Configuring Network IV

Configuring Routing Parameters




You may have noticed that we didn’t configure two important IP
parameters in the preceding topics in this chapter: the default
gateway router address and the DNS server address.



Using the IP protocol, routers do just what their name implies: they
route data across multiple networks to deliver information to a
destination host. Routers operate at the Network layer and are used to
connect various networks together.




Routers are usually implemented in conjunction with a gateway. The
router hardware itself may be as simple as a computer system with two
NICs installed, or it may be a specialized hardware appliance
dedicated to routing.




One of the key jobs performed by routers is to determine the best way
to get information to the right destination host. To do this, a router
maintains a routing table of available routes. Routers use an
algorithm that evaluates distance, cost, and network status to
determine the best route to the destination host. Even if it isn’t
configured as a router, every Linux system maintains a routing table
in RAM that it uses to determine where to send data on a network.




When you’re configuring networking parameters on a Linux system, one
of the key tasks you need to perform is to configure the default
router address. The default router is the default location packets
are sent to if they are addressed to a host that doesn’t reside on the
local network segment.


Your default gateway router address is stored in the 



/etc/sysconfig/network/routes 



file, shown here:




openSUSE:/ # cat /etc/sysconfig/network/routes
default 10.0.0.1 - ens32




The syntax for specifying the default route in this file is


default router_IP_address



Notice in the preceding example that the default gateway router address is set to 10.0.0.1.




TIP  If you have more than one NIC installed and each interface needs
its own routing configuration (for example, if each NIC is connected
to a different network), you can define an additional routing
configuration file in 


/etc/sysconfig/network/ 



named



ifroute-interface_name





The syntax for adding a route to the routes file is shown next:




DESTINATION       GATEWAY          NETMASK   INTERFACE     [TYPE]





DESTINATION

The first column contains the route’s destination. It may contain the
IP address or DNS hostname of a network or host. Entering default in
this column indicates the route is the default route.


NOTE  I recommend you use IP addresses, not DNS names, in this file. If
your DNS server were to go down or become unreachable, routing would
be gone!



GATEWAY

The second column contains the IP address of the router through which
the specified host or a network can be reached. In this column,
specify the IP address of a router that can route the information to
the remote network or host.




NETMASK

The third column contains the netmask for the network or host behind
the router. The fourth column applies the route to a specific
interface. If nothing is specified, the route applies to all
interfaces.


NOTE If you want to leave a column in this file blank, be sure to
enter a dash (-).




[TYPE]

The fifth column is optional. It is used to specify the route type.
You can enter one of the following:


   • unicast The route specifies a real path to the destination route.

   • local The destination is the localhost. Packets sent to this route
     are looped back and delivered to the local machine.

   • broadcast The destination is a broadcast address. Packets sent to
     this route are sent as link broadcasts.

   • multicast Used for multicast routing. This type of route is not
     typically used with most routing tables.

  • unreachable Configures the route destination as unreachable. Packets
    sent to this route are silently dropped.




Here is a sample entry in a routes file:



207.68.156.51     207.68.145.45     255.255.255.0    ens32



After making any changes to the routes file, you will need to restart
your network interface by entering


ifdown interface


followed by


ifup interface




For your LPIC-1/CompTIA Linux+ exam, you also need to be familiar with
how to manage routes with the route command at the shell prompt. You
use the route command to display or modify the routing table on the
Linux host. If you enter


route 


without options, it simply displays the current routing table, as shown in this example:




 openSUSE:/ # route
Kernel IP routing table
Destination    Gateway     Genmask           Flags Metric Ref   Use Iface
default        10.0.0.1    0.0.0.0           UG    0      0     0 ens32
10.0.0.0       *           255.255.255.0     U     0      0     0 ens32
loopback       *           255.0.0.0         U     0      0     0 lo



You can add routes to the host’s route table by entering



route add –net network_address netmask netmask gw router_address



For example, suppose you need to add a route to the 192.168.2.0/24 network through
the router with an IP address of 10.0.0.254. You could do this by
entering



route add –net 192.168.2.0 netmask 255.255.255.0 gw 10.0.0.254 



at the shell prompt.



You can also remove existing routes from the routing table on a Linux
host using the route command. This is done by entering



route del –net network_address netmask netmask gw router_address 



at the shell prompt.


For example, suppose you want to remove the route just added in the
preceding paragraph. You could do this by entering



route del –net 192.168.2.0 netmask 255.255.255.0 gw 10.0.0.254 



at the shell prompt.



You can also use route to set the default route. This is done by
entering



route add default gw router_address 



at the shell prompt. For example, if you want to add 10.0.0.254 as your default gateway router,
you would enter




route add default gw 10.0.0.254 



at the shell prompt.




TIP    Changes made with the route command are not persistent! If you
want the route changes to be persistent across reboots, you need to
add them to your



/etc/sysconfig/network/routes 



file.



