Is there a standard unix program that returns a range of numbers

seq is part of coreutils.

for i in $( seq 1 2 11 ) ; do echo $i ; done

Output:

1
3
5
7
9
11

If you provide only 2 arguments to seq, the increment is 1:

$ seq 4 9
4
5
6
7
8
9

Would Bash suffice?

for i in {10..20}; do echo $i; done

You can do a lot of things with brace expansion. Bash 4 also supports padded ranges, e.g. {01..20}.

Note that Bash is not considered portable, and not a standard Unix utility. Although you can safely assume that it is installed on most modern Linuxes, don't use this in a script that you plan to run on all kinds of Unix-like machines.

If you want something strictly portable (i.e. that does not rely on specific bash extensions or commands not specified by POSIX)

awk 'BEGIN {for(i=10;i<=20;i++) printf "%d ",i; print}'

Use a for loop

for ((i = 10; i <= 20; ++i)); do
    printf '%d\n' "$i"
done