Counting unique values in a column with a shell script
No need to use awk.
$ cut -f2 file.txt | sort | uniq | wc -lshould do it.
This uses the fact that tab is cut's default field separator, so we'll get just the content from column two this way. Then a pass through sort works as a pre-stage to uniq, which removes the duplicates. Finally we count the lines, which is the sought number.
I go for
$ cut -f2 file.txt | sort -u | wc -lAt least in some versions, uniq relies on the input data being sorted (it looks only at adjacent lines).
For example in the Solaris docs:
The uniq utility will read an input file comparing adjacent lines, and write one copy of each input line on the output. The second and succeeding copies of repeated adjacent input lines will not be written.
Repeated lines in the input will not be detected if they are not adjacent.