diff --git a/notebook/editor.neovim.help-system.org b/notebook/editor.neovim.help-system.org
index b7609df..680e633 100644
--- a/notebook/editor.neovim.help-system.org
+++ b/notebook/editor.neovim.help-system.org
@@ -3,13 +3,17 @@
 :END:
 #+title: Neovim help system
 #+date: 2022-04-01 16:20:55 +08:00
-#+date_modified: 2022-04-03 16:59:48 +08:00
+#+date_modified: 2022-04-20 19:04:55 +08:00
 #+language: en
 
 
-- the main command is ~:h~ which should open ~help.txt~ if given no arguments;
-  this should be enough if you want to explore much of Neovim itself as the document has
+- the main command is =:h= which should open =help.txt= if given no arguments;
+  this should be enough if you want to explore much of Neovim itself as the document has an index of tutorials and local additions (most likely from the installed plugins)
+- the help documents are written in a custom formatting language that is recognized as a separate filetype;
+  in addition to navigating the buffer like any other buffer, you can also run =gO= to show the table of contents
 - if given an argument, Vim will open the section of the document with the closest match
 - some points of interest
-  - you can view the complete listing of help tags with ~help-tags~
-  - there is also a index of tags from ~index.txt~
+  - you can view the complete listing of help tags with =help-tags=
+  - there is also a index of tags from =index.txt=
+  - you can easily get the keybind with =CTRL-V= (e.g., =CTRL-V= then =META-D= will quickly print the keymap)
+  - tab completion is present
diff --git a/notebook/editor.neovim.lua-modules.org b/notebook/editor.neovim.lua-modules.org
new file mode 100644
index 0000000..1999653
--- /dev/null
+++ b/notebook/editor.neovim.lua-modules.org
@@ -0,0 +1,19 @@
+:PROPERTIES:
+:ID:       bdcff35e-15e1-4539-9c4e-5fdd5b978c26
+:END:
+#+title: Neovim Lua modules
+#+date: 2022-04-20 18:36:23 +08:00
+#+date_modified: 2022-04-20 18:39:40 +08:00
+#+language: en
+
+
+- Neovim will load the requires files from its runtime paths;
+  see =runtimepath= from the [[id:0a0fe63e-dcf3-4928-9e82-5513784c1244][Neovim help system]]
+- built on top of the already existing [[id:ffb8b08a-ca0a-48a4-bebc-d2bf11aa5ccf][Lua modules]] system
+- while searching for the runtime paths, Neovim will search for certain folders
+  - =plugin/= will load all of the Lua files that are placed in that subdirectory
+  - =lua/= also contains Lua files but does not automatically load until some other module will call ~require~ to that module
+  - =init.lua= is basically the =index.html= for HTML files, =default.nix= for [[id:a57e63a7-6daa-4639-910d-c6648df156a3][Nix language]], or =mod.rs= for [[id:d7d7d8f0-adf9-461d-ace5-c8624dab1083][Rust language]]
+- in module names that are ~require~'d, =.= is treated as a separator and representing as a directory in the file tree;
+  e.g., when running ~require("custom.plugins")~, =custom.plugins= should have a module at =$RUNTIMEPATH/lua/custom/plugins/init.lua=
+- at some point, Neovim can make use of more Lua modules with [[https://luarocks.org/][Luarocks]]
diff --git a/notebook/editor.neovim.lua.org b/notebook/editor.neovim.lua.org
index a9a2a58..6041109 100644
--- a/notebook/editor.neovim.lua.org
+++ b/notebook/editor.neovim.lua.org
@@ -3,34 +3,35 @@
 :END:
 #+title: Neovim Lua integration
 #+date: "2021-07-15 07:45:50 +08:00"
-#+date_modified: "2021-11-16 19:15:13 +08:00"
+#+date_modified: "2022-04-20 18:49:17 +08:00"
 #+language: en
 
