I would like to ask for a review of the following Bash script which cleans up and datestamps file names, with an optional feature to rename the file.

The main use case is renaming files downloaded from Overleaf, so that Angebot_produkt_kunde-2.pdf (resulting from the second download from the project without moving the file Angebot_produkt_kunde.pdf out of the way first) gets renamed to Angebot_produkt_kunde-2-2024-06-10_1248.pdf.

If the file name contains a datestamp, it is updated if the string to append is date.

Another feature is appending custom suffixes, as in

angebot_produkt_Müller-Maßnahmen-5 "Neuer Entwurf" -> Angebot-produkt-mueller-massnahmen-5-neuer-entwurf

In this example, the file name has no extension.

I want to make sure that the file remains in the directory it is in and to skip files whose names start with a dot. Files without extension or with more than one extension are handled correctly.

The file names are also cleaned up so they will not cause problems when archived or moved between file systems. Umlauts and spaces are removed, as are leading, trailing, and excessive internal dots or hyphens. Finally, the new name is in lowercase.

Find test cases at the end of this question.

Specific review questions:

• Can this script be written more concisely? I am particularly unhappy about

     sed -E -e 's/(^-|([\.-]*)$)//g'   \
            -e 's/-\././g'
  

  which is needed to get to the new name Txt.pdf instead of Txt-.pdf when called as follows:

   ~/bin/newname.sh --printonly '/tmp/@#$.txt.pdf' '!'
  

• Is there an accepted name for the base name of a file, but without any extension? That is, in the name /tmp/data.csv, the string /tmp is the directory name, the string data.csv is the base name, the string .csv is the extension, but what is a good name for the string data on its own?

• Should I rename the option --printonly to -d (abbreviating "dry-run")? Is anything off with my handling of options?

• Have I missed any obvious edge cases for testing? (A non-obvious edge case would be a file name consisting entirely of Hanzi characters, for example. Such a case will not occur in the environment the script is expected to run in.)

• What about locales? I am a first-time Cygwin user and am mildly surprised to see echo "$LC_ALL" not produce any output. Should I replace A-Za-z0-9 with [[:alnum:]], or are there any disadvantages to that?

Test cases:

    #!/bin/bash
    
    {    
        ~/bin/newname.sh --printonly Angebot_produkt_kunde_Maßnahmen-6_2024-06-27_1620.pdf date
        ~/bin/newname.sh --printonly angebot_produkt_Müller-Maßnahmen-4__2024-06-27_1806.txt.pdf date
    
        ~/bin/newname.sh --printonly angebot_produkt_Müller-Maßnahmen-5 "Neuer Entwurf"
        ~/bin/newname.sh --printonly angebot_produkt_Müller-Maßnahmen-3.pdf "Neuer Entwurf"
        ~/bin/newname.sh --printonly "Angebot 5.pdf" "  Neuer  Entwurf       "
        ~/bin/newname.sh --printonly Report_kunde_17_06_2024____23_06_2024___LuaLaTeX.pdf date
    
        ~/bin/newname.sh --printonly ~/angebot_produkt_Müller-Maßnahmen-3.pdf "Neuer Entwurf"
        ~/bin/newname.sh --printonly /tmp/files/"Angebot 5.pdf" "  Neuer  Entwurf       "
        ~/bin/newname.sh --printonly /tmp/files/"Angebot 5%% Rabatt.pdf" "  Neuer  Entwurf       "
    
        ~/bin/newname.sh --printonly .configfile date
        ~/bin/newname.sh --printonly /tmp/.configfile date
        
        ~/bin/newname.sh --printonly Angebot_produkt_kunde-2.pdf date
    
        ~/bin/newname.sh --printonly '/tmp/@#$%.txt' '!'
        ~/bin/newname.sh --printonly '/tmp/@#$.txt.pdf' '!'
        ~/bin/newname.sh --printonly '/tmp/@#$*' '!'
    
