tree-wide: code style and print message cleanup

- Continuation of #179, particularly these topics from there:
  - #179 (review)
  - #179 (comment)

- Contains things that were proposed there, but that it was decided to separate out into another PR for organization and easier review

- Replaces backtick command substitution syntax with dollar symbol and parentheses command substitution syntax in shell scripts

- Places double quote symbols around all string (non-numerical) variable references that aren't possibly-empty references in the argument lists of command invocations
  - (placing double quote symbols around a possibly-empty variable in a command invocation is a case where the double quote symbols would cause an error to happen)

- Places double quote symbols around all instances of `@TERMUX_PREFIX@` and other replaced strings in `.sh.in` files

- Remove unused function `hostname()` from `pkg`

- Remove unused variable `PREFIX` from `termux-restore`

- Unify and clarify instances of `login`-related error message in `chsh`

Author: robertkirkman

scripts/chsh.in

@@ -11,11 +11,7 @@ show_usage() {
 	echo "usage: chsh [-s shell]"
 	echo "Change the login shell."
 	echo
-	if [ "${1:-}" = "login" ]; then
-		echo "The shell value must be one of following and cannot be 'login':"
-	else
-		echo "The shell value must be one of following:"
-	fi
+	echo "The shell value must be one of following and cannot be 'login':"
 	echo " - Empty value to restore default shell."
 	echo " - Absolute path to shell starting with a '/'."
 	echo " - Relative path to shell not starting with a '/' relative to '@TERMUX_PREFIX@/bin'."
@@ -28,10 +24,6 @@ set_shell() {
 		exit 0
 	fi
 
-	if [ "$1" = "login" ]; then
-		show_usage "login"
-		exit 1
-	fi
 	mkdir -p "$HOME/.termux"
 
 	unset NEW_SHELL
@@ -44,18 +36,22 @@ set_shell() {
 
 	if ! is_executable_file "$SHELL_TARGET"; then
 		echo "The shell file '$SHELL_TARGET' is not an executable file." 1>&2
+		echo
+		show_usage
 		exit 1
 	fi
 
 	if [ "$SHELL_TARGET" -ef "@TERMUX_PREFIX@/bin/login" ]; then
 		echo "The shell file '$SHELL_TARGET' must not point to the '@TERMUX_PREFIX@/bin/login' script." 1>&2
+		echo
+		show_usage
 		exit 1
 	fi
 
 	ln -sf "$SHELL_TARGET" "$HOME/.termux/shell"
 }
 
-O=`getopt -l help -- hs: "$@"`
+O="$(getopt -l help -- hs: "$@")"
 eval set -- "$O"
 while true; do
 	case "$1" in

scripts/dalvikvm.in

@@ -1,8 +1,8 @@
 #!/bin/sh
 
 # There needs to be a folder at $ANDROID_DATA/dalvik-cache
-export ANDROID_DATA=@TERMUX_PREFIX@/var/android/
-mkdir -p $ANDROID_DATA/dalvik-cache
+export ANDROID_DATA="@TERMUX_PREFIX@/var/android/"
+mkdir -p "$ANDROID_DATA/dalvik-cache"
 
 if [ -x /apex/com.android.art/bin/dalvikvm ]; then
 	DALVIKVM="/apex/com.android.art/bin/dalvikvm"
@@ -11,4 +11,4 @@ else
 fi
 
 unset LD_LIBRARY_PATH LD_PRELOAD
-exec "$DALVIKVM" -Xusejit:true -Xnoimage-dex2oat -Djava.io.tmpdir=@TERMUX_PREFIX@/tmp "$@"
+exec "$DALVIKVM" -Xusejit:true -Xnoimage-dex2oat -Djava.io.tmpdir="@TERMUX_PREFIX@/tmp" "$@"

scripts/login.in

@@ -20,17 +20,17 @@ if tty >/dev/null 2>&1 && [ $# = 0 ] && [ ! -f ~/.hushlogin ] && [ -z "$TERMUX_H
 		[ ! -x ~/.termux/motd.sh ] && chmod u+x ~/.termux/motd.sh
 		~/.termux/motd.sh
 	# Default to termux-tools package provided static motd file if it exists
-	elif [ -f @TERMUX_PREFIX@/etc/motd ]; then
-		cat @TERMUX_PREFIX@/etc/motd
+	elif [ -f "@TERMUX_PREFIX@/etc/motd" ]; then
+		cat "@TERMUX_PREFIX@/etc/motd"
 	fi
 else
 	# This variable shouldn't be kept set.
 	unset TERMUX_HUSHLOGIN
 fi
 