You can also use the ip command at the shell prompt to manage routing.
For example, to view the routing table, you can enter ip route show at
the shell prompt, as shown here:


openSUSE:/ # ip route show
default via 10.0.0.1 dev ens32
10.0.0.0/24 dev ens32  proto kernel  scope link  src 10.0.0.83
127.0.0.0/8 dev lo  scope link




You can also use the ip command to add a static route to the routing
table. This is done by entering



ip route add network/prefix via router_ip_address dev interface 



at the shell prompt. In the following example shown, I’ve added a route to the
192.168.5.0/24 network through a router with an IP address of 10.0.0.254:





openSUSE:/ # ip route add 192.168.5.0/24 via 10.0.0.254 dev ens32
openSUSE:/ # ip route show
default via 10.0.0.1 dev ens32
10.0.0.0/24 dev ens32  proto kernel  scope link  src 10.0.0.83
127.0.0.0/8 dev lo  scope link
192.168.5.0/24 via 10.0.0.254 dev ens32





You can also remove a route from the routing table by entering



ip route del network/prefix 



at the shell prompt.


For example, to remove the 192.168.5.0/24 route I just added, I would enter



ip route del 192.168.5.0/24








LX0-104 Exam Objectives (S)

Printing VIII

Using the Line Printer Daemon (lpd)



By far, CUPS is the preferred printing system for modern Linux
distributions. Many years ago, however, the preferred printing system
was the Line Printer Daemon (lpd). You probably won’t work much with
lpd, but the LPIC-1/Linux+ exam still expects you to know some of the
commands used to manage this daemon.



Most of the lpd commands have functionality similar to that offered by a CUPS command, as shown in Table 16-2.




As an interesting side note,




lpc status




at the shell prompt and it will return the status of your CUPS
printers, if CUPS is installed instead of lpd.











LX0-104 Exam Objectives (Q)

Printing VII

Using Command-Line Tools to Manage CUPS


In addition to the CUPS web-based administration utility, you can also
use a variety of command- line tools to configure CUPS. To view CUPS
printer information, you can use the lpstat utility. One of the most
useful options you can use with



lpstat is –t



This will cause lpstat to display all information about all CUPS
printers on the system, as this next example shows:




openSUSE:~ # lpstat –t

scheduler running

no system default destination

device for HPLJ2: parallel:/dev/lp0

HPLJ2 accepting requests since Fri 13 May 2011 10:57:13 AM MDT

printer HPLJ2 is idle.  enabled since Fri 13 May 2011 10:57:13 AM MDT

       Printer is now online.





This shows the default CUPS printer (HPLJ2), how it’s connected
(/dev/lp0), the print job currently being processed (if any), and a
list of pending print jobs.



To cancel a pending print job, you can use the cancel command. The syntax is



cancel job_ID



For example, suppose I sent a huge print job (a Linux user manual from
/usr/share/doc/manual/) and it was assigned a print ID of HPLJ2-4.
While printing, I decided that this was a real waste of paper. I could
kill the job and remove it from the print queue by entering



cancel HPLJ2-4 



at the shell prompt.



This can also be done from within the CUPS web-based administration
utility. Just go to the Jobs tab and select Show Active Jobs. Locate
the job that needs to be canceled and select Cancel Job, as shown in
Figure 16-10.



If you have more than one CUPS printer connected, you can use the




lpoptions –d printer




command to specify the default printer. For example, to set the HPLJ5
printer as the default, I would enter




lpoptions –d HPLJ5



This sets the default printer for all users on the system. Indi-
vidual users can override this setting, however, by creating a file
named


.lpoptions


 in their home directory and adding the following  directive:



default printer_name



If you want to view your printer’s configuration settings, you can enter



lpoptions –l



at the shell prompt.



In addition to the lpoptions command, you can also use the



cupsaccept printer_name



or


cupsreject printer_name



command to enable or disable a printer’s print queue. For example, I could enter



cupsreject HPLJ2



at the shell prompt to disable the printer’s print queue, as shown in
this example:



openSUSE:~ # cupsreject HPLJ2

openSUSE:~ # lpstat is –t

scheduler is running

system default destination: HPLJ2

device for HPLJ2: parallel:/dev/lp0

HPLJ2 not accepting requests since Fri 13 May 2011 11:03:07 AM MDT -

       Rejecting Jobs

printer HPLJ2 is idle.  enabled since Fri 13 May 2011 11:03:07 AM MDT

       Rejecting Jobs



The printer itself will continue processing queued print jobs, but
cupsd will not allow any new jobs to enter the queue. 


The cupsdisable command also includes the --hold option, which stops printing after
the current job is complete. To enable the queue again, I would enter



cupsaccept HPLJ2



at the shell prompt.


To disable the printer itself, not the queue, I could enter



cupsdisable HPLJ2



at the shell prompt, as this example shows:



openSUSE:~ # cupsdisable HPLJ2

openSUSE:~ # lpstat is –t

scheduler is running

system default destination: HPLJ2

device for HPLJ2: parallel:/dev/lp0

HPLJ2 accepting requests since Fri 13 May 2011 11:15:28 AM MDT

printer HPLJ2 disabled since Fri 13 May 2011 11:15:28 AM MDT -

Paused




The print queue will continue to accept jobs, but none of them will be
printed until I enter



cupsenable HPLJ2



at the shell prompt.

The cupsenable command also includes the --release option to release pending jobs for printing.