    } | sed -e "s/$USER/user/g" | column -s';' -t

Output:

    New filename without renaming: Angebot-produkt-kunde-massnahmen-6-2024-06-28-1644.pdf             dir=.
    New filename without renaming: Angebot-produkt-mueller-massnahmen-4-2024-06-28-1644.txt.pdf       dir=.
    New filename without renaming: Angebot-produkt-mueller-massnahmen-5-neuer-entwurf                 dir=.
    New filename without renaming: Angebot-produkt-mueller-massnahmen-3-neuer-entwurf.pdf             dir=.
    New filename without renaming: Angebot-5-neuer-entwurf.pdf                                        dir=.
    New filename without renaming: Report-kunde-17-06-2024-23-06-2024-lualatex-2024-06-28-1644.pdf    dir=.
    New filename without renaming: Angebot-produkt-mueller-massnahmen-3-neuer-entwurf.pdf             dir=/cygdrive/c/Users/user
    New filename without renaming: Angebot-5-neuer-entwurf.pdf                                        dir=/tmp/files
    New filename without renaming: Angebot-5-prozent-rabatt-neuer-entwurf.pdf                         dir=/tmp/files
    Will not rename dotfile .configfile
    Will not rename dotfile /tmp/.configfile
    New filename without renaming: Angebot-produkt-kunde-2-2024-06-28-1644.pdf                        dir=.
    New filename without renaming: Prozent.txt                                                        dir=/tmp
    New filename without renaming: Txt.pdf                                                            dir=/tmp
    New filename without renaming: Wtf                                                                dir=/tmp

The script:

    #!/bin/bash
    
    # Clean up and datestamp file names; optionally rename the file
    
    # Thure Dührsen, 2024-06-28
    
    # Environment: Cygwin
    
    # Function to modify the base name of the file
    clean_name() {
        if [ "$#" -ne 1 ]; then
            echo "Usage: clean_name <filename>"
            exit 1
        fi
    
        echo "$1" | sed -E -e 's/(Ä|ä)/AE/g'  \
                        -e 's/(Ö|ö)/OE/g'     \
                        -e 's/(Ü|ü)/UE/g'     \
                        -e 's/ß/SS/g'         \
                        -e 's/%+/-Prozent-/g' \
                        -e 's/[[:space:]]/-/g'     |
            tr -dc 'A-Za-z .0-9_-'                 |
            tr '_' '-'                             |
            tr -s '-'                              |
            sed -E -e 's/(^-|([\.-]*)$)//g'   \
                -e 's/-\././g'                     |
            tr '[:upper:]' '[:lower:]'
    }
    
    # Function to append string before the final dot or at the end if no dot exists
    append_string_to_filename() {
        if [ "$#" -ne 2 ]; then
            echo "Usage: append_string_to_filename <filename> <string>"
            exit 1
        fi
    
        local filename="$(clean_name "$1")"
        local append_string="$(clean_name "$2")"
    
        local extension=""
        local base_name=""
    
        if [[ "$filename" =~ ([^.]+)(\..*)$ ]]; then
            base_name="${BASH_REMATCH[1]}"
            extension="${BASH_REMATCH[2]}"
        else
            base_name="$filename"
        fi
    
        local new_base_name="${base_name%.*}"'_'"${append_string}""${extension}"
        local clean_base_name="$(clean_name "$new_base_name")"
        
        # Clean_base_name might no longer contain alphanumeric characters
        if [[ ! "$clean_base_name" =~ ^[A-Za-z0-9]+([-_A-Za-z.0-9]*[A-Za-z0-9])?$ ]]; then
            clean_base_name='wtf'"$extension"
        fi
    
        echo "${clean_base_name^}"
    }
    
    if [[ "$1" == "--printonly" ]]; then
        printonly=true
        shift
    else
        printonly=false
    fi
    
    if [[ "$#" -lt 2 || "$#" -gt 3 ]]; then
        echo "Usage: $0 [--printonly] <file> string_to_append"
        exit 1
    fi
    
    if [ "$printonly" == "false" ]
    then
        if [ ! -e "$1" ]
        then
            echo "file $1 does not exist"
            exit 2
        fi
    fi
    
    fullpath="$1"
    dir="$(dirname "$fullpath")"
    file_name="$(basename -- "$fullpath")"
    string_to_append="$2"
    
    if [[ "$file_name" =~ ^\. ]]
    then
        echo "Will not rename dotfile $fullpath"
        exit 2
    fi
    
    if [[ "$string_to_append" == "date" ]]; then
        string_to_append="$(date '+%F_%H%M')"
        file_name="$(echo "$file_name" | sed -E -e 's/_[[:digit:]]{4}-[[:digit:]]{2}-[[:digit:]]{2}_[[:digit:]]{4}//')"
    fi
    
    new_file_name="$(append_string_to_filename "$file_name" "$string_to_append")"
    
    if [ "$printonly" == "false" ]
    then
        if ! mv -n -v "$dir"/"$file_name" "$dir"/"$new_file_name"
        then
            echo "Failed to rename file $fullpath"
            exit 2
        fi    
    else
        echo "New filename without renaming: $new_file_name ; dir=$dir"
    fi