diff --git a/COPYING b/COPYING
index 623b6258..d159169d 100644
--- a/COPYING
+++ b/COPYING
@@ -1,12 +1,12 @@
-		    GNU GENERAL PUBLIC LICENSE
-		       Version 2, June 1991
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 2, June 1991
 
- Copyright (C) 1989, 1991 Free Software Foundation, Inc.
-     51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  Everyone is permitted to copy and distribute verbatim copies
  of this license document, but changing it is not allowed.
 
-			    Preamble
+                            Preamble
 
   The licenses for most software are designed to take away your
 freedom to share and change it.  By contrast, the GNU General Public
@@ -15,7 +15,7 @@ software--to make sure the software is free for all its users.  This
 General Public License applies to most of the Free Software
 Foundation's software and to any other program whose authors commit to
 using it.  (Some other Free Software Foundation software is covered by
-the GNU Library General Public License instead.)  You can apply it to
+the GNU Lesser General Public License instead.)  You can apply it to
 your programs, too.
 
   When we speak of free software, we are referring to freedom, not
@@ -55,8 +55,8 @@ patent must be licensed for everyone's free use or not licensed at all.
 
   The precise terms and conditions for copying, distribution and
 modification follow.
-
-		    GNU GENERAL PUBLIC LICENSE
+
+                    GNU GENERAL PUBLIC LICENSE
    TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
 
   0. This License applies to any program or other work which contains
@@ -110,7 +110,7 @@ above, provided that you also meet all of these conditions:
     License.  (Exception: if the Program itself is interactive but
     does not normally print such an announcement, your work based on
     the Program is not required to print an announcement.)
-
+
 These requirements apply to the modified work as a whole.  If
 identifiable sections of that work are not derived from the Program,
 and can be reasonably considered independent and separate works in
@@ -168,7 +168,7 @@ access to copy from a designated place, then offering equivalent
 access to copy the source code from the same place counts as
 distribution of the source code, even though third parties are not
 compelled to copy the source along with the object code.
-
+
   4. You may not copy, modify, sublicense, or distribute the Program
 except as expressly provided under this License.  Any attempt
 otherwise to copy, modify, sublicense or distribute the Program is
@@ -225,7 +225,7 @@ impose that choice.
 
 This section is intended to make thoroughly clear what is believed to
 be a consequence of the rest of this License.
-
+
   8. If the distribution and/or use of the Program is restricted in
 certain countries either by patents or by copyrighted interfaces, the
 original copyright holder who places the Program under this License
@@ -255,7 +255,7 @@ make exceptions for this.  Our decision will be guided by the two goals
 of preserving the free status of all derivatives of our free software and
 of promoting the sharing and reuse of software generally.
 
-			    NO WARRANTY
+                            NO WARRANTY
 
   11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
 FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
@@ -277,9 +277,9 @@ YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
 PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGES.
 
-		     END OF TERMS AND CONDITIONS
-
-	    How to Apply These Terms to Your New Programs
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
 
   If you develop a new program, and you want it to be of the greatest
 possible use to the public, the best way to achieve this is to make it
@@ -303,17 +303,16 @@ the "copyright" line and a pointer to where the full notice is found.
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU General Public License for more details.
 
-    You should have received a copy of the GNU General Public License
-    along with this program; if not, write to the Free Software
-    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
-
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
 Also add information on how to contact you by electronic and paper mail.
 
 If the program is interactive, make it output a short notice like this
 when it starts in an interactive mode:
 
-    Gnomovision version 69, Copyright (C) year  name of author
+    Gnomovision version 69, Copyright (C) year name of author
     Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type `show c' for details.
@@ -336,5 +335,5 @@ necessary.  Here is a sample; alter the names:
 This General Public License does not permit incorporating your program into
 proprietary programs.  If your program is a subroutine library, you may
 consider it more useful to permit linking proprietary applications with the
-library.  If this is what you want to do, use the GNU Library General
+library.  If this is what you want to do, use the GNU Lesser General
 Public License instead of this License.
diff --git a/README.rst b/README.rst
index d458d67e..75559047 100644
--- a/README.rst
+++ b/README.rst
@@ -421,7 +421,7 @@ multi-file mode
 
