How to use globbing for loop in bash -c command?

Question

I'm trying to run a for loop inside of bash -c so I can time it. This code writes 23 every time. How can I make it iterate through the numbers?

time bash -c "for i in {0..23}; do echo $i & done"

This code works, but I need to time the whole operation:

for i in {0..23}; do echo $i & done

The echo command is just for simplicity. I am really trying to run dd, that's why I background the process with &

Answer 1

There are two issues.

time bash -c "for i in {0..23}; do echo $i & done"

doesn’t work as you expect because $i is expanded immediately; you need to use single quotes:

time bash -c 'for i in {0..23}; do echo $i & done'

To get time to wait for all the background jobs, you can use wait:

time bash -c 'for i in {0..23}; do sleep $i & done; wait'

This causes the shell to wait for all its jobs to finish, which results in time measuring the time taken for all the operations together.