 # TERMUX_VERSION env variable has been exported since v0.107 and PATH was being set to following value in <0.104. Last playstore version was v0.101.
-if tty >/dev/null 2>&1 && [ $# = 0 ] && [ -f @TERMUX_PREFIX@/etc/motd-playstore ] && [ -z "$TERMUX_VERSION" ] && [ "$PATH" = "@TERMUX_PREFIX@/bin:@TERMUX_PREFIX@/bin/applets" ]; then
-	printf '\033[0;31m'; cat @TERMUX_PREFIX@/etc/motd-playstore; printf '\033[0m'
+if tty >/dev/null 2>&1 && [ $# = 0 ] && [ -f "@TERMUX_PREFIX@/etc/motd-playstore" ] && [ -z "$TERMUX_VERSION" ] && [ "$PATH" = "@TERMUX_PREFIX@/bin:@TERMUX_PREFIX@/bin/applets" ]; then
+	printf '\033[0;31m'; cat "@TERMUX_PREFIX@/etc/motd-playstore"; printf '\033[0m'
 fi
 
 unset SHELL
@@ -74,8 +74,8 @@ elif [ -f "@TERMUX_PREFIX@/lib/libtermux-exec.so" ]; then
 	$SHELL -c "coreutils --coreutils-prog=true" > /dev/null 2>&1 || unset LD_PRELOAD
 fi
 
-if [ -f @TERMUX_PREFIX@/etc/termux-login.sh ]; then
-	. @TERMUX_PREFIX@/etc/termux-login.sh
+if [ -f "@TERMUX_PREFIX@/etc/termux-login.sh" ]; then
+	. "@TERMUX_PREFIX@/etc/termux-login.sh"
 fi
 
 if [ -n "$TERM" ]; then

scripts/pkg.in

@@ -21,7 +21,7 @@ show_help() {
 	elif [ "$TERMUX_APP_PACKAGE_MANAGER" = "pacman" ]; then
 		cache_dir="@TERMUX_PREFIX@/var/cache/pacman/pkg"
 	fi
-	cache_size=$(du -sh "$cache_dir" 2>/dev/null | cut -f1)
+	cache_size="$(du -sh "$cache_dir" 2>/dev/null | cut -f1)"
 
 	echo 'Usage: pkg [--check-mirror] command [arguments]'
 	echo
@@ -96,10 +96,6 @@ check_command() {
 	fi
 }
 
-hostname() {
-	echo "$1" | awk -F'[/:]' '{print $4}'
-}
-
 last_modified() {
 	local mtime
 	local now

scripts/su.in

@@ -4,10 +4,10 @@ unset LD_LIBRARY_PATH LD_PRELOAD
 
 for p in /debug_ramdisk/su /sbin/su /system/sbin/su /system/bin/su /system/xbin/su /su/bin/su /magisk/.core/bin/su
 do
-	if [ -x $p ]; then
+	if [ -x "$p" ]; then
 		# The su tool may require programs in PATH:
 		PATH=/debug_ramdisk:/sbin:/sbin/su:/su/bin:/su/xbin:/system/bin:/system/xbin \
-			exec $p "$@"
+			exec "$p" "$@"
 	fi
 done
 

scripts/termux-backup.in

@@ -31,7 +31,7 @@
 
 set -e -u
 
-export PREFIX=@TERMUX_PREFIX@
+export PREFIX="@TERMUX_PREFIX@"
 
 msg() {
 	echo "$*" >&2

scripts/termux-change-repo.in

@@ -30,7 +30,7 @@ select_repository_group() {
     MIRRORS+=("Mirrors in Oceania" "All in Oceania")
     MIRRORS+=("Mirrors in Russia" "All in Russia")
 
-    local TEMPFILE="$(mktemp @TERMUX_PREFIX@/tmp/mirror.XXXXXX)"
+    local TEMPFILE="$(mktemp "@TERMUX_PREFIX@/tmp/mirror.XXXXXX")"
     dialog \
         --title "termux-change-repo" --clear \
         --menu "Which group of mirrors do you want to use?" 0 0 0 \
@@ -55,31 +55,31 @@ select_repository_group() {
 
     if [ "$mirror_group" == "Mirrors in Asia" ]; then
         echo "[*] Mirrors in Asia (excl. Chinese Mainland and Russia) selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/asia
+        unlink_and_link "${MIRROR_BASE_DIR}/asia"
 
     elif [ "$mirror_group" == "Mirrors in Chinese Mainland" ]; then
         echo "[*] Mirrors in Chinese Mainland selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/chinese_mainland
+        unlink_and_link "${MIRROR_BASE_DIR}/chinese_mainland"
 
     elif [ "$mirror_group" == "Mirrors in Europe" ]; then
         echo "[*] Mirrors in Europe selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/europe
+        unlink_and_link "${MIRROR_BASE_DIR}/europe"
 
     elif [ "$mirror_group" == "Mirrors in North America" ]; then
         echo "[*] Mirrors in North America selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/north_america
+        unlink_and_link "${MIRROR_BASE_DIR}/north_america"
 
