psql -U username -d mydatabase -c 'SELECT * FROM mytable'

If you're new to postgresql and unfamiliar with using the command line tool psql then there is some confusing behaviour you should be aware of when you've entered an interactive session.

For example, initiate an interactive session:

psql -U username mydatabase 
mydatabase=#

At this point you can enter a query directly but you must remember to terminate the query with a semicolon ;

For example:

mydatabase=# SELECT * FROM mytable;

If you forget the semicolon then when you hit enter you will get nothing on your return line because psql will be assuming that you have not finished entering your query. This can lead to all kinds of confusion. For example, if you re-enter the same query you will have most likely create a syntax error.

As an experiment, try typing any garble you want at the psql prompt then hit enter. psql will silently provide you with a new line. If you enter a semicolon on that new line and then hit enter, then you will receive the ERROR:

mydatabase=# asdfs 
mydatabase=# ;  
ERROR:  syntax error at or near "asdfs"
LINE 1: asdfs
    ^

The rule of thumb is: If you received no response from psql but you were expecting at least SOMETHING, then you forgot the semicolon ;