 
-- [[https://github.com/nanotee/nvim-lua-guide][go-to resource when introducing using Lua into Neovim]]
-- similar to VimL configs, really
-- init file is =${XDG_CONFIG_HOME}/nvim/init.lua=
-- Lua configs doesn't configure much by default
-- you can start with =lua-intro= help section in the =lua= help page from Neovim — i.e., =:h lua=
-- you can still execute Vimscript with =vim.cmd=;
-  more information is at =:h lua-vimscript=
-- basics from VimL
-  + to set options, it's mostly in =vim.opt= — e.g., ~set number relativenumber~ versus ~vim.opt.number, vim.opt.relativenumber = true, true~
-  + highlight options are mostly in =vim.highlight= — e.g., ~highlight clear SpellCap~ versus ~vim.highlight~
-  + to set local options, use =vim.opt_local= — e.g., ~setlocal spell~ versus ~vim.opt_local.spell = true~
-  + otherwise, to set global options, use =vim.opt_global=
-  + you can access environment variables through =vim.env= — e.g., ~vim.env.HOME~, ~vim.env.MYVIMRC~
-  + you can manipulate variables of various scales from =vim.{g,b,t}=;
-    to see more details, see =lua-vim-variables= help section
-  + =vim.opt= will return an Option object, it has a common API;
-    to learn more about it, see =vim.opt= and its subsections
-  + to run Vimscript, you can use =vim.cmd= — e.g., ~vim.cmd "colorscheme nord"~
-  + interacting with Neovim API through =vim.api=
-- there are Neovim configurations written in Lua
-  + [[https://github.com/mjlbach/defaults.nvim][defaults.nvim]]
+- with Lua integration, you can create [[id:bdcff35e-15e1-4539-9c4e-5fdd5b978c26][Neovim Lua modules]]
+- you can start with =lua-intro= section from [[id:0a0fe63e-dcf3-4928-9e82-5513784c1244][Neovim help system]];
+  it gives all of the information on the things you need to get started with configuring Neovim with Lua as well as pointers for more things to do with Lua;
+- Neovim also extends the Lua standard library found in =vim= object;
+  see the =lua-stdlib= in [[id:0a0fe63e-dcf3-4928-9e82-5513784c1244][Neovim help system]]
 - comprehensive examples include Neovim plugins that are already written in Lua
   + [[https://github.com/savq/paq-nvim][paq-nvim]] is a simple Neovim package manager
   + [[https://github.com/wbthomason/packer.nvim][packer.nvim]] is a more comprehensive package manager
   + [[https://github.com/L3MON4D3/LuaSnip][LuaSnip]] is a snippet engine
   + [[https://github.com/nvim-telescope/telescope.nvim][telescope.nvim]] is a fuzzy finder integrated inside Neovim
-- for more information to create Lua modules, see =lua-require-example= help section
+
+
+
+
+* Setting configuration with Lua
+- you can still execute Vimscript with =vim.cmd=;
+  more information is at =:h lua-vimscript=
+- to set options, it's mostly in =vim.opt= — e.g., ~set number relativenumber~ versus ~vim.opt.number, vim.opt.relativenumber = true, true~
+- highlight options are mostly in =vim.highlight= — e.g., ~highlight clear SpellCap~ versus ~vim.highlight~
+- to set local options, use =vim.opt_local= — e.g., ~setlocal spell~ versus ~vim.opt_local.spell = true~
+- otherwise, to set global options, use =vim.opt_global=
+- you can access environment variables through =vim.env= — e.g., ~vim.env.HOME~, ~vim.env.MYVIMRC~
+- you can manipulate variables of various scales from =vim.{g,b,t}=;
+  to see more details, see =lua-vim-variables= help section
+- =vim.opt= will return an Option object, it has a common API;
+  to learn more about it, see =vim.opt= and its subsections
+- to run Vimscript, you can use =vim.cmd= — e.g., ~vim.cmd "colorscheme nord"~
+- interacting with Neovim API through =vim.api=
diff --git a/notebook/lang.lua.modules.org b/notebook/lang.lua.modules.org
new file mode 100644
index 0000000..3e705ec
--- /dev/null
+++ b/notebook/lang.lua.modules.org
@@ -0,0 +1,14 @@
+:PROPERTIES:
+:ID:       ffb8b08a-ca0a-48a4-bebc-d2bf11aa5ccf
+:END:
+#+title: Lua modules
+#+date: 2022-04-20 18:36:59 +08:00
+#+date_modified: 2022-04-20 18:37:17 +08:00
+#+language: en
+
+
+- for future references, this is documented from the [[https://www.lua.org/manual][Lua manual]]
+- to use the modules from =lua=, you can call =require= builtin;
+  the ~require~'d module are stored in a table =packages.loaded= and its content will be loaded inside of a function;
+  the module will only load once and it will only cache the return value
+- to unload a module, you can set the package from the package table to null (i.e., ~nil~)
diff --git a/notebook/lang.oil.org b/notebook/lang.oil.org
index 7a462bd..29d05ea 100644
--- a/notebook/lang.oil.org
+++ b/notebook/lang.oil.org
@@ -3,7 +3,7 @@
 :END:
 #+title: Oil shell language
 #+date: "2021-05-09 16:40:50 +08:00"
-#+date_modified: "2021-07-28 10:52:44 +08:00"
+#+date_modified: "2022-04-16 20:18:16 +08:00"
 #+language: en
 #+property: header-args:oil  :eval no
 
