Finding index for new folder - linux

Question

I am given a name and I am supposed to make a dir with this name. If this dir already exists, name of the folder should have _$number as its suffix.
Number is calculated as highest value + 1. Examples:
Name:awesome
Files: dummy awesome awesome_2 awesome_4 dummy_3
New folder: awesome_5
Name:awesome
Files: dummy dummy_1
New folder: awesome
My solution for finding highest value works only for names without special characters. Should the name be for example: "$#&*!(#)(%+#$ asdasd \ ^ sad", it fails.
function max_item() {
local prefix="$1"
local max="0"
shopt -s nullglob
for in_file in * ; do
if [[ "$in_file" =~ ^"$prefix"_(-{0,1}[0-9][0-9]*)$ ]]; then
num="${BASH_REMATCH[1]}";
[[ "$max" -lt "$num" ]] && max="$num";
fi
done
echo "$max"
shopt -u nullglob
return 0
}
I guess it has something to do with special characters in regex but I have exhausted all my ideas.

Answer 1

Since you are looking for a number at the end of the name, prefixed by an _, you could do this instead:
max=0
number='^[[:digit:]]+$'
for in_file in "${prefix}_"* ; do
num="${in_file##*_}"
[[ "$num" =~ $number ]] && [[ "$max" -lt "$num" ]] && max="$num"
done
num=$((max + 1))
I have incorporated #Jens' excellent suggestion to loop through the just the matching files.

Answer 2

Looping in shell code is notoriously slow.
For small numbers, codeforester's solution is fine, but starting at around 30 items (the exact number depends on many factors), the external-utility-based solution below will be faster and scale much better.
(For fewer items, an external-utility solution is slower, but that will rarely matter).
The solution below has the added advantage of being more concise:
max_index() {
printf '%d\n' "$(shopt -s nullglob;
printf '%s\n' "$1_"* |
awk -F_ '{print $NF}' |
sort -rn | head -n 1)"
}
Note: The reasonable assumption is made that your filenames have no embedded newlines.
shopt -s nullglob ensures that if a globbing pattern ("$1_"* in this case) matches nothing, it expands to the null (empty) string.
printf '%s\n' "$1_"* prints all matching filesystem items line by line.
awk -F_ '{print $NF}' outputs the last _-based token on each line, i.e., the trailing number.
Note: cut -d_ -f2 would work too, but makes the assumption that only one _ is present in the filename.
sort -rn sorts the trailing numbers numerically (-n), in reverse (-r).
head -n 1 then extracts only the 1st output line, which is by definition the highest number (if any).
Note that printf '%d\n' '' outputs 0, which is effectively what happens if no existing _<number> suffixes are found.