Using Postgres COPY TO for data export without overwriting file
You could use the meta command \copy in psql (which encapsulates SQL COPY) on the command line, send it to stdout and and redirect output to be appended to a file - with &gt;&gt; instead of &gt;:

psql -c '\copy (SELECT 1, 2) to stdout' &gt;&gt; /path/to/file_to_append_to.csv


(You probably need more connection parameters.)
You can use SQL COPY as well:

psql -c 'COPY (SELECT 1, 2) TO STDOUT' &gt;&gt; /path/to/file_to_append_to.csv


But it's often more convenient to use \copy instead because (quoting the manual):


  file accessibility and access rights depend on the client rather than
  the server when \copy is used.


Or, like Abelisto commented, from inside psql or any other client - if the program cat is available (basically on all *nix OS):

COPY (SELECT 1, 2) TO PROGRAM 'cat &gt;&gt; /path/to/file_to_append_to.csv'


Or, still from inside psql, on any system, you can set the current directory and an output file with:

\cd /path/to/
\o file_to_append_to.csv


The file gets created (only) if it does not exist yet.
Then you can also SQL COPY (or still \copy as well):

COPY (SELECT 1, 2) TO STDOUT;


The output is appended to the file.