@@ -21,7 +21,7 @@ Ripping off from the [[https://www.oilshell.org/blog/2020/01/simplest-explanatio
 Oil is also aimed at people who know say Python or JavaScript, but purposely avoid shell.
 #+end_quote
 
-A modern shell attempting the replace Bash slowly.
+A modern shell attempting the replace [[id:dd9d3ffa-03ff-42a1-8c5d-55dc9fcc70fe][GNU Bash]] slowly.
 The project has an ambitious goal with a wide scope.
 It is known for its [[https://oilshell.org/blog][shell-oriented blog]] and the developer is very responsive and active with shell-related posts.
 
@@ -107,7 +107,7 @@ If you want splitting, you could use =split= Oil function — e.g., ~@split(arra
 Arrays are mostly similar to Bash arrays except you have more options.
 
 - You can create a heterogenous list containing different types of data — e.g., ~var a = ['Dogs', 24, true ]~.
-  Useful for JSON compatability.
+  Useful for JSON compatibility.
 
 - A homogenous array is useful for data consistency.
   It can accept a list of data of the same type — e.g., ~var b = %("foo" "bar" "baz")~.
diff --git a/notebook/linux.systemd.environment-directives.org b/notebook/linux.systemd.environment-directives.org
index 8fb2a34..d420b40 100644
--- a/notebook/linux.systemd.environment-directives.org
+++ b/notebook/linux.systemd.environment-directives.org
@@ -3,16 +3,18 @@
 :END:
 #+title: systemd environment directives
 #+date: 2021-08-07 19:58:29 +08:00
-#+date_modified: 2021-08-07 19:59:45 +08:00
+#+date_modified: 2022-04-16 20:19:15 +08:00
 #+language: en
 
 
 systemd enables setting the environment through environment directives.
-
-Just like how [[id:c7edff80-6dea-47fc-8ecd-e43b5ab8fb1e][systemd at user-level]], you can set it at user-level by placing them in certain user-level load paths (e.g., =$XDG_CONFIG_HOME/environment.d=).
+For some, this is a nice shell-agnostic way of setting environment variables and could replace setting through shell profiles (i.e., =.bashrc=, =.profile=).
 
 It needs a =*.conf= file in one of the load paths (seen from the =environment.d.5= manual page).
-Keep in mind it does not use the shell and instead makes use of shell-like syntax.
+Just like how [[id:c7edff80-6dea-47fc-8ecd-e43b5ab8fb1e][systemd at user-level]], you can set it at user-level by placing them in certain user-level load paths (e.g., =$XDG_CONFIG_HOME/environment.d=).
+
+Keep in mind it does not use the shell directly and instead makes use of shell-like syntax.
+The syntax takes variable substitutions and parameter expansion seen from [[id:dd9d3ffa-03ff-42a1-8c5d-55dc9fcc70fe][GNU Bash]].
 