LX0-104 Exam Objectives (Q)

Printing VI

Configuring a CUPS Printer


All CUPS printers are defined in the


/etc/cups/printers.conf 


file.



Although you can manually edit this file, you really should use the
CUPS web-based administration utility instead. Configuring a CUPS
printer is a snap with it. You can either configure CUPS to service a
locally attached printer (and optionally make it available to other
network users) or connect to a CUPS printer over the network. For
example, to configure CUPS to use a locally attached printer, do the
following:




1) On your Linux system, start a web browser and navigate to
http://localhost:631.



2) Select Administration. The screen in Figure 16-5 is displayed.



3) Under Printers, select Add Printer.



4) When prompted, log in as the administrative user you created
previously. The screen in Figure 16-6 is displayed.



5) Select a locally attached printer type under Local Printers and
then select Continue. A screen similar to that shown in Figure 16-7 is
displayed.



TIP You could also select a network printer in this screen. All
broadcasting CUPS printers on other network hosts are listed under
Discovered Network Printers. To send print jobs to one of these
printers, just select it.




6) In the Name field, enter a name for the printer.



7) In the Description field, enter a description of the printer.



8) In the Location field, enter a location for the printer.



9) If you want to share the printer with other network users, mark
    Share This Printer.



10) Select Continue. The screen in Figure 16-8 is displayed.



11) Select the printer manufacturer; then select Continue.



12) In the Model field, select your printer model; then select Add Printer.



13) Configure your default options for the printer, such as paper
size, color model, media source, print quality, two-sided printing,
and so on. When complete, select Set Default Options.







At this point, a page is displayed indicating your printer has been
added. The current status of your printer is displayed, similar to
that shown in Figure 16-9.



From the Printer Status page, you can manage your CUPS printer. You
can send a test page, stop the printer, kill a print job, modify the
printer configuration, or delete the printer altogether. At this
point, you can send print jobs to the printer. If you’re using a
graphical X application, you can simply select File | Print; then
select the printer and click OK. You can also send print jobs from the
command line to the printer. This is done using the lp command, which
will send a specified file to the printer. The syntax for using lp is




lp –d printer_name filename




For example, if I wanted to print the myfiles file in the current
directory to the HPLJ2 printer I just created, I

would enter




lp –d HPLJ5 ./myfiles




at the shell prompt, as shown here: openSUSE:~ # lp -d HPLJ2 ./myfiles

request id is HPLJ2-2 (1 file(s))





As you can see in this example, the

job is created and assigned an ID (in this case, HPLJ2-2). The job is
added to the print queue and sent to the printer. The lp utility
includes a variety of options besides –d that you can use to create
print jobs, including the following:




• –n x Prints x number of copies

• –m E-mails a confirmation message to my local user account when the
   job is finished printing


• –q x Sets the priority of the print job to x

• –o landscape Prints the file landscape instead of portrait

• –o sides=2 Prints the file double-sided on a printer that supports duplexing




You can also configure other Linux systems to print to the CUPS
printer. Simply configure a new printer, but specify that it listen
for CUPS announcements. The CUPS printer you configured should be
displayed within 30 seconds. After you select it, all print jobs sent
to that printer will be redirected over the network connection to your
CUPS printer.




In addition, if you’ve installed Samba on your system, your CUPS
printers are automatically shared. You can connect to them from
Windows workstations and submit print jobs. Now that’s cool!








LX0-104 Exam Objectives (Q)

Printing V

Configuring the CUPS Service


The CUPS service is configured using several text files within the
/etc/cups directory.


The


/etc/cups/cupsd.conf



file is the main configuration file you will use to configure the cupsd
daemon (calledt he scheduler). Remember that cupsd is also an HTTP server,
like Apache. Accordingly, the cupsd.conf file is very similar to the Apache
web server configuration file.




A sample cupsd .conf file is shown in Figure 16-4.



Figure 16-4 only shows a very small portion of the cupsd.conf file,
which is quite long. The cupsd.conf file is composed of many server
directives, which specify how cupsd operates. We don’t have the time
or space in this book to cover all the configuration options in
cupsd.conf. I’m just going to cover the most important ones here. For
more information, see the man page for cupsd.conf. You can also open



http://localhost:631/help/ 


in a browser on your Linux system to see an extensive list of cupsd.conf directives,
or you can visit



http://www.cups.org/documentation.php/ref-cupsd-conf.html 



Some of the more useful cupsd.conf directives include those shown in Table
16-1.




The way you configure cupsd.conf will largely depend on the particular
network you are implementing the system in. The good news is that you
don’t need to do much with cupsd.conf to configure a basic
implementation that provides local printing.



However, if you want other Linux systems to be able to print through
your CUPS printer, you must


enable BrowseAddress 


or else CUPS won’t announce its printers on the network. This directive is
not enabled by default on many distributions. Sample configurations for this
directive include the following:



BrowseAddress 255.255.255.255:631

BrowseAddress 192.168.1.255:631

BrowseAddress mydom.com:631

BrowseAddress @LOCAL




The BrowseAddress directive is usually set to a value of @LOCAL.



This causes CUPS to send printer announcement broadcasts to all local
network interfaces in the system.


You can set this directive to


@IF(interface_name) 



to limit broadcasts to a specific network interface.




