add linux service

docs/source/index.rst

@@ -32,7 +32,7 @@ Featured
 Some featured pages:
 
 * to learn about the Linux commands, read the
-  `Unix and Linux Commands <reference/commands/unix-and-linux-commands.html>`_
+  `Unix and Linux Commands <reference/commands/linux-and-linux-commands.html>`_
 * to see how easy setting up `iptables` and `neftables` for Debian is, read the
   `Debian Firewall nftables and iptables
   <reference/devops/debian-firewall-nftables-and-iptables.html>`_
@@ -68,7 +68,7 @@ The organization of this site is as following:
    :caption: Command Reference
    :name: sec-reference-command
 
-   reference/commands/unix-and-linux-commands
+   reference/commands/linux-and-linux-commands
    reference/commands/windows-commands
    reference/commands/docker-commands
    reference/commands/git-commands
@@ -86,17 +86,18 @@ The organization of this site is as following:
 
 .. toctree::
    :maxdepth: 1
-   :caption: Unix Reference
-   :name: sec-reference-unix
-
-   reference/unix/unix-permissions
-   reference/unix/unix-user-and-group
-   reference/unix/disk-basics
-   reference/unix/unix-disk-management
-   reference/unix/unix-file-system
-   reference/unix/mount-and-unmount
-   reference/unix/unix-network-config
-   reference/unix/introduction-to-logs-and-rsyslog
+   :caption: Linux Reference
+   :name: sec-reference-linux
+
+   reference/linux/linux-permissions
+   reference/linux/linux-user-and-group
+   reference/linux/disk-basics
+   reference/linux/linux-disk-management
+   reference/linux/linux-file-system
+   reference/linux/mount-and-unmount
+   reference/linux/linux-network-config
+   reference/linux/introduction-to-logs-and-rsyslog
+   reference/linux/linux-service
 
 
 .. toctree::