 Before download starts, a complete directory structure is created if
 needed. By default, aria2 opens at most 100 files mentioned in
-.torrent file, and directly writes to and reads from these files. 
+.torrent file, and directly writes to and reads from these files.
 The number of files to open simultaneously can be controlled by
 ``--bt-max-open-files`` option.
 
diff --git a/android-release b/android-release
index cbc46bfb..9ea99e9a 100755
--- a/android-release
+++ b/android-release
@@ -32,7 +32,7 @@
 # files in the program, then also delete it here.
 
 # cd to the directory where this script exists.
-cd $(dirname $0)
+cd "$(dirname $0)"
 . ./script-helper
 
 get_version
@@ -43,8 +43,8 @@ if [ -z "$VERSION" ]; then
 fi
 
 DIST_DIR=aria2-$VERSION-aarch64-linux-android-build1
-mkdir $DIST_DIR
+mkdir "$DIST_DIR"
 cp AUTHORS COPYING ChangeLog LICENSE.OpenSSL NEWS README.html README.android \
-    android-out/aria2c $DIST_DIR
-zip -9 -r $DIST_DIR.zip $DIST_DIR
-rm -rf $DIST_DIR
+    android-out/aria2c "$DIST_DIR"
+zip -9 -r "$DIST_DIR.zip" "$DIST_DIR"
+rm -rf "$DIST_DIR"
diff --git a/build_test.sh b/build_test.sh
index e0c090da..63fded82 100755
--- a/build_test.sh
+++ b/build_test.sh
@@ -6,27 +6,27 @@ BUILDDIR=/tmp/aria2buildtest
 
 if [ ! -d "$BUILDDIR" ]; then
     mkdir "$BUILDDIR" \
-	|| { echo "Failed to create directory $BUILDDIR" && exit -1; }
+	|| { echo "Failed to create directory $BUILDDIR" && exit 1; }
 fi
 
 echo -n "Starting build test "
-echo `date`
+echo "$(date)"
 
 # build CONFIGURE_OPTS BIN_SUFFIX DESC
 build()
 {
     echo -n "new build() started at "
-    echo `date`
+    echo "$(date)"
     echo "*** configure opts=$1"
     BIN_NAME="aria2c_$2"
     if [ -f "$BUILDDIR/$BIN_NAME" ]; then
 	echo "$BIN_NAME exists, skipping"
 	return
     fi
-    ./configure $1 2>&1 | tee "$BUILDDIR/configure_$2.log"\
+    ./configure $1 2>&1 | tee "$BUILDDIR/configure_$2.log" \
 	&& cp config.log "$BUILDDIR/config.log_$2" \
 	&& LANG=C make clean \
-	&& LANG=C make -j2 check 2>&1 |tee "$BUILDDIR/aria2c_$2.log" \
+	&& LANG=C make -j2 check 2>&1 | tee "$BUILDDIR/aria2c_$2.log" \
 	&& cp src/aria2c "$BUILDDIR/aria2c_$2"
 
     if [ -f "test/aria2c.log" ]; then
@@ -36,8 +36,8 @@ build()
 
 clear()
 {
-    for file in `ls $BUILDDIR`; do
-	rm -f "$BUILDDIR/$file";
+    for file in $(ls "$BUILDDIR"); do
+	rm -f "$BUILDDIR/$file"
     done
 }
 
diff --git a/import-po b/import-po
index dea98de2..cca7b57e 100755
--- a/import-po
+++ b/import-po
@@ -33,7 +33,7 @@ echo -n "en@quot en@boldquot" > "$PO_DIR"/LINGUAS
 for file in "$WORK_DIR"/*.po; do
     # First remove useless '\r' in messages
     sed -i -e 's/\\r//' "$file"
-    bn=`basename "$file"`
+    bn=$(basename "$file")
     bn=${bn#aria2-}
     dst="$PO_DIR"/"$bn"
     # copy file to po directory
diff --git a/makerelease-osx.mk b/makerelease-osx.mk
index 28548a73..2c23abb1 100644
--- a/makerelease-osx.mk
+++ b/makerelease-osx.mk
@@ -238,7 +238,7 @@ endef
 export ARIA2_DISTXML
 
