describe unix services

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                                         Hardware
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 ```
+
+## Services
+
+* init: first process started. performs startup chores.
+  * single user mode
+  * multi user mode aka run levels
+    * 0: system halt
+    * 1: single user mode
+    * 3: multi user mode
+    * 5: start gui
+    * 6: system reboot
+  * starts `getty`
+  * init -> getty -> login -> shell
+* syslog: aggregates logs
+* cron: runs program regularly on a schedule
+* at: runs program at a specific time once.
+* GUI:
+  * X Window system: X
+  * Window managers: fvwm, icewm, blackbox, windowmaker
+  * Desktop manager: KDE, Gnome
+* Networking:
+  * telnet
+  * ssh
+  * rlogin
+* Network file systems
+  * NFS supported by kernel
+  * CIFS supported through Samba
+* Mail
+  * Mail Transfer Agent (MTA): delivers email to local or remote mailbox. e.g sendmail, postfix
+  * Mail User Agent (MUA): used to read mail. e.g. pine, evolution
+  * usually stored in /var/spool/mail
+* Printing
+  * print queue: manages which job to print.
+  * print queue software spools the printouts on disk.
+* File system
+  * /bin
+  * /lib
+  * /etc
+  * /dev
+  * /usr: unchanging data.
+  * /var: changing data like logs