How to run mongo --eval?

I run mongo --help and it has an options for evaluate javascript.

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mongo --eval  arg

However, I haven't found any example of how to use it. I try to run it but with no success. anyone know how to do it?

the reason is that, I want to delete a collection through command line ( so that I can reuse the code and put it into shell script). I know I can write another nodejs and call the db to run it.. but instead of 20 lines of code, it would be nice if there is a solution with only few lines and be faster. :)

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That would be:

mongo <server>:<port>/<database> --eval "db.<some_collection>.drop();"

Or if you want to do something in more lines you can call a file (must end with .js):

mongo <server>:<port>/<database> some_instructions.js