 The following code block is an example of setting Nix-related environment variables to enable desktop integrations.
 
diff --git a/notebook/linux.systemd.org b/notebook/linux.systemd.org
index a59a1cc..6e71be2 100644
--- a/notebook/linux.systemd.org
+++ b/notebook/linux.systemd.org
@@ -3,7 +3,7 @@
 :END:
 #+title: systemd
 #+date: "2021-05-20 22:37:22 +08:00"
-#+date_modified: "2022-01-03 22:03:26 +08:00"
+#+date_modified: "2022-04-19 20:21:56 +08:00"
 #+language: en
 #+property: header-args  :eval no
 
@@ -21,6 +21,7 @@ Among other things, it has the following list of features.
 - [[id:e4dba4ef-71dd-4d30-9a2c-4ad97223510b][systemd-networkd]] is the network configuration manager in case you want to do [[id:a208dd50-2ebc-404d-b407-3ec2f556535e][Network configuration in Linux]].
 - [[id:8505f1f0-f15b-4b04-91fc-12be01913ce6][systemd-boot]] is a bootloader mainly for UEFI-based systems.
 - [[id:d83c099a-fc11-4ccc-b265-4de97c85dcbe][systemd-journald]] is the system logging service providing a structured way to manage your logs from different units.
+- [[id:7fce893f-418f-42aa-b2b1-59d9f0993406][systemd unit hardening]] can help your services secure.
 
 
 
@@ -58,5 +59,7 @@ systemctl --user start $SERVICE
   Also contains the related manual pages for a deeper references.
   [fn:: How did I pass a year without knowing this?]
 
+- On a similar note, =systemd.index.7= is an alphabetical index of the important keywords found in systemd.
+
 - =systemd.mount= units require the filename to be the mountpoint.
   Though, it has to be converted to what systemd accepts (e.g., =systemd-escape --path $PATH=).
diff --git a/notebook/linux.systemd.timers.org b/notebook/linux.systemd.timers.org
index ed1dc86..36c7bf1 100644
--- a/notebook/linux.systemd.timers.org
+++ b/notebook/linux.systemd.timers.org
@@ -3,20 +3,40 @@
 :END:
 #+title: systemd timers
 #+date: 2021-07-19 21:45:47 +08:00
-#+date_modified: 2021-08-07 19:29:15 +08:00
+#+date_modified: 2022-04-16 19:37:37 +08:00
 #+language: en
 
 
 You can schedule tasks with timers.
 If systemd is compiled with the feature, it makes cron unnecessary.
 
+
+
+
+* Timer management
+
+In a fully-installed systemd-enabled system, there are multiple ways to manage your timers.
+
+While managing them is practically the same as any other units (see [[id:20830b22-9e55-42a6-9cef-62a1697ea63d][systemd]]), there are timer-specific ways to manage them easier.
+
+- ~systemctl list-timers~ is the go-to command that displays an overview of all active timers.
+  Just give the =--all= flag to list all timers including disabled timers.
+
+- ~systemctl status ${TIMER_UNIT}~ is another way for a specific timer unit.
+  It gives the same details as the =list-timers= subcommand so you'll rarely use this subcommand in practice.
+
+
+
+
+* Timedate formats
+
 systemd has different ways to denote time.
 
 - Timespans denote the duration — e.g., =100 seconds=, =5M 3w=.
 - Timestamps refer to a specific point in time — e.g., =2021-04-02=, =today=, =now=.
 - Calendar events can refer to more than one point of time — e.g., =*-*-4/2=, =Sun,Wed,Fri *-1/2-1/8=.
 
-To find more details about time notation, you can view the =systemd.time.5= manual page.
+To find more details about time notation, you can view the =systemd.time.7= manual page.
 
