36.5 Alphabetic and Numeric Sorting

sort performs two fundamentally different kinds of sorting operations: alphabetic sorts and numeric sorts. An alphabetic sort is performed according to the traditional "dictionary order," using the ASCII (51.3 ) collating sequence. Uppercase letters come before lowercase letters (unless you specify the -f option, which "folds" uppercase and lowercase together), with numerals and punctuation interspersed.

This is all fairly trivial and common sense. However, it's worth belaboring the difference, because it's a frequent source of bugs in shell scripts. Say you sort the numbers 1 through 12. A numeric sort gives you these numbers "in order," just like you'd expect. An alphabetic sort gives you:

1
11
12
2
...

Of course, this is how you'd sort the numbers if you applied dictionary rules to the list. Numeric sorts can handle decimal numbers (for example, numbers like 123.44565778); they can't handle floating-point numbers (for example, 1.2344565778E+02).

What happens if you include alphabetic characters in a numeric sort? Although the results are predictable, I would prefer to say that they're "undefined." Including alphabetic characters in a numeric sort is a mistake, and there's no guarantee that different versions of sort will handle them the same way. As far as I know, there is no provision for sorting hexadecimal numbers.

One final note: Under System V, the numeric sort treats initial blanks as significant - so numbers with additional spaces before them will be sorted ahead of numbers without the additional spaces. This is an incredibly stupid misfeature. There is a workaround; use the -b (ignore leading blanks) and always specify a sort field. [2] That is: sort -nb +0 will do what you expect; sort -n won't.

[2] Stupid misfeature number 2: -b doesn't work unless you specify a sort field explicitly, with a +n option.