     elif [ "$mirror_group" == "Mirrors in Oceania" ]; then
         echo "[*] Mirrors in Oceania selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/oceania
+        unlink_and_link "${MIRROR_BASE_DIR}/oceania"
 
     elif [ "$mirror_group" == "Mirrors in Russia" ]; then
         echo "[*] Mirrors in Russia selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/russia
+        unlink_and_link "${MIRROR_BASE_DIR}/russia"
 
     elif [ "$mirror_group" == "All mirrors" ]; then
         echo "[*] All mirrors selected"
-        unlink_and_link ${MIRROR_BASE_DIR}/all
+        unlink_and_link "${MIRROR_BASE_DIR}/all"
 
     else
         echo "[!] Error: unknown mirror group: '$1'. Exiting"
@@ -102,7 +102,7 @@ get_mirror_description() {
 }
 
 select_individual_mirror() {
-    mirrors=($(find ${MIRROR_BASE_DIR}/{asia,chinese_mainland,europe,north_america,oceania,russia}/ -type f ! -name "*\.dpkg-old" ! -name "*\.dpkg-new" ! -name "*~"))
+    mirrors=($(find "${MIRROR_BASE_DIR}"/{asia,chinese_mainland,europe,north_america,oceania,russia}/ -type f ! -name "*\.dpkg-old" ! -name "*\.dpkg-new" ! -name "*~"))
 
     # Choose default mirror per default
     MIRRORS=("$(get_mirror_url "${MIRROR_BASE_DIR}/default")" "$(get_mirror_description "${MIRROR_BASE_DIR}/default")")
@@ -114,7 +114,7 @@ select_individual_mirror() {
         MIRRORS+=("$mirror_url" "$(get_mirror_description "$mirror")")
     done
 
-    local TEMPFILE="$(mktemp @TERMUX_PREFIX@/tmp/mirror.XXXXXX)"
+    local TEMPFILE="$(mktemp "@TERMUX_PREFIX@/tmp/mirror.XXXXXX")"
     dialog \
         --title "termux-change-repo" --clear \
         --menu "Which mirror do you want to use?" 0 0 0 \
@@ -137,7 +137,7 @@ select_individual_mirror() {
     mirror="$(cat "$TEMPFILE")"
 
     echo "[*] Mirror $(get_mirror_url "$mirror") selected"
-    unlink_and_link "$(find ${MIRROR_BASE_DIR} -name $mirror)"
+    unlink_and_link "$(find "${MIRROR_BASE_DIR}" -name "$mirror")"
     rm "$TEMPFILE"
 }
 
@@ -160,12 +160,12 @@ fi
 if [ "$TERMUX_APP_PACKAGE_MANAGER" = "pacman" ]; then
     read -p "Warning: termux-change-repo can only change mirrors for apt, is that what you intended? [y|n] " -n 1 -r
     echo
-    [[ ${REPLY} =~ ^[Nn]$ ]] && exit
+    [[ "${REPLY}" =~ ^[Nn]$ ]] && exit
 fi
 
 mkdir -p "@TERMUX_PREFIX@/tmp" || exit $?
 
-TEMPFILE="$(mktemp @TERMUX_PREFIX@/tmp/termux-change-repo.XXXXXX)"
+TEMPFILE="$(mktemp "@TERMUX_PREFIX@/tmp/termux-change-repo.XXXXXX")"
 
 MODES=()
 MODES+=("Mirror group" "Rotate between several mirrors (recommended)")
@@ -180,7 +180,7 @@ clear
 
 case $retval in
     0)
-        case "$(cat $TEMPFILE)" in
+        case "$(cat "$TEMPFILE")" in
         "Mirror group")
             select_repository_group
             ;;

scripts/termux-open.in

@@ -12,11 +12,11 @@ show_usage () {
     exit 0
 }
 
-TEMP=`getopt \
-     -n $SCRIPTNAME \
-     -o h \
-     --long send,view,chooser,content-type:,help\
-     -- "$@"`
+TEMP="$(getopt \
+    -n "$SCRIPTNAME" \
+    -o h \
+    --long send,view,chooser,content-type:,help \
+    -- "$@")"
 eval set -- "$TEMP"
 
 ACTION=android.intent.action.VIEW

scripts/termux-restore.in

@@ -36,8 +36,6 @@
 
 set -e -u
 
-export PREFIX=@TERMUX_PREFIX@
-
 msg() {
 	echo "$*" >&2
 }
@@ -80,7 +78,7 @@ fi
 
 case "$1" in
 	-\?|-h|--help|--usage) show_usage; exit 0;;
-	*) BACKUP_FILE_PATH=$1;;
+	*) BACKUP_FILE_PATH="$1";;
 esac
 
 if [ "$BACKUP_FILE_PATH" != "-" ] && [ ! -e "$BACKUP_FILE_PATH" ]; then