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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Major services in a UNIX system</TITLE
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>The Linux System Administrators' Guide</TH
></TR
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><TD
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ALIGN="left"
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><A
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></TD
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><HR
ALIGN="LEFT"
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN94"
>Major services in a UNIX system</A
></H1
><P
>This section describes some of the more important UNIX
services, but without much detail. They are described more
thoroughly in later chapters.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN97"
><B
CLASS="COMMAND"
>init</B
></A
></H2
><P
>The single most important service in a UNIX system is
provided by <B
CLASS="COMMAND"
>init</B
>. <B
CLASS="COMMAND"
>init</B
>
is started as the first process of every UNIX system, as the last
thing the kernel does when it boots. When <B
CLASS="COMMAND"
>init</B
>
starts, it continues the boot process by doing various startup
chores (checking and mounting filesystems, starting daemons,
etc).</P
><P
>The exact list of things that <B
CLASS="COMMAND"
>init</B
>
does depends on which flavor it is; there are several to choose
from. <B
CLASS="COMMAND"
>init</B
> usually provides the concept of
<I
CLASS="GLOSSTERM"
>single user mode</I
>, in which no one can
log in and root uses a shell at the console; the usual mode is
called <I
CLASS="GLOSSTERM"
>multiuser mode</I
>. Some flavors
generalize this as <I
CLASS="GLOSSTERM"
>run levels</I
>; single
and multiuser modes are considered to be two run levels, and
there can be additional ones as well, for example, to run X on
the console.</P
><P
>In normal operation, <B
CLASS="COMMAND"
>init</B
> makes sure
<B
CLASS="COMMAND"
>getty</B
> is working (to allow users to log in),
and to adopt orphan processes (processes whose parent has died; in
UNIX <I
CLASS="EMPHASIS"
>all</I
> processes <I
CLASS="EMPHASIS"
>must</I
>
be in a single tree, so orphans must be adopted).</P
><P
>When the system is shut down, it is <B
CLASS="COMMAND"
>init</B
>
that is in charge of killing all other processes, unmounting all
filesystems and stopping the processor, along with anything else
it has been configured to do.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN117"
>Logins from terminals</A
></H2
><P
>Logins from terminals (via serial lines) and the console
(when not running X) are provided by the <B
CLASS="COMMAND"
>getty</B
>
program. <B
CLASS="COMMAND"
>init</B
> starts a separate instance
of <B
CLASS="COMMAND"
>getty</B
> for each terminal for which
logins are to be allowed. <B
CLASS="COMMAND"
>getty</B
> reads
the username and runs the <B
CLASS="COMMAND"
>login</B
> program,
which reads the password. If the username and password
are correct, <B
CLASS="COMMAND"
>login</B
> runs the shell.
When the shell terminates, i.e., the user logs out, or when
<B
CLASS="COMMAND"
>login</B
> terminated because the username
and password didn't match, <B
CLASS="COMMAND"
>init</B
> notices
this and starts a new instance of <B
CLASS="COMMAND"
>getty</B
>.
The kernel has no notion of logins, this is all handled by the
system programs.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN129"
>Syslog</A
></H2
><P
>The kernel and many system programs produce error, warning, and
other messages. It is often important that these messages can
be viewed later, even much later, so they should be written to
a file. The program doing this is <B
CLASS="COMMAND"
>syslog</B
>. It can be
configured to sort the messages to different files according to
writer or degree of importance. For example, kernel messages
are often directed to a separate file from the others, since
kernel messages are often more important and need to be read
regularly to spot problems.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN133"
>Periodic command execution: <B
CLASS="COMMAND"
>cron</B
> and
<B
CLASS="COMMAND"
>at</B
></A
></H2
><P
>Both users and system administrators often need
to run commands periodically. For example, the system
administrator might want to run a command to clean the
directories with temporary files (<TT
CLASS="FILENAME"
>/tmp</TT
>
and <TT
CLASS="FILENAME"
>/var/tmp</TT
>) from old files, to keep the
disks from filling up, since not all programs clean up after
themselves correctly.</P
><P
>The <B
CLASS="COMMAND"
>cron</B
> service is set up to do this.
Each user has a <TT
CLASS="FILENAME"
>crontab</TT
> file, where he
lists the commands he wants to execute and the times they should
be executed. The <B
CLASS="COMMAND"
>cron</B
> daemon takes care of
starting the commands when specified.</P
><P
>The <B
CLASS="COMMAND"
>at</B
> service is similar to
<B
CLASS="COMMAND"
>cron</B
>, but it is once only: the command is
executed at the given time, but it is not repeated.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN147"
>Graphical user interface</A
></H2
><P
>UNIX and Linux don't incorporate the user interface
into the kernel; instead, they let it be implemented by user
level programs. This applies for both text mode and graphical
environments.</P
><P
>This arrangement makes the system more flexible, but has
the disadvantage that it is simple to implement a different
user interface for each program, making the system harder to
learn.</P
><P
>The graphical environment primarily used with Linux
is called the X Window System (X for short). X also does
not implement a user interface; it only implements a window
system, i.e., tools with which a graphical user interface can
be implemented. The three most popular user interface styles
implemented over X are Athena, Motif, and Open Look.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN152"
>Networking</A
></H2
><P
>Networking is the act of connecting two or more computers
so that they can communicate with each other. The actual methods
of connecting and communicating are slightly complicated, but
the end result is very useful.</P
><P
>UNIX operating systems have many networking features.
Most basic services (filesystems, printing, backups, etc) can
be done over the network. This can make system administration
easier, since it allows centralized administration, while
still reaping in the benefits of microcomputing and distributed
computing, such as lower costs and better fault tolerance.</P
><P
>However, this book merely glances at networking; see the
<I
CLASS="CITETITLE"
>Linux Network Administrators' Guide</I
> for
more information, including a basic description of how networks
operate.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN158"
>Network logins</A
></H2
><P
>Network logins work a little differently than normal logins.
