Shell: Create a Comma Separated String

I recently needed to generate a string with comma separated values, based on iterating a range of numbers (e.g. we should get the following output where n = 3).

foo-0,foo-1,foo-2

I only had Shell available to me so I couldn’t shell out into Python or Ruby, for example. That means it’s Bash scripting time!

If we want to iterate a range of numbers and print them out on the screen we can write the following code:

n=3
for i in $(seq 0 $(($n > 0? $n-1: 0))); do 
  echo "foo-$i"
done

foo-0
foo-1
foo-2

Combining them into a string is a bit more tricky, but luckily I found a great blog post by Andreas Haupt which shows what to do. Andreas is solving a more complicated problem than me, but these are the bits of code that we need from the post.

n=3
combined=""

for i in $(seq 0 $(($n > 0? $n-1: 0))); do 
  token="foo-$i"
  combined="${combined}${combined:+,}$token"
done
echo $combined

foo-0,foo-1,foo-2

This won’t work if you set n<0, but that’s ok for me! I’ll let Andreas explain how it works:

• ${combined:+,} will return either a comma (if combined exists and is set) or nothing at all.
• In the first invocation of the loop combined is not yet set and nothing is put out.
• In the next rounds combined is set and a comma will be put out.

We can see it in action by printing out the value of $combined after each iteration of the loop:

n=3
combined=""

for i in $(seq 0 $(($n > 0 ? $n-1: 0))); do 
  token="foo-$i"
  combined="${combined}${combined:+,}$token"
  echo $combined
done

foo-0
foo-0,foo-1
foo-0,foo-1,foo-2

Looks good to me!