diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7046b3d4..f914a16e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,20 +1,25 @@
 ## next
 
+### New features
+
 * Single script can now execute within `POSIX` or `non-POSIX`
   environment and it detects/loads features dynamically.
+* `lib/opkg`: Add support for `OpenWrt` (and alike)
+
+### Fixes and Updates
 
+* `tests/`: Support new `ubuntu`/`debian` systems
 * `lib/yum`, `lib/dpkg`, `lib/apk`: Fix #96, #143
   by adding  `-q` (quiet) option for `Qs`.
-* `tests/`: Support new `ubuntu`/`debian` systems
 * `lib/00_core.sh`: POSIX going well
 * `lib/zz_main.sh`: POSIX going well
-
 * `lib/apk`:
+  * Fix many implementation issues.
   * `Q` prints version information.
   * Fix `--noconfirm` issue (#150)
-  * Fix many implementation issues.
-  * POSIX going well
   * POSIX version (`pacapt-POSIX` works perfectly without `bash`)
+* Remove `{apk,dnf,zypper}_Sw` methods
+  and fix the `--download-only` for them
 
 ## v2.4.4
 
diff --git a/Makefile b/Makefile
index 8d5d0926..d9a277a2 100644
--- a/Makefile
+++ b/Makefile
@@ -9,10 +9,12 @@ default:
 	@echo "  pacapt      : Generate stable script."
 	@echo '  install     : Install stable script into $$BINDIR.'
 	@echo "  clean       : (Experimental) Remove git-ignored files."
-	@echo "  shellcheck  : Syntax and style checking. Use http://shellcheck.net/."
-	@echo "  docker.i    : Launch interactive Docker container which mounts."
-	@echo '                your local 'pacapt.dev' script to $$BINDIR/pacman.'
-	@echo '                Please use DISTRO= to specify Docker image'
+	@echo "  shellcheck  : It's shellcheck."
+	@ecoh "  POSIX       : Use shellcheck with POSIX checks for some scripts."
+	@echo "  docker.i    : Launch interactive Docker container which mounts"
+	@echo '                your local working directory to /src/'
+	@echo '                and create symlink /bin/pacman to the local pacapt.dev'.
+	@echo '                Please use DISTRO= to specify Docker image.'
 	@echo "  tests       : Run all tests. Please read tests/README.md first."
 	@echo "                Use TESTS= to specify a package. Docker is required."
 	@echo "  stats       : Generate table of implemented operations in development branch."
@@ -75,8 +77,8 @@ $(BINDIR)/pacapt: pacapt
 .PHONY: docker.i
 docker.i:
 	@docker run --rm -ti \
-    -v $(PWD)/pacapt.dev:$(BINDIR)/pacman \
-    $(DISTRO) /bin/bash
+    -v $(PWD)/:/src/ \
+    $(DISTRO) /bin/sh -c 'ln -s /src/pacapt.dev /bin/pacman && exec sh'
 
 .PHONY: update_stats
 update_stats:
diff --git a/README.md b/README.md
index 720d7404..9657a85c 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@
 
 `pacapt` is a wrapper for many package managers.
 Simply install package with `pacapt -S htop` or `pacapt install htop`
-on any `Linux`, `BSD`, `Mac OS` machines.
+on any `Linux`, `BSD`, `OpenWrt` or `Mac OS` machines.
 It supports the following package managers:
 
 * `pacman`        by `Arch Linux`, `ArchBang`, `Manjaro`, etc.
@@ -19,6 +19,7 @@ It supports the following package managers:
 * `pkg_tools`     by `OpenBSD`
 * `sun_tools`     by `Solaris(SunOS)`
 * `apk`           by `Alpine Linux`
+* `opkg`          by `OpenWrt`
 * `tazpkg`        by `SliTaz Linux`
 * `swupd`         by `Clear Linux`
 * `tlmgr`         by `TeX Live`
@@ -44,7 +45,7 @@ It supports the following package managers:
 ## Installation
 
 1. This script shouldn't be installed on an Arch-based system;
-2. On `FreeBSD` and `Alpine Linux`, please install `bash` package first.
+2. On `FreeBSD`, please install `bash` package first.
 