 Here's an example of setting a timer for an example backup service.
 The following timer unit sets it to execute every day at 18:00.
diff --git a/notebook/linux.systemd.transient-units.org b/notebook/linux.systemd.transient-units.org
index 5bb1c14..a184e7a 100644
--- a/notebook/linux.systemd.transient-units.org
+++ b/notebook/linux.systemd.transient-units.org
@@ -3,13 +3,13 @@
 :END:
 #+title: systemd transient units
 #+date: 2021-08-02 12:01:49 +08:00
-#+date_modified: 2021-08-02 12:05:06 +08:00
+#+date_modified: 2022-04-16 20:01:51 +08:00
 #+language: en
 
 
 You can create units on-the-go with =systemd-run=.
 Very useful for quickly creating and scheduling one-off services.
-This tool involves [[id:cd5f0d04-d9bb-44e8-a0f2-630ea58c1e94][systemd services]] and [[id:f1b21fc8-86a5-47cd-b3d8-da6ac7a34427][systemd timers]].
+This tool mainly involves [[id:cd5f0d04-d9bb-44e8-a0f2-630ea58c1e94][systemd services]] and [[id:f1b21fc8-86a5-47cd-b3d8-da6ac7a34427][systemd timers]].
 
 Like most systemd-related binaries, this can be run at system- and user-level (see [[id:c7edff80-6dea-47fc-8ecd-e43b5ab8fb1e][systemd at user-level]]).
 
@@ -23,3 +23,5 @@ systemd-run --user borg-backup@external-drive.service --on-calendar=12:00
 
 The result should give you the generated name of the unit.
 Then, they can be managed like an ordinary unit.
+Unit generated this way will persist until the next boot.
+If you want to manage them on a permanent basis, create the appropriate unit files for them.
diff --git a/notebook/linux.systemd.unit-hardening.org b/notebook/linux.systemd.unit-hardening.org
new file mode 100644
index 0000000..22c739e
--- /dev/null
+++ b/notebook/linux.systemd.unit-hardening.org
@@ -0,0 +1,32 @@
+:PROPERTIES:
+:ID:       7fce893f-418f-42aa-b2b1-59d9f0993406
+:END:
+#+title: systemd unit hardening
+#+date: 2022-04-19 20:19:26 +08:00
+#+date_modified: 2022-04-19 20:21:27 +08:00
+#+language: en
+
+
+- main command to interact is ~systemd-analyze security~;
+  this will give a list of units along with their exposure score (lower is better);
+  - take note the goal to a 1.0 score shouldn't be taken as a goal since not all units need are the same;
+    security, after all, is about mitigating against your threat model
+  - the only unit possible to attain the lowest score is a simple "Hello world" program or similar so don't go for a 1.0
+- several systemd unit options are only available in certain units such as system services
+- here is a list of sandboxing-related options;
+  for more information, see ~systemd.exec.5~ manual page
+  - ~ProtectHome~ will restrict process to interact with ~/home~, ~/root~, and ~/run/user~;
+    can accept a boolean or certain values: ~read-only~ will set certain directories to read-only and ~tmpfs~ will mount the temporary filesystems to the directories as read-only;
+  - ~ProtectControlGroups~ will make the control group filesystem (i.e., ~/sys/fs/cgroup~) to read-only
+  - ~PrivateUsers~, if enabled, will run the processes through another user
+  - ~ProtectClock~ prohibits interacting with the system clock
+  - ~ProtectKernelModules~ restricts loading of kernel modules
+  - ~ProtectKernelLogs~ prevents logging into the kernel ring buffer
+  - ~PrivateTmp~ will create a new temporary filesystem for the unit
+  - ~PrivateNetwork~ will create a new set of network devices only composing of a loopback network device;
+    this will disallow network access and thus should only use for processes with no business with network access
+  - ~PrivateDevices~ will create a new set of devices with only the pseudo-devices (e.g., ~/dev/null~, ~/dev/zero~);
+    this will restrict device access and should be used for processes with no device access
+- extra resources
+  - [[https://www.ctrl.blog/entry/systemd-service-hardening.html][systemd service hardening]] from ctrl.blog
+  - also, a [[https://www.ctrl.blog/entry/systemd-opensmtpd-hardening.html][follow-up post that uses a real-life example for service hardening a web server with recent exploits]]