why am I getting this bash scripting for loop error?

Question

This is my script

#!/bin/bash
a=1
for [ $a -ge 1 ]
do
        touch ~/file.$a
        a=`expr $a + 1`
done

When I execute it it gives me below error

./script.sh: line 3: syntax error near unexpected token `$a'

./script.sh: line 3: `for [ $a -ge 1 ]'

But it works fine when using "while" instead of "for".

Could you please help me understand why it works with while loop and not with for loop?

Thanks

Answer 1

for loop syntax is not like that mate, do this instead.

for a in {1..10}
do
touch /file.$a
done

you need to declare the range of the variable first.

Answer 2

You meant to use while not for in that syntax. Also since you're using bash, use a for (( )) loop:

for (( i = 1; i <= 10; ++i )); do
    ...
done

The loop by brace expansion has also been suggested in the other answer but I'm not a fan of it since it expands iteration values to words before looping.