 ### Install stable `Bash` script 2.4.x from Github
 
@@ -113,23 +114,24 @@ A long list of options and operations can be found from [`ArchLinux`'s wiki](htt
 ### Implemented operations
 
 ```
-           Q Qc Qe Qi Qk Ql Qm Qo Qp Qs Qu R Rn Rns Rs S Sc Scc Sccc Sg Si Sii Sl Ss Su Suy Sw Sy U
-      apk  ~        *     *     *     *  * *  *   *  * *  *   *    *     *   *  *  *  *   *  *  * *
-     cave  *        *     *     *  *  *  * *  *   *  * *  *   *    x     *         *  *   *     * x
+           Q Qc Qe Qi Qk Ql Qm Qo Qp Qs Qu R Rn Rns Rs S Sc Scc Sccc Sg Si Sii Sl Ss Su Suy Sy U
+      apk  ~        *     *     *     *  * *  *   *  * *  *   *    *     *   *  *  *  *   *  * *
+     cave  *        *     *     *  *  *  * *  *   *  * *  *   *    x     *         *  *   *  * x
     conda  *                               *           *  *              *         *      *
-      dnf  ~  *  *  *     *  *  *  *  *  * *           *  *   *    *  *  *      *  *  *   *  *  * *
-     dpkg  ~        *     *     *  *  *  * *  *   *  ~ *  *   *    *     *   *     *  *   *     * *
- homebrew  ~  *     *     *     *     *  * *         * *  *   *    *     *         *  *   *     *
- macports     *           *     *        * *         ~ *  *   *          *         *  *   *     *
-    pkgng  *        *     *     *  *     * *         * *  *   *          *         *  *   *     *
-pkg_tools  ~        *     *     *  *     * *  *   *  ~ *  *   x          *      *  ~  *   *     x
-  portage  *  *     *     *     *        * *         * *  *   *    *     *         *  *   *     *
-sun_tools  *        *     *     *     *    *                                                      *
-    swupd              *        *     *    *           *                           *  *   *     *
-   tazpkg  *        *     *     *          *           *  *   *                    *  *   *     * *
-    tlmgr           *  *  *                *           *                 *      *  *      *       *
-      yum  *  *     *     *  *  *  *  *  * *         * *  *   *    *     *   *     *  *   *     * *
-   zypper  *  *     *     *  *  *  *  *  * *  *   *  * *  *   *    *     *   *  *  *  *   *  *  * *
+      dnf  ~  *  *  *     *  *  *  *  *  * *           *  *   *    *  *  *      *  *  *   *  * *
+     dpkg  ~        *     *     *  *  *  * *  *   *  ~ *  *   *    *     *   *     *  *   *  * *
+ homebrew  ~  *     *     *     *     *  * *         * *  *   *    *     *         *  *   *  *
+ macports     *           *     *        * *         ~ *  *   *          *         *  *   *  *
+     opkg  *        *     *     *     *  * *           *                 *   *     *  *      * *
+    pkgng  *        *     *     *  *     * *         * *  *   *          *         *  *   *  *
+pkg_tools  ~        *     *     *  *     * *  *   *  ~ *  *   x          *      *  ~  *   *  x
+  portage  *  *     *     *     *        * *         * *  *   *    *     *         *  *   *  *
+sun_tools  *        *     *     *     *    *                                                   *
+    swupd              *        *     *    *           *                           *  *   *  *
+   tazpkg  *        *     *     *          *           *  *   *                    *  *   *  * *
+    tlmgr           *  *  *                *           *                 *      *  *      *    *
+      yum  *  *     *     *  *  *  *  *  * *         * *  *   *    *     *   *     *  *   *  * *
+   zypper  *  *     *     *  *  *  *  *  * *  *   *  * *  *   *    *     *   *  *  *  *   *  * *
 ```
 
 **Notes:**
diff --git a/bin/check.sh b/bin/check.sh
index 95f2f311..3e491ad0 100755
--- a/bin/check.sh
+++ b/bin/check.sh
@@ -61,36 +61,33 @@ _has_shellcheck() {
 }
 
 _check_file() {
-  local _file="${1:-/x/x/x/x/x/x/x/}"
+  local_file="${1:-/x/x/x/x/x/x/x/}"
+  local_shell="${SHELLCHECK_SHELL:-bash}"
 
-  echo >&2 ":: ${FUNCNAME[0]}: $1"
+  echo >&2 ":: ${FUNCNAME[0]} (${local_shell}): $1"
 
-  [[ -f "$_file" ]] \
+  [[ -f "$local_file" ]] \
   || {
-    echo >&2 ":: File not found '$_file'"
+    echo >&2 ":: File not found '$local_file'"
     return 1
   }
 
-  _simple_check "$_file" || return
+  _simple_check "$local_file" || return
 
-  shellcheck -s "${SHELLCHECK_SHELL:-bash}" -f json "$_file" \
-  | _shellcheck_output_format "$_file"
+  shellcheck -s "${local_shell}" -f json "$local_file" \
+  | _shellcheck_output_format "$local_file"
 