After making any changes to cupds.conf, be sure to restart the cupsd
daemon. 



After configuring your cupsd.conf file, you next need to set
up a Linux user account that will be used as the CUPS administrator.


CUPS does not use the same user accounts that your Linux system uses.


Instead, CUPS is configured to use the


/etc/cups/passwd.md5 


file to store user accounts.


To create an administrative user in the passwd.md5 file named root that
is a member of the CUPS administration group named sys, you would enter



lppasswd –g sys –a root









LX0-104 Exam Objectives (Q)

Printing IV

Configuring CUPS


The CUPS service appears complicated, and under the hood, it is.
Fortunately, the developers who wrote CUPS made it very easy for you
and me to configure and manage. In this part of this chapter, you’re
going learn how to configure CUPS by learning about the following
topics:




• Configuring the CUPS service
• Configuring a CUPS printer
• Using command-line tools to manage CUPS




Let’s begin by discussing how to configure the cupsd daemon.








LX0-104 Exam Objectives (Q)

MTA Management Commands: mail III

Using MTA Management Commands on Linux


Let’s first look at reading messages stored in your local MTA. When
you log in to a shell session, you will receive a notification if
there are mail messages waiting for you. You can read messages for
local users from the local MTA directly from the command line using
the mail command at the shell prompt. When you do, a list of messages
is displayed. An example is shown in Figure 16-15.



Some services running on Linux are configured to send notification
messages to the root user.



These messages are stored in your user’s mail queue, which is located
in the


/var/spool/mail/ 


directory. The mail utility reads yourmessages directly out of your user’s queue file.
Because you’rerunning the mail utility on the same system where your queue resides,
you don’t need POP3 or IMAP support configured. You can enter the mailcommands shown in
Table 16-4 at the ? prompt.



An example of viewing a received message with the t command is shown
in Figure 16-16.



To send a message, you can also enter


mail recipient_address


at the shell prompt. You can then enter a subject line and the text of
your message. Press

ctrl-d 

when you’re done to actually send the message. When you do, the message is delivered to the other user’s mail queue by your local MTA.


To view a list of unread messages in your mail queue, you can enter


mailq


at the shell prompt.



In addition to mail, many other packages are available that you can
install to read mail from the shell prompt. The key thing to remember
is that the user must run the mail command from the local shell
prompt. If the user isn’t using the local computer system, they must
ssh into the system to read mail.


You can also configure aliases for the MTA running on your Linux
system. Mail aliases redi- rect mail addressed to one user to another
user’s account. You use the

/etc/aliases 


file to configure aliases.


This file defines one alias per line. The alias you define must point
to an existing e-mail address. The syntax for this file follows:


alias: list of real e-mail addresses (separated by commas)


For example, the following two aliases must be present in this file on
most Linux distributions:


postmaster:  root

mailer-daemon:     postmaster



These aliases cause any e-mail messages sent to the postmaster to be
automatically redirected to the root user. Likewise, any e-mail
messages sent to mailer-daemon will be redirected to post- master
(which will then be redirected to root). Depending on your
distribution, you will probably find that many aliases are defined for
you by default.


Here is an example:

# General redirections for pseudo accounts in /etc/passwd.

administrator:     root

daemon:     root

lp:     root

news:     root

uucp:     root

games:     root

man:     root

at:     root

postgres:     root

mdom:     root

amanda:     root

ftp:     root

wwwrun:     root

squid:     root

msql:     root

gnats:     root

nobody:     root


# "bin" used to be in /etc/passwd

bin:         root


Of course, you can enter your own custom aliases if needed. Just open
the aliases file in a text editor and add the appropriate aliases, one
per line. When done configuring aliases, you must run the



newaliases



command at the shell prompt as root to enable them.



You can also use the



~/.forward 



file in your user’s home directory to configure forwarding. Most Linux MTAs 
check for the existence of this file in the user’s home directory to configure 
forwarding of messages.



You can open/create the file in a text editor and enter the e-mail
address to which you want to forward e-mail. If you are forwarding to
a local user, just enter the username. If you’re forwarding to a
remote user account, enter username@domain.com. If you need to forward
messages to multiple recipients, separate them with a comma.



The MTA will treat the addresses you enter in this file as an alias.
This causes all e-mail to be forwarded to the forwarding e-mail
address. Messages will not be delivered to the original user’s
mailbox.





LX0-104 Exam Objectives (P)

Tunnel Your X server traffic

Tunnel Your X server traffic


You can also tunnel your X server traffic to remote X clients using an
SSH connection. This is important because unencrypted X traffic
provides an attacker with a gold mine of information that he or she
can use to compromise your systems.


To configure a remote X client without encryption, you can use the
following procedure:



1) On the remote

    X client, enter xhost +X_server_hostname.

    This tells  the client to accept connections from the X server.


2) On the X server, enter

    DISPLAY=X_client_hostname:0.0 

    and then enter

    export DISPLAY

    This tells the X server to display its output on the remote X client.


3) From the X client, use the ssh client to access the shell prompt on
   the X server and then run the graphical application you want displayed
   on the X client. For example, you could enter gedit at the shell
   prompt to remotely display the gedit text editor. You could also enter
   office at the shell prompt to remotely display the OpenOffice.org suite.



