Fix small issues reported by shellcheck

- Use subshells instead of "cd -".
- Use quotes around variables.
- Add "set -eo pipefail" to catch errors.

histd.sh

@@ -3,6 +3,8 @@
 # Histd: how I spent this day.
 # A simple but useful personal diary CLI utility.
 
+set -eo pipefail
+
 # Check if a text editor is specified
 function _check_editor {
     if [[ $EDITOR == "" ]]; then
@@ -16,26 +18,28 @@ function _check_editor {
 function edit_note {
     # Create dirs (base_dir/year/month)
     WORKDIR="$BASE_DIR/$(date +%Y)/$(date +%m)"
-    mkdir -p $WORKDIR
+    mkdir -p "$WORKDIR"
 
     # Generate file name
     PATH_TO_FILE="$WORKDIR/$(date +%d).md"
 
     # Open editor
     _check_editor
-    cd $BASE_DIR
-    $EDITOR $PATH_TO_FILE
-    cd - > /dev/null
+    (
+        cd $BASE_DIR
+        $EDITOR "$PATH_TO_FILE"
+    )
 }
 
 # Create an archive with all notes
 function backup {
     ARCHIVE_PATH=~/histd-$(date +%F).tar.xz
-    cd $BASE_DIR
-    tar cfJ $ARCHIVE_PATH ../histd
-    cd - > /dev/null
+    (
+        cd $BASE_DIR
+        tar cfJ "$ARCHIVE_PATH" ../histd
+    )
 
-    echo Saved to $ARCHIVE_PATH
+    echo "Saved to $ARCHIVE_PATH"
 }
 
 # Concatenate all files and prefix each with the filename.
@@ -54,7 +58,7 @@ function last {
     _check_editor
     LAST_FILE=$(find $BASE_DIR -type f -name "*.md" | tail -1)
     cd $BASE_DIR
-    $EDITOR $LAST_FILE
+    $EDITOR "$LAST_FILE"
     cd - > /dev/null
 }