   [[ "${PIPESTATUS[0]}" == "0" ]]
 }
 
-_check_POSIX_file() {
-  if ! grep -Eiqe "^# +POSIX.*:.*Ready" -- "$1" ; then
-    >&2 echo ":: $1: POSIX is not required."
-  else
-    _check_file "$1" || return 1
-  fi
-}
-
 _check_POSIX_files() {
   export SHELLCHECK_SHELL="sh"
   while (( $# )); do
-    _check_POSIX_file "$1" || return 1
+    if awk 'NR==1' < "$1" \
+      | grep -Eiqe '^#!/usr/bin/env sh' ;
+    then
+      _check_file "$1" || return 1
+    fi
     shift
   done
 }
diff --git a/bin/compile.sh b/bin/compile.sh
index 5f1087e9..c15ea0ee 100755
--- a/bin/compile.sh
+++ b/bin/compile.sh
@@ -116,7 +116,7 @@ library_files() {
 }
 
 library_POSIX_ready() {
-  grep -Eqie "^# +POSIX.*:.*Ready" -- "$@"
+  grep -Eqie '#!/usr/bin/env sh' -- "$@"
 }
 
 ########################################################################
@@ -129,6 +129,7 @@ for L in $(library_files); do
   || continue
 
   if library_POSIX_ready "$L"; then
+    >&2 echo ":: POSIX library '$L'"
     $GREP -v '^#' "$L"
   else
     $GREP -v '^#' "$L" | awk '{printf("#_!_POSIX_# %s\n", $0)}'
diff --git a/lib/00_core.sh b/lib/00_core.sh
index 854db0e8..ba1cef28 100644
--- a/lib/00_core.sh
+++ b/lib/00_core.sh
@@ -1,6 +1,5 @@
-#!/bin/bash
+#!/usr/bin/env sh
 
-# POSIX  : Ready
 # Purpose: Provide some basic functions
 # Author : Anh K. Huynh
 # License: Fair license (http://www.opensource.org/licenses/fair)
@@ -98,6 +97,7 @@ _PACMAN_detect() {
   _issue2pacman pkg_tools "OpenBSD" && return
   _issue2pacman pkg_tools "Bitrig" && return
   _issue2pacman apk "Alpine Linux" && return
+  _issue2pacman opkg "OpenWrt" && return
 
   [ -z "$_PACMAN" ] || return
 
@@ -121,6 +121,7 @@ _PACMAN_detect() {
   [ -x "/usr/sbin/pkg_add" ] && _PACMAN="pkg_tools" && return
   [ -x "/usr/sbin/pkgadd" ] && _PACMAN="sun_tools" && return
   [ -x "/sbin/apk" ] && _PACMAN="apk" && return
+  [ -x "/bin/opkg" ] && _PACMAN="opkg" && return
   [ -x "/usr/bin/tazpkg" ] && _PACMAN="tazpkg" && return
   [ -x "/usr/bin/swupd" ] && _PACMAN="swupd" && return
 
@@ -141,6 +142,7 @@ _translate_w() {
   case "$_PACMAN" in
   "dpkg")     local_opt="-d";;
   "cave")     local_opt="-f";;
+  "dnf")      local_opt="--downloadonly";;
   "macports") local_opt="fetch";;
   "portage")  local_opt="--fetchonly";;
   "zypper")   local_opt="--download-only";;
@@ -156,6 +158,7 @@ _translate_w() {
     local_ret=1
     ;;
   "apk")      local_opt="fetch";;
+  "opkg")     local_opt="--download-only";;
   *)
     local_opt=""
     local_ret=1
diff --git a/lib/00_external.sh b/lib/00_external.sh
index 02a9f073..d5e71963 100644
--- a/lib/00_external.sh
+++ b/lib/00_external.sh
@@ -1,6 +1,5 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
-# POSIX   : Ready
 # Purpose : Provide some basic settings for external package managers
 # Author  : Ky-Anh Huynh
 # License : MIT
diff --git a/lib/apk.sh b/lib/apk.sh
index 9283341f..3278aa53 100644
--- a/lib/apk.sh
+++ b/lib/apk.sh
@@ -1,6 +1,5 @@
-#!/bin/bash
+#!/usr/bin/env sh
 
-# POSIX  : Ready
 # Purpose: Support next-generation Alpine Linux apk package manager
 # Author : Carl X. Su <bcbcarl@gmail.com>
 #          Cuong Manh Le <cuong.manhle.vn@gmail.com>
@@ -20,6 +19,7 @@ _apk_init() {
 }
 
 # apk_Q may _not_implemented
+# FIXME: Need to support a small list of packages
 apk_Q() {
   case "$_TOPT" in
   "")
@@ -162,11 +162,6 @@ apk_Sy() {
   apk update
 }
 
