SED - Align - Align all text flush right on a 79-column width

sed -e :a -e 's/^.\{1,78\}$/ &/;ta'  # Set at 78 plus 1 space.

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Running that command again this file:

testfile

aaa
bbb
ccc

returns:

testfile

                                                                            aaa
                                                                            bbb
                                                                            ccc