This procedure works, but all the X traffic is transmitted unencrypted. This isn’t good.

Instead, you should use SSH to tunnel the X server traffic between the X server and the X client.

You can do this using one of the following options:


• Use the –X option with the ssh client program.

• Set the ForwardX11 option to a value of yes in the
  /etc/ssh/ssh_config file on the X client system.



Once this is done, you then need to set the X11Forwarding option to
yes in the /etc/ssh/ sshd_config file on the X server system.



LX0-104 Exam Objectives (H)

Manipulating SQL Data

Manipulating SQL Data


After installing your MySQL packages, you next need to set up your
MySQL server’s granttables. All MySQL access controls are managed from within the MySQL
service itself. It’s important to understand that MySQL has its own unique set of user accounts
defined in its grant tables; it doesn’t use the accounts defined on your Linux system. Five tables
are implemented within the MySQL database to do this:


user

Specifies whether a user is allowed to connect to the MySQL server


db

Defines which databases a user is allowed to access


host

Specifies which hosts are allowed to access a particular database


tables_priv

Defines access privileges for a given table


columns_priv

Specifies access privileges for specific columns of data for a given table




These tables must be initialized before you can use MySQL. This is
done by changing to the

/usr/bin directory

and running the

mysql_install_db

command at the shell prompt.


With the grant tables created, you next need to start the database
service.

If your distribution uses init, you can use the mysql init script located in your init
script directory. Then you can use the insserv or chkconfig

command to ensure the database service starts
every time the system

boots. If your distribution uses systemd, you can use the systemctl
command to enable and start the mysql service.

To verify that the server is running, you can enter the

mysqladmin version

command at the shell prompt.


you can view the databases currently on the server by entering

mysqlshow

 at the command prompt.


You can also view the tables within any of the databases shown in the
output using the

mysqlshow table_name

 command.


Now that MySQL is running properly, you need to assign passwords to
your MySQL user accounts.

After you run msql_install_db, your root database user account has been created but has no password assigned. To remedy this, enter


mysqladmin –u root password  ‘your_new_password'



 at the shell prompt. Then restrict root access to the system where MySQL is running

by entering


mysqladmin –u root –h system_hostname password  mysql_root_user_password


at the shell prompt.


LX0-104 Exam Objectives (G)

Commands to manage data in an SQL database

Managing SQL Data


You can use the following commands to manage data in an SQL database:

• SELECT

  Retrieves information from a table


• UPDATE

  Modifies information in a table


• DELETE

  Removes information from a table


• INSERT INTO

  Adds new data to a table


• CREATE TABLE

  Creates a new table


• ALTER TABLE

  Modifies an existing table


• DROP TABLE

  Deletes and existing table



A key feature of relational databases is the fact that you can create

relationships between tables, which allows you to create interrelated data sets.




LX0-104 Exam Objectives (G)

find but exclude directory and exclude user

find . ! -path "/path/folder/*" -name "*" ! -user kenmsipe -ls

find all files that are not located in /path/folder/* and do not have a user of kenmsipe

fsck options

fsck options



 

y



a



r



n



 


-y     For  some filesystem-specific checkers, the -y option will cause the fs-specific fsck to always
attempt to fix any detected filesystem corruption automatically.  Sometimes an  expert  may  be able  to  do  better driving the fsck manually.  Note that not all filesystem-specific checkers
implement this option.  In particular fsck.minix(8) and fsck.cramfs(8) do not  support  the  -y
option as of this writing.






-a     Automatically repair the filesystem without any questions (use this option with caution).  Note that  e2fsck(8) supports -a for backward compatibility only.  This option is mapped to e2fsck's -p option which is safe to use, unlike the -a option that some filesystem checkers support.








-r     Interactively  repair the filesystem (ask for confirmations).  Note: It is generally a bad idea
to use this option if multiple fsck's are being run  in  parallel.   Also  note  that  this  is
e2fsck's default behavior; it supports this option for backward compatibility reasons only.






-n     For some filesystem-specific checkers, the -n option will cause the fs-specific fsck  to  avoid
attempting  to repair any problems, but simply report such problems to stdout.  This is however not true for all filesystem-specific checkers.  In particular, fsck.reiserfs(8) will not report
any corruption if given this option.  fsck.minix(8) does not support the -n option at all.

Shell commands you can use to manage kernel modules

Shell commands you can use to manage kernel modules. These include the following:

lsmod  {Views loaded kernel modules}

modinfo {Views module information}

depmod  {Builds a module dependency list}

insmod {Installs a kernel module but doesn’t factor in module dependencies}

modprobe {Installs or removes a kernel module while taking module dependencies into account}

rmmod {Removes a kernel module but doesn’t factor in module dependencies}

Command-line tools to view information about the hardware

You can also use the following command-line tools to view information about the hardware in your system:

hdparm /dev/device

sg_scan

sginfo –l

hwinfo

lshw

lsusb

lspci

Systemctl

https://www.freedesktop.org/wiki/
 www/ Software/ systemd/   FrequentlyAskedQuestions 


Frequently Asked Questions
Also check out the Tips & Tricks!

