How do I iterate over a range of numbers defined by variables in Bash?

To iterate over a range of numbers defined by variables in Bash, use one of the following methods:

1. C-style for Loop (Recommended)

The most efficient and modern approach, supported in Bash 3.0+.

Syntax:

start=3
end=7
for ((i = start; i <= end; i++)); do
    echo "$i"
done

Output:

3
4
5
6
7

With a Step Value:

step=2
for ((i = start; i <= end; i += step)); do
    echo "$i"
done

Output:

3
5
7

2. Using seq (Portable)

Works on most systems but relies on the external seq command.

Basic Usage:

for i in $(seq "$start" "$end"); do
    echo "$i"
done

With a Step Value:

step=2
for i in $(seq "$start" "$step" "$end"); do
    echo "$i"
done

3. Brace Expansion with eval (Caution)

Use eval to expand variables in brace syntax. Only use this if you control the input.

Syntax:

for i in $(eval echo "{$start..$end}"); do
    echo "$i"
done

Key Notes

• C-style Loop: Best for modern scripts (no external dependencies).
• seq: Useful for stepping or backward ranges (e.g., seq 10 -2 3).
• Error Handling: Ensure start and end are integers.
• Reverse Ranges: Use for ((i = start; i >= end; i--)).

Examples

Backward Iteration

start=5
end=1
for ((i = start; i >= end; i--)); do
    echo "$i"
done

Output:

5
4
3
2
1

Negative Numbers

start=-2
end=2
for ((i = start; i <= end; i++)); do
    echo "$i"
done

Output:

-2
-1
0
1
2

Common Pitfalls

• Variables Not Integers: Validate input (e.g., [[ $start =~ ^-?[0-9]+$ ]]).
• Start > End: Handle reverse ranges explicitly (use i-- instead of i++).

Choose the method that best fits your script’s requirements!