docs/source/reference/linux/linux-service.md

@@ -0,0 +1,244 @@
+# Linux Service
+
+A service is one or a set of applications running in the background doing
+specific tasks.
+
+Services usually do not have GUI.
+
+They run and accept requests all the time. And the reliability
+often refers to the how long they can work without errors.
+
+## System V and systemd
+
+SysVinit is an old way for Linux to manage system and services.
+
+Systemd replaces SysVinit and is the system and service manager used in common distributions now.
+
+But it seems that a lot of people dislike systemd.
+
+* [systemd - Wikipedia](https://en.wikipedia.org/wiki/Systemd#Criticism)
+* [Linus Torvalds and others on Linux's systemd](https://www.zdnet.com/article/linus-torvalds-and-others-on-linuxs-systemd/)
+* [The Biggest Myths](http://0pointer.de/blog/projects/the-biggest-myths.html)
+* [ELI5: The difference between SystemV and Systemd in Linux and why it's a big deal with Debian](https://www.reddit.com/r/explainlikeimfive/comments/2jormj/eli5_the_difference_between_systemv_and_systemd/)
+* ... a lot more
+
+## systemd
+
+```bash
+man systemd
+man systemd-system.conf
+```
+
+`systemd` uses "unit files" to manage each service and other things (from man page):
+
+* Service, daemons
+* Socket, local IPC or network sockets
+* Target, group units or synchronization points
+* Device, kernel devices
+* Mount, mount points
+* Automount
+* Timer, triggering based on times
+* Swap, similar to "mount", memory swap partitions or files
+* Path, activate other services on change
+* Slice, group system process management units
+* Scope, similar to "Service", but manage foreign processes
+
+Some `systemctl` commands:
+
+```bash
+systemctl status <service>
+
+# specific ones depend on whether the unit supports the operations
+sudo systemctl start <service>
+sudo systemctl stop <service>
+sudo systemctl reload <service>
+sudo systemctl restart <service>
+sudo systemctl enable <service>
+sudo systemctl disable <service>
+
+# list enabled
+systemctl list-unit-files --state=enabled
+systemctl list-unit-files | grep enabled
+
+# list running
+systemctl list-units --type=service --state=running
+systemctl | grep running
+
+# list active
+systemctl list-units --type=service --state=active
+```
+
+If you know SysVinit, [SysVinit to Systemd Cheatsheet](https://fedoraproject.org/wiki/SysVinit_to_Systemd_Cheatsheet)
+
+### Write a Unit File
+
+General recommendation:
+
+1. read existing files or examples
+2. read man page for interested directories
+    * if you want your thing truly works, always read man page even it's overlength
+
+```bash
+man systemd.unit # https://www.freedesktop.org/software/systemd/man/systemd.unit.html
+```
+
+Where should I put the file?
+
+Paths in descending priority (config in `etc` overwrites others):
+
+```bash
+# system
+/etc/systemd/system.conf, /etc/systemd/system.conf.d/*.conf, /run/systemd/system.conf.d/*.conf, /lib/systemd/system.conf.d/*.conf
+# user
+/etc/systemd/user.conf, /etc/systemd/user.conf.d/*.conf, /run/systemd/user.conf.d/*.conf, /usr/lib/systemd/user.conf.d/*.conf
+```
+
+### Unit Section
+
+```bash
+[Unit]
+...
+```
+
+* Description=
+  * short and descriptive
+* Documentation=
+  * docs
+  * local or online
+  * displayed in `systemctl status`
+* Requires=
+  * essentially depends
+  * must be activate simultaneously
+  * started in parallel by default
+* Wants=
+  * less strictly depends
+  * continue to work even if other units it "wants" not working
+  * recommended for most dependencies
+  * started in parallel by default
+* BindsTo=
+  * similar to Requires
+  * this unit stops when the units it "Binds to" stops
+* Before= After=
+  * ordering, not implying dependencies
+  * common to list units listed in both the After= and Requires= option
+    * this unit defined in this file depends on those units and will start after them
+* Conflicts=
+  * negative requirement dependencies
+* Conditions...=
+  * a set of directives, see man page for all of them (plenty of)
+  * will be verified (is true?) before starting the current unit
+  * if not true, unit will be (mostly silently) skipped but not failure
+  * read the man page (again) for each option
+* Asserts...=
+  * similiar to Conditions...= but failure
+
+For `enable`, read about the [Install] section.
+
+If you are truly tired of reading man page, maybe [this](https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files) might be helpful.
+
+### Example Unit Files
+
+From man page, read man page for more.
+
+```bash
+[Unit]
+Description=Foo
+
+[Service]
+ExecStart=/usr/sbin/foo-daemon
+
+[Install]
+WantedBy=multi-user.target
+```
+
+## SysVinit
+
+Very brief
+
+### Run Levels
+
+* 0 Halt
+* 3 multi-user, non-graphical
+* 5 multi-user, graphical
+* 6 reboot
+
+Each run level has different services to start.
+
+3 and 5 are most common.
+
+### Management
+
+Write System V scripts
+
+Path:
+
+```bash
+/etc/rc.d/init.d # stores the actual scripts
+/etc/rc[0-6].d/  # scripts to run for each level, links to init.d
+
+# s in link name: start at boot
+# k in link name: not start at boot
+
+# number in the name means the order, 01-99
+```
+
+Commands:
+
+```bash
+service <some-service> [start|restart|stop|status]
+```
+
+#### `chkconfig`
+
+Control services started at boot or not
+
+```bash
+chkconfig cups [on/off]
+chkconfig cups on
+chkconfig cups off
+chkconfig --list
+```
+
+### `xinetd`
+
+* service
+  * daemon, always (almost) up
+* xinted
+  * not always up
+
+`xinted` is a daemon, when requests sent to services it controls,
+it will start that service and stop it afterwards.
+
+`xinted` can also controls access
+
+Usually legacy ones, rare.
+
+Common `xinetd` services:
+
+* telnet
+* tftp
+* rsync
+
+Still controlled by `chkconfig`
+
+#### `xinetd` config
+
+`/etc/xinetd.d/<service-name>`
+
+`/etc/xinetd.d/tftp` example:
+
+```bash
+service tftp
+{
+    disable          = yes
+    socket_type      = dgram
+    protocol         = udp
+    wait             = yes
+    user             = root
+    server           = /usr/sbin/in.tftpd # the read server behind
+    server_args      = -c -s   /tftpboot
+    per_source       = 11
+    cps              = 100 2
+    flags            = IPv4
+}
+```