Q: How do I change the current runlevel?
A: In systemd runlevels are exposed via "target units". You can change them like this:
# systemctl isolate runlevel5.target

Note however, that the concept of runlevels is a bit out of date, and it is usually nicer to use modern names for this. e.g.:
# systemctl isolate graphical.target

This will only change the current runlevel, and has no effect on the next boot.

Q: How do I change the default runlevel to boot into?

A: The symlink /etc/systemd/system/default.target controls where we boot into by default. Link it to the target unit of your choice. For example, like this:
# ln -sf /usr/lib/systemd/system/multi-user.target /etc/systemd/system/default.target

or
# ln -sf /usr/lib/systemd/system/graphical.target /etc/systemd/system/default.target

Q: How do I figure out the current runlevel?

A: Note that there might be more than one target active at the same time. So the question regarding the runlevel might not always make sense. Here's how you would figure out all targets that are currently active:
$ systemctl list-units --type=target

If you are just interested in a single number, you can use the venerable runlevel command, but again, its output might be misleading.

Q: I want to change a service file, but rpm keeps overwriting it in /usr/lib/systemd/system all the time, how should I handle this?
A: The recommended way is to copy the service file from /usr/lib/systemd/system to /etc/systemd/system and edit it there. The latter directory takes precedence over the former, and rpm will never overwrite it. If you want to use the distributed service file again you can simply delete (or rename) the service file in /etc/systemd/system again.

Q: My service foo.service as distributed by my operating system vendor is only started when (a connection comes in or some hardware is plugged in). I want to have it started always on boot, too. What should I do?

A: Simply place a symlink from that service file in the multi-user.target.wants/ directory (which is where you should symlink everything you want to run in the old runlevel 3, i.e. the normal boot-up without graphical UI. It is pulled in by graphical.target too, so will be started for graphical boot-ups, too):
# ln -sf /usr/lib/systemd/system/foobar.service /etc/systemd/system/multi-user.target.wants/foobar.service
# systemctl daemon-reload

Q: I want to enable another getty, how would I do that?

A: Simply instantiate a new getty service for the port of your choice (internally, this places another symlink for instantiating another serial getty in the getty.target.wants/ directory).
# systemctl enable serial-getty@ttyS2.service
# systemctl start serial-getty@ttyS2.service

Note that gettys on the virtual console are started on demand. You can control how many you get via the NAutoVTs= setting in logind.conf(7). Also see this blog story.

Q: How to I figure out which service a process belongs to?
A: You may either use ps for that:
$ alias psc='ps xawf -eo pid,user,cgroup,args'
$ psc
...

Or you can even check /proc/$PID/cgroup directly. Also see this blog story.

Q: Why don't you use inotify to reload the unit files automatically on change?

A: Unfortunately that would be a racy operation. For an explanation why and how we tried to improve the situation, see the bugzilla report about this.

Q: I have a native systemd service file and a SysV init script installed which share the same basename, e.g. /usr/lib/systemd/system/foobar.service vs. /etc/init.d/foobar -- which one wins?

A: If both files are available the native unit file always takes precedence and the SysV init script is ignored, regardless whether either is enabled or disabled. Note that a SysV service that is enabled but overridden by a native service does not have the effect that the native service would be enabled, too. Enabling of native and SysV services is completely independent. Or in other words: you cannot enable a native service by enabling a SysV service by the same name, and if a SysV service is enabled but the respective native service is not, this will not have the effect that the SysV script is executed.

Q: How can I use journalctl to display full (= not truncated) messages even if less is not used?
A: Use:
# journalctl --full

Q: Whenever my service tries to acquire RT scheduling for one of its threads this is refused with EPERM even though my service is running with full privileges. This works fine on my non-systemd system!

A: By default, systemd places all systemd daemons in their own cgroup in the "cpu" hierarchy. Unfortunately, due to a kernel limitation, this has the effect of disallowing RT entirely for the service. See My Service Can't Get Realtime! for a longer discussion and what to do about this.

Q: My service is ordered after network.target but at boot it is still called before the network is up. What's going on?

A: That's a long story, and that's why we have a wiki page of its own about this: Running Services After the Network is up

Q: My systemd system always comes up with /tmp as a tiny tmpfs. How do I get rid of this?
A: That's also a long story, please have a look on API File Systems
 Last edited Sun May 26 10:17:02 2013

Processing Text Streams

Processing Text Streams

When you’re processing text streams within a script or when piping output at the shell prompt, there may be times when you need to filter the output of one command so that only certain portions of the text stream are actually passed along to the stdin of the next command. You can use a variety of tools to do this. In the last part of this chapter, we’ll look at using the following commands:

•cut

•expand and unexpand

•fmt

•join and paste

•nl

•od

•pr    

•sed and awk

•sort

•split

•tr

•uniq

•wc

cut

The cut command is used to print columns or fields that you specify from a file to the standard output. By default, the tab character is used as a delimiter. The following options can be used with cut:

–blist Select only these bytes.

–clist Select only these characters.

–ddelim Use the specified character instead of tab for the field delimiter.

–flist Select only the specified fields. Print any line that contains no delimiter character,

unless the –s option is specified.

–s Do not print lines that do not contain delimiters.