 
-# Detect numer of CPUs to be used with make -j
+# Detect number of CPUs to be used with make -j
 CPUS = $(shell sysctl hw.ncpu | cut -d" " -f2)
 
 
diff --git a/mingw-release b/mingw-release
index 61a07899..722a7fe5 100755
--- a/mingw-release
+++ b/mingw-release
@@ -36,7 +36,7 @@ test -z "$BUILD_VER" && BUILD_VER=1
 test -z "$LABEL" && LABEL=$HOST
 
 # cd to the directory where this script exists.
-cd $(dirname $0)
+cd "$(dirname "$0")"
 . ./script-helper
 
 get_version
@@ -48,8 +48,8 @@ fi
 
 DIST_DIR=aria2-${VERSION}-${LABEL}-build${BUILD_VER}
 
-mkdir $DIST_DIR
+mkdir "$DIST_DIR"
 cp AUTHORS COPYING ChangeLog LICENSE.OpenSSL NEWS README.html README.mingw \
-    mingw-out/aria2c.exe $DIST_DIR
-zip -9 -r $DIST_DIR.zip $DIST_DIR
-rm -rf $DIST_DIR
+    mingw-out/aria2c.exe "$DIST_DIR"
+zip -9 -r "$DIST_DIR.zip" "$DIST_DIR"
+rm -rf "$DIST_DIR"
diff --git a/script-helper b/script-helper
index 30460b66..4f8e9c30 100644
--- a/script-helper
+++ b/script-helper
@@ -29,7 +29,7 @@
 # version.  If you delete this exception statement from all source
 # files in the program, then also delete it here.
 
-get_version () {
-  VERSION=`grep AC_INIT configure.ac | sed '/AC_INIT/ s/AC_INIT(\[aria2\],\[\([^]]\+\)\],.*/\1/'`
+get_version() {
+  VERSION=$(grep AC_INIT configure.ac | sed '/AC_INIT/ s/AC_INIT(\[aria2\],\[\([^]]\+\)\],.*/\1/')
   echo "Version: $VERSION"
 }
diff --git a/src/AppleTLSContext.cc b/src/AppleTLSContext.cc
index 82bfe84c..ff749f8c 100644
--- a/src/AppleTLSContext.cc
+++ b/src/AppleTLSContext.cc
@@ -131,7 +131,7 @@ struct hash_finder {
 
 std::string errToString(OSStatus err)
 {
-  std::string rv = "Unkown error";
+  std::string rv = "Unknown error";
   CFRef<CFStringRef> cerr(SecCopyErrorMessageString(err, nullptr));
   if (!cerr) {
     return rv;
diff --git a/src/DHTSetup.h b/src/DHTSetup.h
index 30eb8a42..7a04d8a1 100644
--- a/src/DHTSetup.h
+++ b/src/DHTSetup.h
@@ -52,7 +52,7 @@ public:
   ~DHTSetup();
 
   // Returns two vector of Commands.  First one contains regular
-  // commands.  Secod one contains so called routine commands, which
+  // commands.  Second one contains so called routine commands, which
   // executed once per event poll returns.
   std::pair<std::vector<std::unique_ptr<Command>>,
             std::vector<std::unique_ptr<Command>>>
diff --git a/src/DiskWriter.h b/src/DiskWriter.h
index 03a4147b..2fed66d6 100644
--- a/src/DiskWriter.h
+++ b/src/DiskWriter.h
@@ -62,7 +62,7 @@ public:
   virtual void closeFile() = 0;
 
   /**
-   * Opens a file.  If the file doesnot exists, an exception may be
+   * Opens a file.  If the file does not exist, an exception may be
    * thrown.
    */
   virtual void openExistingFile(int64_t totalLength = 0) = 0;
diff --git a/src/HttpHeaderProcessor.h b/src/HttpHeaderProcessor.h
index deff890e..f66e0a20 100644
--- a/src/HttpHeaderProcessor.h
+++ b/src/HttpHeaderProcessor.h
@@ -61,7 +61,7 @@ public:
   bool parse(const std::string& data);
 