There is a separate physical serial line for each terminal via
which it is possible to log in. For each person logging in via
the network, there is a separate virtual network connection,
and there can be any number of these.
<A
NAME="AEN161"
HREF="#FTN.AEN161"
>[1]</A
>
It is therefore not possible to run a separate
<B
CLASS="COMMAND"
>getty</B
> for each possible virtual connection.
There are also several different ways to log in via a network,
<B
CLASS="COMMAND"
>telnet</B
> and <B
CLASS="COMMAND"
>rlogin</B
> being
the major ones in TCP/IP networks.</P
><P
>Network logins have, instead of a herd of
<B
CLASS="COMMAND"
>getty</B
>s, a single daemon per way of logging in
(<B
CLASS="COMMAND"
>telnet</B
> and <B
CLASS="COMMAND"
>rlogin</B
> have
separate daemons) that listens for all incoming login attempts.
When it notices one, it starts a new instance of itself to
handle that single attempt; the original instance continues to
listen for other attempts. The new instance works similarly
to <B
CLASS="COMMAND"
>getty</B
>.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN171"
>Network file systems</A
></H2
><P
>One of the more useful things that can be done with
networking services is sharing files via a <I
CLASS="GLOSSTERM"
>network
file system</I
>. The one usually used is called the
Network File System, or NFS, developed by Sun.</P
><P
>With a network file system any file operations done by
a program on one machine are sent over the network to another
computer. This fools the program to think that all the files
on the other computer are actually on the computer the program
is running on. This makes information sharing extremely simple,
since it requires no modifications to programs.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN176"
>Mail</A
></H2
><P
>Electronic mail is usually the most important method for
communicating via computer. An electronic letter is stored in a
file using a special format, and special mail programs are used
to send and read the letters.</P
><P
>Each user has an <I
CLASS="GLOSSTERM"
>incoming mailbox</I
>
(a file in the special format), where all new mail is stored.
When someone sends mail, the mail program locates the receiver's
mailbox and appends the letter to the mailbox file. If the
receiver's mailbox is in another machine, the letter is sent to
the other machine, which delivers it to the mailbox as it best
sees fit.</P
><P
>The mail system consists of many programs. The
delivery of mail to local or remote mailboxes is done by one
program (the <I
CLASS="GLOSSTERM"
>mail transfer agent</I
> or
<I
CLASS="GLOSSTERM"
>MTA</I
>, e.g., <B
CLASS="COMMAND"
>sendmail</B
>
or <B
CLASS="COMMAND"
>smail</B
>), while the programs users use
are many and varied (<I
CLASS="GLOSSTERM"
>mail user agent</I
>
or <I
CLASS="GLOSSTERM"
>MUA</I
>, e.g., <B
CLASS="COMMAND"
>pine</B
>
or <B
CLASS="COMMAND"
>elm</B
>). The mailboxes are usually stored
in <TT
CLASS="FILENAME"
>/var/spool/mail</TT
>.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN191"
>Printing</A
></H2
><P
>Only one person can use a printer at one time, but it is
uneconomical not to share printers between users. The printer is
therefore managed by software that implements a <I
CLASS="GLOSSTERM"
>print
queue</I
>: all print jobs are put into a queue and
whenever the printer is done with one job, the next one is sent
to it automatically. This relieves the users from organizing
the print queue and fighting over control of the printer.
<A
NAME="AEN195"
HREF="#FTN.AEN195"
>[2]</A
>
</P
><P
>The print queue software also <I
CLASS="GLOSSTERM"
>spools</I
>
the printouts on disk, i.e., the text is kept in a file while
the job is in the queue. This allows an application program
to spit out the print jobs quickly to the print queue software;
the application does not have to wait until the job is actually
printed to continue. This is really convenient, since it
allows one to print out one version, and not have to wait for
it to be printed before one can make a completely revised new
version.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN200"
>The filesystem layout</A
></H2
><P
>The filesystem is divided into many parts;
usually along the lines of a root filesystem with
<TT
CLASS="FILENAME"
>/bin</TT
>, <TT
CLASS="FILENAME"
>/lib</TT
>,
<TT
CLASS="FILENAME"
>/etc</TT
>, <TT
CLASS="FILENAME"
>/dev</TT
>, and
a few others; a <TT
CLASS="FILENAME"
>/usr</TT
> filesystem with
programs and unchanging data; a <TT
CLASS="FILENAME"
>/var</TT
>
filesystem with changing data (such as log files); and a
<TT
CLASS="FILENAME"
>/home</TT
> filesystem for everyone's personal
files. Depending on the hardware configuration and the decisions
of the system administrator, the division can be different;
it can even be all in one filesystem.</P
><P
><A
HREF="c212.html"
>Chapter 3</A
> describes the filesystem
layout in some detail; the Linux Filesystem Standard covers it
in somewhat more detail.</P
></DIV
></DIV
><H3
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN161"
HREF="x94.html#AEN161"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Well, at least there can be many. Network
bandwidth still being a scarce resource, there is still
some practical upper limit to the number of concurrent
logins via one network connection. </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN195"
HREF="x94.html#AEN195"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Instead, they form a new queue
<I
CLASS="EMPHASIS"
>at</I
> the printer, waiting for their
printouts, since no one ever seems to be able to get the
queue software to know exactly when anyone's printout is
really finished. This is a great boost to intra-office
social relations.</P
></TD
></TR
></TABLE
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>Home</A
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>Next</A
></TD
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><TR
><TD
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>Important parts of the kernel</TD
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