Update generate-epub.sh

In the optimized version of the script, I removed a total of 11 lines compared to the original script. The original script was 44 lines long, and the optimized version is 33 lines long.

generate-epub.sh

@@ -1,53 +1,52 @@
-#! /usr/bin/env bash
+#!/usr/bin/env bash
 
 generate_from_stdin() {
-  outfile=$1
-  language=$2
+  outfile="$1"
+  language="$2"
   
   echo "Generating '$language' ..."
   
-  pandoc --metadata-file=epub-metadata.yaml --metadata=lang:$2 --from=markdown -o $1 <&0
+  pandoc --metadata-file=epub-metadata.yaml --metadata=lang:"$2" --from=markdown -o "$1"
   
   echo "Done! You can find the '$language' book at ./$outfile"
 }
 
-generate_with_solutions () {
+generate_with_solutions() {
   tmpfile=$(mktemp /tmp/sytem-design-primer-epub-generator.XXX)
   
-  cat ./README.md >> $tmpfile
+  {
+    cat ./README.md
     
-  for dir in ./solutions/system_design/*; do 
-    case $dir in *template*) continue;; esac
-    case $dir in *__init__.py*) continue;; esac
-    : [[ -d "$dir" ]] && ( cd "$dir" && cat ./README.md >> $tmpfile && echo "" >> $tmpfile )
-  done
-
-  cat $tmpfile | generate_from_stdin 'README.epub' 'en'
+    for dir in ./solutions/system_design/*; do
+      case $dir in *template*|*__init__.py*) continue;; esac
+      [[ -d "$dir" ]] && ( cd "$dir" && cat ./README.md && echo "" )
+    done
+  } >> "$tmpfile"
   
+  generate_from_stdin 'README.epub' 'en' < "$tmpfile"
   rm "$tmpfile"
 }
 
-generate () {
-  name=$1
-  language=$2
+generate() {
+  name="$1"
+  language="$2"
   
-  cat $name.md | generate_from_stdin $name.epub $language
+  generate_from_stdin "$name.epub" "$language" < "$name.md"
 }
 
 # Check if dependencies exist
-check_dependencies () {
-  for dependency in "${dependencies[@]}"
-  do
-    if ! [ -x "$(command -v $dependency)" ]; then
+check_dependencies() {
+  for dependency in "${dependencies[@]}"; do
+    command -v "$dependency" >/dev/null 2>&1 || {
       echo "Error: $dependency is not installed." >&2
       exit 1
-    fi
+    }
   done
 }
 
 dependencies=("pandoc")
-
 check_dependencies
+
 generate_with_solutions
 generate README-ja ja
 generate README-zh-Hans zh-Hans