Replace backticks with $() command substitution

nvm.sh

@@ -1168,7 +1168,7 @@ nvm_install_node_binary() {
       fi
       t="$VERSION-$NVM_OS-$NVM_ARCH"
       url="$NVM_NODEJS_ORG_MIRROR/$VERSION/node-${t}.tar.gz"
-      sum=`nvm_download -L -s "$NVM_NODEJS_ORG_MIRROR/$VERSION/SHASUMS.txt" -o - | command grep "node-${t}.tar.gz" | command awk '{print $1}'`
+      sum=$(nvm_download -L -s "$NVM_NODEJS_ORG_MIRROR/$VERSION/SHASUMS.txt" -o - | command grep "node-${t}.tar.gz" | command awk '{print $1}')
       local tmpdir
       tmpdir="$NVM_DIR/bin/node-${t}"
       local tmptarball
@@ -1236,10 +1236,10 @@ nvm_install_node_source() {
   local tmptarball
   tmptarball="$tmpdir/node-$VERSION.tar.gz"
 
-  if [ "`nvm_download -L -s -I "$NVM_NODEJS_ORG_MIRROR/$VERSION/node-$VERSION.tar.gz" -o - 2>&1 | command grep '200 OK'`" != '' ]; then
+  if [ "$(nvm_download -L -s -I "$NVM_NODEJS_ORG_MIRROR/$VERSION/node-$VERSION.tar.gz" -o - 2>&1 | command grep '200 OK')" != '' ]; then
     tarball="$NVM_NODEJS_ORG_MIRROR/$VERSION/node-$VERSION.tar.gz"
-    sum=`nvm_download -L -s "$NVM_NODEJS_ORG_MIRROR/$VERSION/SHASUMS.txt" -o - | command grep "node-${VERSION}.tar.gz" | command awk '{print $1}'`
-  elif [ "`nvm_download -L -s -I "$NVM_NODEJS_ORG_MIRROR/node-$VERSION.tar.gz" -o - | command grep '200 OK'`" != '' ]; then
+    sum=$(nvm_download -L -s "$NVM_NODEJS_ORG_MIRROR/$VERSION/SHASUMS.txt" -o - | command grep "node-${VERSION}.tar.gz" | command awk '{print $1}')
+  elif [ "$(nvm_download -L -s -I "$NVM_NODEJS_ORG_MIRROR/node-$VERSION.tar.gz" -o - | command grep '200 OK')" != '' ]; then
     tarball="$NVM_NODEJS_ORG_MIRROR/node-$VERSION.tar.gz"
   fi
 
@@ -1710,7 +1710,7 @@ nvm() {
       echo "$NVM_SUCCESS_MSG"
 
       # rm any aliases that point to uninstalled version.
-      for ALIAS in `command grep -l "$VERSION" "$(nvm_alias_path)/*" 2>/dev/null`
+      for ALIAS in $(command grep -l "$VERSION" "$(nvm_alias_path)/*" 2>/dev/null)
       do
         nvm unalias "$(command basename "$ALIAS")"
       done