for loop control in bash using a string

Question

I want to use a string to control a for loop in bash. My first test code produces what I would expect and what I want:

$ aa='1 2 3 4'
$ for ii in $aa; do echo $ii; done
1
2
3
4

I'd like to use something like the following instead. This doesn't give the output I'd like (I can see why it does what it does).

$ aa='1..4'
$ for ii in $aa; do echo $ii; done
1..4

Any suggestions on how I should modify the second example to give the same output as the first?

Thanks in advance for any thoughts. I'm slowly learning bash but still have a lot to learn.

Mike

Answer 1

The notation could be written out as:

for ii in {1..4}; do echo "$ii"; done

but the {1..4} needs to be written out like that, no variables involved, and not as the result of variable substitution. That is brace expansion in the Bash manual, and it happens before string expansions, etc. You'll probably be best off using:

for ii in $(seq 1 4); do echo "$ii"; done

where either the 1 or the 4 or both can be shell variables.

Answer 2

You could use seq command (see man seq).

$ aa='1 4'
$ for ii in $(seq $aa); do echo $ii; done

Answer 3

Bash won't do brace expansion with variables, but you can use eval:

$ aa='1..4'
$ for ii in $(eval echo {$aa}); do echo $ii; done
1
2
3
4

You could also split aa into an array:

IFS=. arr=($aa) 
for ((ii=arr[0]; ii<arr[2]; ii++)); do echo $ii; done

Note that IFS can only be a single character, so the .. range places the numbers into indexes 0 and 2.