For example, you could use the cut command to display all group names from the /etc/group file. Remember, the name of each group is contained in the first field of each line of the file.

However, the group file uses colons as the delimiter between fields, so you must specify a colon instead of a tab as the delimiter. The command to do this is cut –d: –f1 /etc/group

.

expand and unexpand

The expand command is used to process a text stream and remove all instances of the tab character and replace them with the specified number of spaces (the default is eight). You can use the –t number option to specify a different number of spaces. The syntax is

expand –t number filename

.

In Figure 14-3, the tab characters in the tabfile file are replaced with five spaces.

You can also use the unexpand command. The unexpand command works in the opposite manner as the expand command. It converts spaces in a text stream into tab characters. By default, eight contiguous spaces are converted into tabs. However, you can use the –t option to specify a different number of spaces.

It’s important to note that, by default, unexpand will only convert leading spaces at the beginning of each line. To force it to convert all spaces of the correct number to tabs, you must include the –a option with the unexpand command.

fmt

You can use the fmt command to reformat a text file. It is commonly used to change the wrapping of long lines within the file to a more manageable width. The syntax for using fmt is fmt option filename

For example, you could use the –w option with the fmt command to narrow the text of a file

to 80 columns by entering fmt –w 80 filename

join and paste

The join command prints a line from each of two specified input files that have identical join fields. The first field is the default join field, delimited by white space. You can specify a different join field using the –j field option.

For example, suppose you have two files. The first file (named firstnames) contains the following content:

1 Mike

2 Jenny

3 Joe

The second file (named lastnames) contains the following content:

1 Johnson

2 Doe

3 Jones

You can use the join command to join the corresponding lines from each file by entering

join –j 1 firstnames lastnames

. This is shown here:

rtracy@openSUSE:~> join -j 1 firstnames lastnames

1 Mike Johnson

2 Jenny Doe

3 Joe Jones

The paste command works in much the same manner as the join command. It pastes togethe

corresponding lines from one or more files into columns. By default, the tab character is used to

separate columns. You can use the –dn option to specify a different delimiter character. You can also use the –s option to put the contents of each file into a single line.

For example, you could use the paste command to join the corresponding lines from the firstnames and lastnames files by entering

paste firstnames lastnames

. An example is shown here:

rtracy@openSUSE:~> paste firstnames lastnames

1 Mike   1 Johnson

2 Jenny  2 Doe

3 Joe    3 Jones

nl

The nl command determines the number of lines in a file. When you run the command, the

output is written with a line number added to the beginning of each line in the file. The syntax is nl filename

For example, in the example shown here, the nl command is used to add a number to the beginning of each line in the tabfile.txt file:

rtracy@openSUSE:~> nl tabfile.txt

     1 This file uses tabs.

     2         This line used a tab.

     3         This line used a tab.

     4 After using expand, the tabs will be replaced with spaces.

od

The od (octal dump) command is used to dump a file, including binary files. This utility can

dump a file in several different formats, including octal, decimal, floating point, hex, and character format. The output from od is simple text, so you can use the other stream-processing tools we’ve been looking at to further filter it.

The od command can be very useful. For example, you can perform a dump of a file to locate stray characters in a file. Suppose you created a script file using an editor on a different operating system (such as Windows) and then tried to run it on Linux. Depending on which editor you used, there may be hidden formatting characters within the script text that aren’t displayed by your text editor. However, they will be read by the bash shell when you try to run the script, thus causing errors. When you look at the script in an editor, everything seems fine.

You could use the od command to view a dump of the script to isolate where the problem-

causing characters are located in the file. The syntax for using od is od options filename

. Some of the more commonly used options include the following:

–b  Octal dump

–d  Decimal dump

–x  Hex dump

–c  Character dump

For example, “Hello World” script has been created in the LibreOffice word processor and saved as an .odt file. As such, it has a myriad of hidden characters embedded in the text.

These characters obviously cannot be viewed from within LibreOffice. However, they can be viewed using the od –c helloworld.odt command.

pr

The pr command is used to format text files for printing. It formats the file with pagination, headers, and columns. The header contains the date and time, filename, and page number. You can use the following options with pr:

–d  Double-space the output.

–l  page_length  Set the page length to the specified number of lines. The default is 66.

–o margin  Offset each line with the specified number of spaces. The default margin is 0.

sed and awk

The sed command is a stream text editor. Unlike the interactive text editors that you’ve already learned how to use in this book, such as vi, a stream editor takes a stream of text as its stdin and then performs operations on it that you specify. Then, sed sends the results to stdout. You can use the following commands with sed:

s  Replaces instances of a specified text string with another text string. The syntax for

using the s command is sed s/term1/term2/

For example,  I’ve used the cat command to display a file in the tux user’s home directory named lipsum.txt. I then use cat to read lipsum.txt and then pipe the stdout to the stdin of the sed command and specify that the term “ipsum” be replaced with “IPSUM.”

d  Deletes the specified text. For example, to delete every line of text from the stdin that

contains the term “eos,” you would enter sed /eos/d

.

