How can I declare global variables and have a main loop like i can have in C?

I know there is GNU make utility or Cmake(what I use usually). But I am learning dash/posix shell and I want to write a simple script to compile and build my solution. This is what I have so far.

#!/bin/sh

#THIS BUILD SOLUTION SHOULD WORK ANY PROGRAMMING SOLUTION AS LONG AS IT CONFORMS THE SAME FOLDER STRUCTURE
#THIS BUILD SOLUTION WILL ONLY WORK IN ANY POSIX CONFORMING SHELL
#THIS SOLUTION IS IDEAL FOR VERY SMALL PROGRAMS

PROGRAM_NAME="count_vowels"
SOURCE_DIR="src"
files_timestamp="files_timestamp.txt"

compile() {
    for file in $SOURCE_DIR/*.c; do
        files="${files:=$files} $file"
    done
    gcc -ggdb3 -std=c17 $files -Wall -o $1 && echo "An executable $1 has been created" || echo "Failed to complie!!!"
    unset file files
    return 0
}

compile_with_msg() {
    CURRENT_MOD_TIME="$( stat -c %Z $SOURCE_DIR $SOURCE_DIR/headers $SOURCE_DIR/*.c $SOURCE_DIR/headers/*.h )"
    echo "$CURRENT_MOD_TIME" > $files_timestamp
    echo "Timestamps of files attempting to complie: \n$CURRENT_MOD_TIME\n"
    echo "Compiling and linking files..."
    compile $PROGRAM_NAME
    return 0
}

compile_with_msg_changes(){
    echo "Changes in source files detected"
    rm $PROGRAM_NAME $files_timestamp && echo "Deleted existing $PROGRAM_NAME executable"
    compile_with_msg
    return 0
}

build() {       
        [ ! -d $SOURCE_DIR ] && echo "Cannot find the source directory\nPlease open the build scipt in an editor and check if the source directory information is correct" && return 2
        if [ ! -f $PROGRAM_NAME ]; then
            [ -f $files_timestamp ] && rm $files_timestamp && echo "Error: $PROGRAM_NAME does not exist but $files_timestamp from last compilation exists\nRemoved $files_timestamp" && return 1
            compile_with_msg
        else
            [ ! -f $files_timestamp ] && rm "$PROGRAM_NAME" && echo "Error: $files_timestamp not found\nDeleted $PROGRAM_NAME" && return 1
            CURRENT_MOD_TIME="$( stat -c %Z $SOURCE_DIR $SOURCE_DIR/headers $SOURCE_DIR/*.c $SOURCE_DIR/headers/*.h )"
            LAST_MOD_TIME="$( cat $files_timestamp )"
            [ "$CURRENT_MOD_TIME" = "$LAST_MOD_TIME" ] && echo "$PROGRAM_NAME is up to date" || compile_with_msg_changes
            
        fi
        return 0
}

main() {
    err_check=1
    while [ "$err_check" -eq 1 ]; do
        build
        err_check=$?
    done
    return 0
}
main

Problem is I do not know how I can set a global variable and main fucntion in dash shell. LAST_MOD_TIME has to be global for it to work and not get updated outside the scope of the build fucntion which is my main function.

Update: this code worked for me. thanks