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

wrong:

pavel(at)nemesis ~]$ psql postgres -v x=10 -c "select :x"
ERROR: syntax error at or near ":"
LINE 1: select :x
^

good (workaround):
[pavel(at)nemesis ~]$ echo 'select :x' | psql postgres -v x=10
?column?
──────────
10
(1 row)

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

Regards
Pavel Stehule