !
    ln /tmp/x /tmp/y
    sort -c /tmp/x
    join --check-order /tmp/x /tmp/y
    # Note: The two files do not have to be the same.

Output is

    sort: /tmp/x:2: disorder: a
    join: file 1 is not in sorted order

Thanks for your consideration.




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