-apk_Sw() {
-  # shellcheck disable=2086
-  apk fetch $_TOPT -- "$@"
-}
-
 apk_U() {
   # shellcheck disable=2086
   apk add --allow-untrusted $_TOPT -- "$@"
diff --git a/lib/dnf.sh b/lib/dnf.sh
index 6fd28d02..b357a992 100644
--- a/lib/dnf.sh
+++ b/lib/dnf.sh
@@ -62,10 +62,6 @@ dnf_Suy() {
   dnf upgrade "$@"
 }
 
-dnf_Sw() {
-  dnf download "$@"
-}
-
 dnf_Sy() {
   dnf clean expire-cache && dnf check-update
 }
diff --git a/lib/dpkg.sh b/lib/dpkg.sh
index ea027a39..a91f9ca2 100644
--- a/lib/dpkg.sh
+++ b/lib/dpkg.sh
@@ -18,6 +18,7 @@ _dpkg_init() {
 }
 
 # dpkg_Q may _not_implemented
+# FIXME: Need to support a small list of packages
 dpkg_Q() {
   if [[ "$_TOPT" == "q" ]]; then
     dpkg -l \
diff --git a/lib/opkg.sh b/lib/opkg.sh
new file mode 100644
index 00000000..92c69abb
--- /dev/null
+++ b/lib/opkg.sh
@@ -0,0 +1,129 @@
+#!/usr/bin/env sh
+
+# Purpose: Support opkg (e.g. OpenWrt)
+# Author : Ky-Anh Huynh
+# License: Fair license (http://www.opensource.org/licenses/fair)
+# Source : http://github.com/icy/pacapt/
+
+# Copyright (C) 2021 Ky-Anh Huynh
+#
+# Usage of the works is permitted provided that this instrument is
+# retained with the works, so that any entity that uses the works is
+# notified of this instrument.
+#
+# DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY.
+
+_opkg_init() {
+  :
+}
+
+opkg_Sy() {
+  opkg update
+}
+
+opkg_Q() {
+  # shellcheck disable=SC2016
+  case "$_TOPT" in
+  "q")
+    opkg list-installed "$@" | "$AWK" '{print $1}'
+    ;;
+  "")
+    opkg list-installed "$@"
+    ;;
+  *)
+    _not_implemented
+    ;;
+  esac
+}
+
+opkg_Qi() {
+  for  pkg in $(opkg__get_local_pkgs "${@}"); do
+    opkg info "$pkg"
+  done
+}
+
+# Get list of installed-packages from user list.
+opkg__get_local_pkgs() {
+  if [ "$#" -eq 0 ]; then
+    # shellcheck disable=SC2016
+    opkg list-installed | "$AWK" '{print $1}'
+  else
+    # `opkg status` returns empty if package is not installed/removed.
+    # shellcheck disable=SC2016
+    for pkg in "${@}"; do
+      opkg status "$pkg"
+    done \
+    | "$AWK" '/^Package: / {print $NF}'
+  fi
+}
+
+opkg_Ql() {
+  for pkg in $(opkg__get_local_pkgs "${@}"); do
+    # shellcheck disable=SC2016
+    opkg files "$pkg" \
+    | PKG="$pkg" "$AWK" \
+        '{ if (NR>1) {printf("%s %s\n", ENVIRON["PKG"], $0)} }'
+  done
+}
+
+opkg_Qo() {
+  if cmd="$(command -v -- "$@")"; then
+    opkg search "$cmd"
+  else
+    opkg search "$@"
+  fi
+}
+
+opkg_Qs() {
+  if command -v sort >/dev/null; then
+    local_filter="sort -u"
+  else
+    local_filter="cat"
+  fi
+
+  # FIXME: opkg doesn't work with wildcard by default.
+  case "$@" in
+  *\**) local_pattern="$*" ;;
+  *)    local_pattern="*${*}*" ;;
+  esac
+
+  opkg search "$local_pattern" \
+  | ${local_filter} \
+  | _quiet_field1
+}
+
+# FIXME: It's not easy to test this method =))
+opkg_Qu() {
+  opkg list-upgradable
+}
+
+opkg_R() {
+  opkg remove "$@"
+}
+
+opkg_S() {
+  opkg install "$@"
+}
+
+opkg_Si() {
+  # shellcheck disable=2086
+  opkg list $_TOPT "$@"
+}
+
+opkg_Sii() {
+  # shellcheck disable=2086
+  opkg list $_TOPT "$@"
+  opkg whatdepends "$@"
+}
+
+opkg_Ss() {
+  opkg list "$@"
+}
+
+opkg_Su() {
+  opkg upgrade "$@"
+}
+
+opkg_U() {
+  opkg install "$@"
+}
diff --git a/lib/sun_tools.sh b/lib/sun_tools.sh
index ce63e862..09974fbe 100644
--- a/lib/sun_tools.sh
+++ b/lib/sun_tools.sh
@@ -1,6 +1,5 @@
-#!/bin/bash
+#!/usr/bin/env sh
 
