psql command line variables are unknown when -c SQL statement are executed

wrong:

pavel@nemesis ~]$ psql postgres -v x=3D10 -c "select :x"
ERROR:  syntax error at or near ":"
LINE 1: select :x
               ^

good (workaround):
[pavel@nemesis ~]$ echo 'select :x' | psql postgres -v x=3D10
 ?column?
=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=94=80=E2=
=94=80=E2=94=80
       10
(1 row)

psql have to have process external variables first and then execute query.

Regards
Pavel Stehule