Check if database exists in PostgreSQL using shell

Question

I was wondering if anyone would be able to tell me about whether it is possible to use shell to check if a PostgreSQL database exists?

I am making a shell script and I only want it to create the database if it doesn't already exist but up to now haven't been able to see how to implement it.

Answer 1

I use the following modification of Arturo's solution:

psql -lqt | cut -d \| -f 1 | grep -qw <db_name>

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What it does

psql -l outputs something like the following:

                                        List of databases
     Name  |   Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+-----------+----------+------------+------------+-----------------------
 my_db     | my_user   | UTF8     | en_US.UTF8 | en_US.UTF8 | 
 postgres  | postgres  | LATIN1   | en_US      | en_US      | 
 template0 | postgres  | LATIN1   | en_US      | en_US      | =c/postgres          +
           |           |          |            |            | postgres=CTc/postgres
 template1 | postgres  | LATIN1   | en_US      | en_US      | =c/postgres          +
           |           |          |            |            | postgres=CTc/postgres
(4 rows)

Using the naive approach means that searching for a database called "List, "Access" or "rows" will succeed. So we pipe this output through a bunch of built-in command line tools to only search in the first column.

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The -t flag removes headers and footers:

 my_db     | my_user   | UTF8     | en_US.UTF8 | en_US.UTF8 | 
 postgres  | postgres  | LATIN1   | en_US      | en_US      | 
 template0 | postgres  | LATIN1   | en_US      | en_US      | =c/postgres          +
           |           |          |            |            | postgres=CTc/postgres
 template1 | postgres  | LATIN1   | en_US      | en_US      | =c/postgres          +
           |           |          |            |            | postgres=CTc/postgres

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The next bit, cut -d \| -f 1 splits the output by the vertical pipe | character (escaped from the shell with a backslash), and selects field 1. This leaves:

 my_db             
 postgres          
 template0         

 template1         

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grep -w matches whole words, and so won't match if you are searching for temp in this scenario. The -q option suppresses any output written to the screen, so if you want to run this interactively at a command prompt you may with to exclude the -q so something gets displayed immediately.

Note that grep -w matches alphanumeric, digits and the underscore, which is exactly the set of characters allowed in unquoted database names in postgresql (hyphens are not legal in unquoted identifiers). If you are using other characters, grep -w won't work for you.

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The exit status of this whole pipeline will be 0 (success) if the database exists or 1 (failure) if it doesn't. Your shell will set the special variable $? to the exit status of the last command. You can also test the status directly in a conditional:

if psql -lqt | cut -d \| -f 1 | grep -qw <db_name>; then
    # database exists
    # $? is 0
else
    # ruh-roh
    # $? is 1
fi