-# POSIX  : Ready
 # Purpose: SunOS support
 # Author : Daniel YC Lin <dlin.tw@gmail.com>
 # License: Fair license (http://www.opensource.org/licenses/fair)
diff --git a/lib/yum.sh b/lib/yum.sh
index 71afc0c3..9f0f8dca 100644
--- a/lib/yum.sh
+++ b/lib/yum.sh
@@ -17,6 +17,7 @@ _yum_init() {
   :
 }
 
+# FIXME: Need to support a small list of packages
 yum_Q() {
   if [[ "$_TOPT" == "q" ]]; then
     rpm -qa --qf "%{NAME}\\n"
diff --git a/lib/zypper.sh b/lib/zypper.sh
index 0402a1a8..1d88acef 100644
--- a/lib/zypper.sh
+++ b/lib/zypper.sh
@@ -166,10 +166,6 @@ zypper_S() {
   zypper install $_TOPT "$@"
 }
 
-zypper_Sw() {
-  zypper install --download-only "$@"
-}
-
 zypper_U() {
   zypper install "$@"
 }
diff --git a/lib/zz_main.sh b/lib/zz_main.sh
index b35b6141..544e77df 100644
--- a/lib/zz_main.sh
+++ b/lib/zz_main.sh
@@ -1,6 +1,5 @@
-#!/bin/bash
+#!/usr/bin/env sh
 
-# POSIX  : Ready
 # Purpose: A wrapper for all Unix package managers
 # Author : Anh K. Huynh
 # License: Fair license (http://www.opensource.org/licenses/fair)
@@ -71,6 +70,7 @@ _PACMAN_detect \
 if [ -z "${__PACAPT_FORKED__:-}" ]; then
   case "$_PACMAN" in
   "apk")  ;;
+  "opkg")  ;;
   "sun_tools" ) ;;
   *)
     _die "pacapt($_PACMAN) library is not ready for pure-POSIX features."
diff --git a/tests/opkg.txt b/tests/opkg.txt
new file mode 100644
index 00000000..623dfc7c
--- /dev/null
+++ b/tests/opkg.txt
@@ -0,0 +1,91 @@
+#!/bin/sh
+
+# Purpose : Testing script for okpg support
+# Author  : Ky-Anh Huynh
+# Date    : June 19th, 2021
+# License : MIT
+
+im oofnik/openwrt:snapshot-x86-64
+im oofnik/openwrt:19.07.7-x86-64
+im oofnik/openwrt:19.07.6-x86-64
+im oofnik/openwrt:19.07.5-x86-64
+
+in -Sy
+ou Updated list of available packages in /var/opkg-lists/openwrt_core
+ou Downloading.*downloads.openwrt.org
+ou Signature check passed.
+
+in -Q
+ou busybox - .*
+ou libc - .*
+
+in -Q busybox
+ou busybox
+
+in -Qq
+ou ^busybox$
+
+in -Qi
+ou busybox
+ou libc
+ou Status: install user installed
+
+in -Ql
+ou libc
+ou busybox
+
+in -Ql libc
+ou libc
+
+in -Ql ruby
+ou empty
+
+in -Qo
+ou empty
+
+in -Qo busybox
+ou busybox - .*
+
+in -Qo /bin/sh
+ou busybox - .*
+
+in -Qs sh
+ou base-files - .*
+ou busybox - .*
+
+in -Qqs sh
+ou ^busybox$
+
+in -Sw htop
+in ! htop
+ou Downloading.*htop
+ou htop: not found
+
+in -S htop
+in ! command -v htop
+ou /usr/bin/htop
+
+in -R htop
+in ! htop -h
+ou Removing package htop from root
+ou htop: not found
+
+in -Sw htop
+in -U ./htop*.ipk
+in ! htop -h
+ou Hisham Muhammad
+
+in -Si ruby
+in ! ruby -e true
+ou ruby: not found
+ou Ruby is .* for quick and easy
+
+in -Sii librt
+ou e2fsprogs.*depends on.*
+
+in -Ss
+ou zoneinfo-core
+ou ^zsh
+
+in -Ss perl
+ou Perl is a stable.* programming language.