Add quoting (based on ShellCheck)

99bottles.sh

     #
     local numeral=$1    # number in word form
     case $numeral in
-        *nine)      echo ${numeral%nine}eight;;
-        *eight)     echo ${numeral%eight}seven;;
-        *seven)     echo ${numeral%seven}six;;
-        *six)       echo ${numeral%six}five;;
-        *five)      echo ${numeral%five}four;;
-        *four)      echo ${numeral%four}three;;
-        *three)     echo ${numeral%three}two;;
-        *two)       echo ${numeral%two}one;;
-        one)        echo zero;;
-        *-one)      echo ${numeral%-one};;
-        *one)       echo ${numeral%one};;
-        ten)        echo nine;;
-        eleven)     echo ten;;
-        twelve)     echo eleven;;
-        *teen)      teenpred $numeral;;
-        *ty)        tenspred $numeral;;
+        *nine)      echo "${numeral%nine}eight" ;;
+        *eight)     echo "${numeral%eight}seven" ;;
+        *seven)     echo "${numeral%seven}six" ;;
+        *six)       echo "${numeral%six}five" ;;
+        *five)      echo "${numeral%five}four" ;;
+        *four)      echo "${numeral%four}three" ;;
+        *three)     echo "${numeral%three}two" ;;
+        *two)       echo "${numeral%two}one" ;;
+        one)        echo "zero" ;;
+        *-one)      echo "${numeral%-one}" ;;
+        *one)       echo "${numeral%one}" ;;
+        ten)        echo "nine" ;;
+        eleven)     echo "ten" ;;
+        twelve)     echo "eleven" ;;
+        *teen)      teenpred "$numeral" ;;
+        *ty)        tenspred "$numeral" ;;
         zero)       echo "";  #to terminate
     esac
 }
     local numeral=$1    # number in word form
     case $numeral in
         thirteen)   echo twelve;;
-        *)          echo $(crunchprefix $(pred $(uncrunchprefix ${numeral%teen})))teen;;
+        *)          echo "$(crunchprefix "$(pred "$(uncrunchprefix "${numeral%teen}")")")teen" ;;
     esac
 }
 
     local numeral=$1    # number in word form
     case $numeral in
         twenty)     echo nineteen;;
-        *) echo $(crunchprefix --tens $(pred $(uncrunchprefix ${numeral%ty})))ty-nine;;
+        *) echo "$(crunchprefix --tens "$(pred "$(uncrunchprefix "${numeral%ty}")")")ty-nine";;
     esac
 }
 
     #
     local numeral       # number in word form
     local tensop        # 'true' if --tens option is active
-    [ $1 = --tens ] && { tensop=true; shift; }
+    [ "$1" = --tens ] && { tensop=true; shift; }
     numeral=$1
     case $numeral in
-        two)    [ -n "$tensop" ] && echo twen || echo $numeral;;
+        two)    [ -n "$tensop" ] && echo twen || echo "$numeral";;
         three)  echo thir;;
-        four)   [ -n "$tensop" ] && echo 'for' || echo $numeral;;
+        four)   [ -n "$tensop" ] && echo 'for' || echo "$numeral";;
         five)   echo fif;;
-        eight)  [ -n "$tensop" ] && echo eigh || echo $numeral;;
-        *)      echo $numeral;;
+        eight)  [ -n "$tensop" ] && echo eigh || echo "$numeral";;
+        *)      echo "$numeral" ;;
     esac
 }
 
         'for')  echo four;;
         fif)    echo five;;
         eigh)   echo eight;;
-        *)      echo $prefix;;
+        *)      echo "$prefix";;
     esac
 }
 
         case "$line" in
             *"one of those bottles"*)   line="$(
                                             [ $oneBottle = true ] \
-                                             && echo ${line/one of those bottles/that lone bottle} \
-                                             || echo $line
+                                             && echo "${line/one of those bottles/that lone bottle}" \
+                                             || echo "$line"
                                         )"
                                         ;;
             *"one down"*)               line="$(
                                             [ $oneBottle = true ] \
-                                             && echo ${line/one down/it down} \
-                                             || echo $line
+                                             && echo "${line/one down/it down}" \
+                                             || echo "$line"
                                         )"
                                         ;;
             *bottles*)                  oneBottle=false;;
         esac
         #Some say the twenties should have no hyphen
         line="${line/twenty-/twenty }"
-        echo $line
+        echo "$line"
     done
 }
 
     #
     local line      # every line
     while read line; do
-        echo -n ${line:0:1} | tr '[:lower:]' '[:upper:]'
-        echo ${line#?}
+        echo -n "${line:0:1}" | tr '[:lower:]' '[:upper:]'
+        echo "${line#?}"
     done
 }
 
     local line      # every line
     while read line; do
         case "${line}" in
-            [Ii]f*)     echo ${line},;;
+            [Ii]f*)     echo "${line},";;
             '')         echo;;
-            *)          echo ${line}.;;
+            *)          echo "${line}.";;
         esac
     done
 }
     # Write one verse with number $1
     #
     local nb=$1     # numeral
-    echo $nb bottles of beer on the wall
-    echo $nb bottles of beer
-    if [ $nb = zero ]; then
-        echo Go to the store and buy some more
+    echo "$nb bottles of beer on the wall"
+    echo "$nb bottles of beer"
+    if [ "$nb" = zero ]; then
+        echo "Go to the store and buy some more"
         nb=ninety-nine
     else
-        echo $breakLine
-        nb=$(pred $nb)
+        echo "$breakLine"
+        nb=$(pred "$nb")
     fi
-    echo $nb bottles of beer on the wall
+    echo "$nb bottles of beer on the wall"
 }
 
 poeticize() {
         case "$rest" in
             *beer*)
                 first=${first/zero/no}
-                syl=$(syllables ${first% *})
+                syl=$(syllables "${first% *}")
                 case $syl in  #improve meter
-                    1|2)    echo $first 'more' $rest;;
-                    *)      echo $first $rest;;
+                    1|2)    echo "$first more $rest";;
+                    *)      echo "$first $rest" ;;
                 esac
                 ;;
-            *)  echo $first $rest
+            *)  echo "$first $rest"
         esac
     done
 }
     case $word in
         *seven*)    let $((n = n+1));;
     esac
-    echo $n
+    echo "$n"
 }
 
 main() {
     wastefulBreakLine="If one of those bottles should happen to fall"
     breakLine=$(
         [ "$1" = --careless ] \
-         && echo $wastefulBreakLine \
-         || echo $standardBreakLine
+         && echo "$wastefulBreakLine" \
+         || echo "$standardBreakLine"
     )
     nb=ninety-nine
     while [ -n "$nb" ]; do
-        verse $nb
+        verse "$nb"
         echo
-        nb=$(pred $nb)
+        nb=$(pred "$nb")
     done | poeticize | grammar | punctuate | capitalize
 }