Using sed and mv to add characters to files - sed

Question

First off, I'd like to say that I know this is almost an exact duplicate of some posts that I've read, but have not had any luck with referencing.
I have 100+ files that all follow a very strict naming convention of 5_##_<name>.ext My issue was that when originally making these files I failed to realise that 5_100_ and above would mess up my ordering.
I am now trying to append a 0 in front of every number between 01 and 99. I've written a bash script using sed that works for the file contents (the file name is in the file as well):
#!/bin/bash
for fl in *.tcl; do
echo Filename: $fl
#sed -i 's/5_\(..\)_/5_0\1_/g' $fl
done
However, this only changes the contents and not the filename itself. I've read that mv is the solution (rename is simpler but I do not have it on my system). My current incarnation of my multiple attempts is:
mv "$fl" $(echo "$file" | sed -e 's/5_\(..\)_/5_0\1_/g') but it gives me an error: mv: missing destination file operand after <filename>
Again, I'm sorry about the duplicate but I wasn't able to solve my issue by reading it. I'm sure I'm just using the combination of mv and sed incorrectly.
Solution was entered in the comments. I was using $file instead of $fl.

Answer 1

Something like this might be useful:
for n in $(seq 99)
do
prefix2="5_$(printf "%02d" ${n})_"
prefix3="5_$(printf "%03d" ${n})_"
for f in ${prefix2}*.tcl
do
suffix="${f#${prefix2}}"
[[ -r "${prefix3}${suffix}" ]] || mv "${prefix2}${suffix}" "${prefix3}${suffix}"
done
done
Rather than processing every single file, it only looks at the ones that currently have a "5_XX_" prefix, and only renames them if the corresponding "5_XXX_" file doesn't already exist...

Answer 2

#!/bin/bash
for fl in *.tcl
do
NewName="$(echo "${fl} | sed '/^5_[0-9]\{2\}_/ s/../&0/' )"
#echo "Filename: ${fl} -> ${NewName}
[ ! "${fl}" = "${NewName}" ] && mv ${fl} ${NewName}
done
With a bit a securisation a allow to pass several time on same folder (changing only needed one).
Under linux (non posix sed by default), use sed --posix instead of simple sed call