Technically speaking, this may be a case of undefined behavior. From my man page: -u, --unique Unique keys. Suppress all lines that have a key that is

Technically speaking, this may be a case of undefined behavior. From my man page: -u, --unique Unique keys. Suppress all lines that have a key that is equal to an already processed one. This option, similarly to -s, implies a stable sort. If used with -c or -C, sort also checks that there are no lines with duplicate keys. ... -n, --numeric-sort, --sort=numeric Sort fields numerically by arithmetic value. Fields are supposed to have optional blanks in the beginning, an optional minus sign, zero or more digits (including decimal point and possible thousand separators). When you use -n, without a key fields specification, the whole line does not meet the requirement for numeric sorting. This sort does give me the intended output: $sort -k1,1n -k2 -u ~/tmp/sort.txt 1 a 5 which 10 exotically 15 aerodynamically 15 differentiation 20 electroencephalogram Whether or not deduplication on keys is ideal behavior, it is what is specified here. What is not explicitly specified is what is considered to be the key when you try to sort a non-numeric line numerically. This is the sort of problem that you get with duck typing: it does what you expect and intend, except in those corner cases where it doesn't.