Remember, sed doesn’t actually modify the source of the information—in this case, the lipsum.txt file. It takes its stdin, makes the changes, and sends it to the stdout. If you want to save the changes made by sed, you need to redirect its stdout to a file using >

For example, I could redirect the output from the command in Figure 14-8 to a file named lipsum_out.txt by entering cat lipsum.txt | sed s/ipsum/IPSUM/ > lipsum_out.txt at the shell prompt.

In addition to sed, you can also use awk to manipulate output. Like sed, awk can be used to

receive output from another command as its stdin and manipulate it in a manner you specify.

However, the way awk does this is a little bit different. The awk command treats each line of text it receives as a record. Each word in the line, separated by a space or tab character, is treated as a separate field within the record.

For example, consider the following text file:

Lorem ipsum dolor sit amet, consectetur adipisicing elit,

sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.

Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris

nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in

reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.

Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia

deserunt mollit anim id est laborum

According to awk, this file has seven records because it has seven separate lines of text. Each line of text has a carriage return/linefeed character at the end that creates a new line. This is the character awk uses to define the end of a record. The first record has eight fields, the second record has 11 fields, and so on.

Notice that white space, not punctuation, delimits the fields. Each field is referenced as $

field_number.

For example, the first field of any record is referenced as $1, the second as $2, and so on.

Using awk, we can specify a field in a specific record and manipulate it in some manner. The syntax for using awk is awk ‘pattern{manipulation}’

For example, we could enter

cat lipsum2.txt | awk ‘{print $1,$2,$3}’

to print out the first three words (“fields”) of each line (“records”).

Because we didn’t specify a pattern to match on, awk simply prints out the first three words of every line.

You can also include a pattern to specify exactly which records to search on. For example,

suppose we only wanted to display the first three fields of any record that includes the text “do” somewhere in the line. To do this, you add a pattern of /do/ to the command.

You can also add your own text to the output. Just add it to the manipulation part of the command within quotes. In fact, you can also add control characters to output as well. Use the following:

\t  Inserts a tab character

\n  Adds a newline character

\f  Adds a form feed character

\r  Adds a carriage return character

For example, in Figure 14-11, I’ve entered

cat lipsum.txt | awk ‘/do/ {print "Field 1: "$1"\t", "Field 2: "$2"\t",

"Field 3: "$3"\t"}’

which causes each field to be labeled Field 1, Field 2, and Field 3

.

 It also inserts a tab character between each field. As with sed, awk doesn’t modify the original file. It sends its output to stdout (the screen). If you want to send it to a file, you can redirect it using >

sort

The sort command sorts the lines of a text file alphabetically. The output is written to the standard output. Some commonly used options for the sort command include the following:

–f Fold lowercase characters to uppercase characters.

–M  Sort by month.

–n  Sort numerically.

–r  Reverse the sort order.

For example, the sort –n –r firstnames

command sorts the lines in the firstnames file numerically in reverse order. This is shown here:

rtracy@openSUSE:~> sort –n –r firstnames

3 Joe

2 Jenny

1 Mike

The sort command can be used to sort the output of other commands (such as ps) by piping

the standard output of the first command to the standard input of the sort command.

split

The split command splits an input file into a series of files (without altering the original input file). The default is to split the input file into 1,000-line segments. You can use the –n option to specify a different number of lines.

For example, the split –1 firstnames outputfile_ command can be used to split the firstnames file into three separate files, each containing a single line.

tr

The tr command is used to translate or delete characters. However, be aware that this command does not work with files. To use it with files, you must first use a command such as cat to send the text stream to the standard input of tr. The syntax is

tr options X Y

Some commonly used options for the tr command include the following:

–c  Use all characters not in X.

–d  Delete characters in X; do not translate.

–s  Replace each input sequence of a repeated character that is listed in X with a single occurrence of that character.

–t  First truncate X to the length of Y.

For example, to translate all lowercase characters in the lastnames file to uppercase characters, you could enter cat lastnames | tr a-z A-Z

, as shown in this example:

rtracy@openSUSE:~> cat lastnames | tr a-z A-Z

1 JOHNSON

2 DOE

3 JONES

uniq

The uniq command reports or omits repeated lines. The syntax is

uniq options input output

 You can use the following options with the uniq command:

–d  Only print duplicate lines.

–u  Only print unique lines.

For example, suppose our lastnames file contained duplicate entries:

1 Johnson

1 Johnson

2 Doe

3 Jones

You could use the uniq lastnames command to remove the duplicate lines. This is shown in

the following example:

rtracy@openSUSE:~> uniq lastnames

1 Johnson

2 Doe

3 Jones

Be aware that the uniq command only works if the duplicate lines are adjacent to each other.

If the text stream you need to work with contains duplicate lines that are not adjacent, you can use the sort command to first make them adjacent and then pipe the output to the standard input of uniq.

wc

The wc command prints the number of newlines, words, and bytes in a file. The syntax is

wc options files

. You can use the following options with the wc command:

–c  Print the byte counts.

–m  Print the character counts.

–l  Print the newline counts.

–L  Print the length of the longest line.

–w  Print the word counts.

For example, to print all counts and totals for the firstnames file, you would use the

wc firstnames

 command, as shown in this example:

rtracy@openSUSE:~> wc firstnames

  3 6 21 firstnames