I'm making a command line tool that allows end users to query a statically-schemed database; however, I want users to be able to specify boolean matchers in their query (effectively things like "get rows where (field1=abcd && field2=efgh) || field3=1234"). I did Googling a solution, but I couldn't find anything suitable for end users--still, this seems like it would be a very common problem so I suspect there is a standard solution.

So:

1. What (if any) standard query "languages" are there that might be appropriate for end users?
2. What (if any) de facto standards are there (for example, Unix tools that solve similar problems).
3. Failing the previous two options, can you suggest a syntax that would be simple, concise, and easy to validate?