fix: avoid echo usage

Echo can interpret operand as an option and checking every variable to
be echoed is troublesome while with printf, if the format specifier is
present before the operand, printing as string can be enforced.

scripts/toc-gen.sh

@@ -8,7 +8,7 @@
 set -eu
 
 usage(){
-  echo "Usage: ${0##*/} <file> [file ...]"
+  printf '%s\n' "Usage: ${0##*/} <file> [file ...]"
   exit 1
 }
 
@@ -21,18 +21,19 @@ esac
 ## update on save.
 if ! vim -e -c 'setf markdown' -c 'if !exists(":GenTocGFM") | cq | endif' -c q
 then
-  echo "Error: Vim Plugin mzlogin/vim-markdown-toc is not installed." >&2
+  err_msg="Error: Vim Plugin mzlogin/vim-markdown-toc isn't installed."
+  printf '%s\n' "${err_msg}" >&2
   exit 1
 fi
 
 
 for f in "${@}"; do
   if ! test -f "${f}"; then
-    echo "Error: Not a regular file: ${f}" >&2
+    printf '%s\n' "Error: Not a regular file: ${f}" >&2
     exit 1
   fi
   if ! grep -q -e "^## Table of Contents$" -- "${f}"; then
-    echo "Could not find table of contents in file: ${f}, skipping" >&2
+    printf '%s\n' "Could not find TOC in file: ${f}, skipping" >&2
     continue
   fi
   ## This is fragile, the table of contents should have at least one block
@@ -40,5 +41,5 @@ for f in "${@}"; do
   ## the rest of the file.
   vim -c 'norm zRgg' -c '/^## Table of Contents$' -c 'norm jd}k' \
       -c ':GenTocGFM' -c 'norm ddgg' -c wq -- "${f}"
-  echo "Updated TOC in file: ${f}"
+  printf '%s\n' "Updated TOC in file: ${f}"
 done