How to run parallel with multiplying increment

Question

I know that I can use "parallel" to run multiple instances of whatever script/application in parallel over a variable with a given increment, eg:

parallel "echo hello {}" ::: {1..16..2}

Output would be:

hello 1
hello 3
hello 5
hello 7
hello 9
hello 11
hello 13
hello 15

I want to use an increment that multiplies the run variable so that I get an output like this:

hello 1
hello 2
hello 4
hello 8
hello 16

What should I write in the {1..16..#}?

Thanks!

Answer 1

Try this:

parallel "echo hello {}" ::: $(awk 'BEGIN {for(i=0; i<=16; i++) printf 2**i" "}')

awk is used to print out a list of powers of 2 which will then be used by parallel .

Alternatively:

parallel "echo hello {}" ::: $(printf '%s\n' 2^{0..16} | bc | tr '\n' ' ')

This prints out the numbers 1 to 16 as part of the string x^2 , printf ensures each number is on a separate line. bc then computes the actual numbers, and tr removes the newlines again.

Answer 2

Use {= =} (available in version 20140822 and later):

seq 1 2 16 | parallel echo hello {}
parallel echo hello '{= $_=2*$_ =}' ::: {1..16}
seq 1 16 | parallel echo hello '{= $_=2**$_ =}'
parallel echo hello '{= $_=2**$_ =}'  ::: {1..16}