PostgreSQL: How to pass parameters from command line?

Question

I have a somewhat detailed query in a script that uses ? placeholders. I wanted to test this same query directly from the psql command line (outside the script

Answer 1

I would like to offer another answer inspired by @malcook's comment (using bash).

This option may work for you if you need to use shell variables within your query when using the -c flag. Specifically, I wanted to get the count of a table, whose name was a shell variable (which you can't pass directly when using -c).

Assume you have your shell variable

$TABLE_NAME='users'

Then you can get the results of that by using

psql -q -A -t -d databasename -c <<< echo "select count(*) from $TABLE_NAME;"

(the -q -A -t is just to print out the resulting number without additional formatting)

I will note that the echo in the here-string (the <<< operator) may not be necessary, I originally thought the quotes by themselves would be fine, maybe someone can clarify the reason for this.