Let’s say you want to dump your MongoDB database. There’s a handy tool that does just that, mongodump.

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mongodump

If executed without any arguments it will try to connect to localhost:27017 and dump all databases. You can specify a single database that you’re interested in and it will dump just this database.

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mongodump -d mydb

But in some cases you don’t want to dump the whole database. In my case, it’s an analytics application and 99% of data is in raw event collections (events20120320, events20120321, …). I am interested only in a small number of “important” collections. But mongodump doesn’t provide us with an option to specify several collections. You can only dump one collection at a time. If you don’t mind some typing, that’s easy.

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mongodump -d mydb -c mycoll1
mongodump -d mydb -c mycoll2
mongodump -d mydb -c mycoll5

But we’re all programmers, so let’s automate this stuff. I, personally, never used bash loops before, and it seems like a good use case for them. Let’s do it.

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colls=( mycoll1 mycoll2 mycoll5 )

for c in ${colls[@]}
do
  mongodump -d mydb -c $c
done

First line defines a bash array literal. Don’t use commas to delimit array elements, they’ll become part of the element. The ${colls[@]} string means “all array elements” and it can be used anywhere where a variable is expected. The rest is pretty straightforward, I think.