How to pass timestamp from bash to psql - bash

Question

I am having problem with passing timestamp parameter to psql. In $since variable I can have any string formatted according to SQL standard and I pass this value to sql like this:
First I check if $since is in correct format (if it fails it won't continue):
1) psql --command "SELECT ($since)::TIMESTAMPTZ;"
Second I use the value in my function (it takes timestamptz as input parameter):
2) cmd="SELECT myfunc($since);"
psql --command "$cmd" $DBNAME
Works: if since="NOW() - INTERVAL '5 months'"
Does not work: if since="2017-10-23 10:42:48" (it fails on LINE 1: SELECT (2017-10-23 10:42:48)::TIMESTAMPTZ; error)
I tried to escape the $since string somehow with ', ", \ characters, but after many combinations both in bash and sql I gave up.
What is the correct way to escape in such case?

Answer 1

If you need to cast a string to TIMESTAMPTZ, so you need to enclose the value of $since in ' ', either when creating the variable
since="'2017-10-23 10:42:48'"
or when passing it to psql:
since="2017-10-23 10:42:48"
psql --command "SELECT '$since'::TIMESTAMPTZ ;"
If you need to pass either a string or an expression like NOW() - INTERVAL '1 day' you would better decide about the quoting when assigning the value to the variable, so:
$ since="'2017-10-23 10:42:48'"
$ psql postgres --command "SELECT $since::TIMESTAMPTZ ;"
timestamptz
------------------------
2017-10-23 10:42:48+02
(1 row)
$ since="(NOW() - INTERVAL '1 day')"
$ psql postgres --command "SELECT $since::TIMESTAMPTZ ;"
timestamptz
-------------------------------
2018-03-24 08:49:24.577356+01
(1 row)