Defining and calling variables in shell script using sed steps

I have an HTML file that lays out images of text to create interesting typographic patterns. The images are placed in the local web page with

<img src="[path-to-project-folder]/DIR/[filename].png">.

The project folder has 15 folders with variations of all the images in them, named 001, 002 etc. I want to replace DIR with a random number from 001-015 and have tried the following in a shell script to get sed to replace the variable number in steps across my source file;

for RUN in {1..15};
do 
var1=`shuf -i 001-015 -n 1`
step=$((step + 1));
while [ ${#var1} -ne 3 ];
 do
   var1="0"$var1
 done

echo $var1
echo $step

sed  -i -e "s/DIR/$var1/"{100..1..$step} temp.htm
done

Unfortunately that results in an error of

sed: -e expression #1, char 11: unknown option to `s'

I know from attempting to debug that $var1 is replaced okay. It is $step that is not getting through. I have tried things like ${step} and combinations of backslashes but that's just copying other scripts that work rather than truly understanding what is going on after my sed s/// argument to explain the error. The expansion section in the bash manual doesn't seem to explain the sed error.

Can anyone help please? I'd like the output to be random like

<img src="[path-to-project-folder]/001/[filename].png"><img src="[path-to-project-folder]/014/[filename].png"><img src="[path-to-project-folder]/003/[filename].png">.

and so on. I'm using sed 4.4 on Ubuntu 18.10 with bash 4.4.20

Solution

{100..1..$step} does not do what you think it does.

From the bash man page:

Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and pathname expansion.

Notice that brace expansion happens before parameter expansion. The literal sequence of characters $step is not a valid number (increment), so the brace expansion fails.

Consider:

$ step=1
$ echo "preamble"{10..1..$step}
preamble{10..1..1}
$

shellcheck can provide useful diagnostics. It finds the problem in your code and reports:

sed  -i -e "s/DIR/$var1/"{100..1..$step} temp.htm
                         ^-------------^ SC2051 (warning): Bash doesn't support variables in brace range expansions.

A quick rewrite of your code:

for ((step=1; step<=15; step++)); do

    printf -v var1 '%03d' $(( (RANDOM%15)+1 ))

    echo $var1
    echo $step

    for (( n=100; n>=1; n-=step )); do
        echo "s/DIR/$var1/$n"
    done |
    sed  -i -f - temp.htm

done