   /**
-   * Retruns the number of bytes processed in the last invocation of
+   * Returns the number of bytes processed in the last invocation of
    * parse().
    */
   size_t getLastBytesProcessed() const;
diff --git a/src/Netrc.cc b/src/Netrc.cc
index 005f3162..d6cbbe52 100644
--- a/src/Netrc.cc
+++ b/src/Netrc.cc
@@ -179,7 +179,7 @@ void Netrc::parse(const std::string& path)
         else {
           if (!authenticator) {
             throw DL_ABORT_EX(
-                fmt("Netrc:parse error. %s encounterd where 'machine'"
+                fmt("Netrc:parse error. %s encountered where 'machine'"
                     " or 'default' expected.",
                     std::string((*iter).first, (*iter).second).c_str()));
           }
diff --git a/src/PeerListenCommand.h b/src/PeerListenCommand.h
index 13003c01..ecf5f2ae 100644
--- a/src/PeerListenCommand.h
+++ b/src/PeerListenCommand.h
@@ -60,7 +60,7 @@ public:
   virtual bool execute() CXX11_OVERRIDE;
 
   /**
-   * Binds port. If successful, the bound port number is assinged to port and
+   * Binds port. If successful, the bound port number is assigned to port and
    * returns true, otherwise port is undefined and returns false.
    */
   bool bindPort(uint16_t& port, SegList<int>& seq);
diff --git a/src/SeedCriteria.h b/src/SeedCriteria.h
index 77c98826..0765b5c0 100644
--- a/src/SeedCriteria.h
+++ b/src/SeedCriteria.h
@@ -49,7 +49,7 @@ public:
   virtual bool evaluate() = 0;
 
   /**
-   * Used for reseting status.
+   * Used for resetting status.
    */
   virtual void reset() = 0;
 };
diff --git a/src/WrDiskCache.h b/src/WrDiskCache.h
index c70d1ccc..a9cede2f 100644
--- a/src/WrDiskCache.h
+++ b/src/WrDiskCache.h
@@ -52,7 +52,7 @@ public:
   // Adds the cache entry |ent| to the storage. The size of cached
   // data of ent is added to total_.
   bool add(WrDiskCacheEntry* ent);
-  // Removes the cache entry |ent| from the stroage. The size of
+  // Removes the cache entry |ent| from the storage. The size of
   // cached data of ent is subtracted from total_.
   bool remove(WrDiskCacheEntry* ent);
   // Updates the already added entry |ent|. The |delta| means how many
diff --git a/src/bittorrent_helper.h b/src/bittorrent_helper.h
index a8886012..ea1a6277 100644
--- a/src/bittorrent_helper.h
+++ b/src/bittorrent_helper.h
@@ -210,7 +210,7 @@ void createPeerMessageString(unsigned char* msg, size_t msgLength,
                              size_t payloadLength, uint8_t messageId);
 
 /**
- * Creates compact form(packed addresss + 2bytes port) and stores the
+ * Creates compact form(packed address + 2bytes port) and stores the
  * results in compact.  This function looks addr and if it is IPv4
  * address, it stores 6bytes in compact and if it is IPv6, it stores
  * 18bytes in compact.  So compact must be at least 18 bytes and
diff --git a/src/option_processing.cc b/src/option_processing.cc
index f9891fed..a671b23a 100644
--- a/src/option_processing.cc
+++ b/src/option_processing.cc
@@ -275,7 +275,7 @@ error_code::Value option_processing(Option& op, bool standalone,
     // we must clear eof bit and seek to the beginning of the buffer.
     cmdstream.clear();
     cmdstream.seekg(0, std::ios::beg);
-    // finaly let's parse and store command-iine options.
+    // finally let's parse and store command-line options.
     op.setParent(confOption);
     oparser->parse(op, cmdstream);
     oparser->parse(op, options);