Thread: Variable + string concatenation?

How can I concatenate a variable to a string?
Something like:

\set input_directory /dsk2/pg_input/
concatenate input_directory + "filename"

copy horses from :input_directory 'horses.txt' with NULL as '';

I am going to have the "\set" in a file which I am going to "\i" include
into several scripts. This way i would be able to easily change the source
directory for all scripts.