Writing a bash for-loop with a variable top-end

Question

I frequently write for-loops in bash with the well-known syntax:

for i in {1..10}  [...]

Now, I'm trying to write one where the top is defined by a variable:

TOP=10
for i in {1..$TOP} [...]

I've tried a variety of parens, curly-brackets, evaluations, etc, and typically get back an error "bad substitution".

How can I write my for-loop so that the limit depends on a variable instead of a hard-coded value?

Solution 1

You can use for loop like this to iterate with a variable $TOP:

for ((i=1; i<=$TOP; i++))
do
   echo $i
   # rest of your code
done

Solution 2

If you have a gnu system, you can use seq to generate various sequences, including this.

for i in $(seq $TOP); do
    ...
done

Solution 3

Answer is partly there : see Example 11-12. A C-style for loop.

Here is a summary from there, but be aware the final answer to your question depends on your bash interpreter (/bin/bash --version):

# Standard syntax.
for a in 1 2 3 4 5 6 7 8 9 10

# Using "seq" ...
for a in `seq 10`

# Using brace expansion ...
# Bash, version 3+.
for a in {1..10}

# Using C-like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++))  # Double parentheses, and "LIMIT" with no "$".

# another example
lines=$(cat $file_name | wc -l)
for i in `seq 1 "$lines"`

# An another more advanced example: looping up to the last element count of an array :
for element in $(seq 1 ${#my_array[@]})

Comments

[Daenyth]

seq is not recommended over brace expansion. The behavior between bsd/gnu isn't quite the same.

[Kevin]

@daenyth good to note for when brace expansion is a viable alternative, but the whole point of this question is that brace expansion doesn't work with variable bounds.