How do I rename and overwrite part of file name in Bash at the same time?

Question

I have multiple JS files in folder, for example, foo.js, this_is_foo.js, foo_is_function.js.

I want to append a number which is passed as parameter in front of extension ".js", such as foo.1.js, this_is_foo.1.js, foo_is_function.1.js

I write a script to append a number successfully the first time, but if I run the script twice, it does not overwrite the first number but append right after that.

Actual result: foo.js --> foo.1.js (1st run) --> foo.1.2.js (2nd run).

Expected result: foo.js --> foo.1.js (1st run) --> foo.2.js (2nd run).

This is my script:

#!/bin/sh
param=$1
for file in *.js; do
    ext="${file##*.}";
    filename="${file%.*}";
    mv "$file" "${filename}.${param}.${ext}";
done

How can I do that? I want to write pure bash script, not to use any tools.

Answer 1

Before doing the rename you can check if what is after the last dot in filename (${filename%.*}) is numeric. If so switch that with param instead of appending a new param

Since you are writing that you want to use pure bash I assume that it is ok to change the shebang to #!/bin/bash:

#!/bin/bash
param=$1
for file in *.js; do
    ext="${file##*.}";
    filename="${file%.*}";

    # Check if what is after the last dot in filename is numeric
    # Then assume that it should be switched to param
    if [[ ${filename##*.} =~ ^[0-9]+$ ]]; then
        mv "$file" "${filename%.*}.${param}.${ext}"

    ## Else add param
    else
        mv "$file" "${filename}.${param}.${ext}"
    fi  
done

Testrun

$> touch a.js && find -type f -name *.js && \
 ./test.sh 1 && find -type f -name *.js && \
 ./test.sh 2 && find -type f -name *.js

./a.js
./a.1.js
./a.2.js