Using a variable in a bash script for loop - bash

Question

I have a file which has the number of .pdfs in my folder. I assign this number to a variable, fileNum like so:
fileNum=$(ls -l *.pdf | wc -l)
echo $fileNum returns this number without any problem.
Now I need to use fileNum in a for loop and I am having problems with it.
My for loop is:
for i in {1..$fileNum}
do
var=$(awk 'NR=='$i 'pdfs.file')
gs \
-sOutputFile="exgs_"$var \
-sDEVICE=pdfwrite \
-sColorConversionStrategy=Gray \
-dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 \
-dNOPAUSE \
-dBATCH \
$var
done
The $ at the beginning of fileNum gives me an error message which is:
awk: line 1: syntax error at or near {
Things are fine when I actually use the number itself (which in this case is 17).
Obviously, awk doesn't like this because of ... something.... I don't know what. What should I do about this?
I tried other forms such as $(fileNum) and filenum with the single quotes around it.
Is it something to do with strings?

Answer 1

I'd use bash to read the file instead of running awk for every line.
while read -r file; do
gs -sOutputFile="exgs_$file" \
-sDEVICE=pdfwrite \
-sColorConversionStrategy=Gray \
-dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 \
-dNOPAUSE \
-dBATCH \
"$file"
done < pdfs.file
See also http://mywiki.wooledge.org/BashFAQ/001
Otherwise, for the general case where you want to iterate from 1 to n, I'd use a C-style for-loop.
n=10
for (( i=1; i <= n; i++)); do
...
done

Answer 2

This is because Bash will do the expansion on the braces before the variable. You need to use eval in this case so that Bash expands the variable first.
for i in $(eval echo {1..$fileNum})
do
var=$(awk 'NR=='$i 'pdfs.file')
gs \
-sOutputFile="exgs_"$var \
-sDEVICE=pdfwrite \
-sColorConversionStrategy=Gray \
-dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 \
-dNOPAUSE \
-dBATCH \
$var
done

Answer 3

I would rather write:
for i in $(seq $fileNum)
do
....
done