Operating Systems and Linux II

7/25/2019 Operating Systems and Linux II

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Operating Systems and Linux

Partha Sarathi Dasgupta

MIS GroupIndian Institute of Management Calcutta


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Partitions

Major partitions on a Linux system:data partition

: normal Linux system data, including the root partitioncontaining allthe data to start up and run the system

swap partition: expansion of the computer's physical memory, extra memory on harddisk

!sually, systems contain a root partition, one or more data partitions and one or more

s"ap partitions

fdisk

#"ap space is only accessi$le for the system itself, and is hidden from %ie" duringnormal operation &ny pro$lem "ith using theswappartition

(ypically,system datais separate from user data Programs that offer ser%ices are kept in a different place than the data handled $y this ser%ice )ifferent partitions created:

a partition "ith all data necessary to $oot the machinea partition "ith configuration data and ser%er programsone or more partitions containing the data$ase ta$les, user mails, an ftp archi%e etca partition "ith user programs and applicationone or more partitions for the user specific files *home directories+one or more s"ap partitions


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-o" to check the mounted Partitions

nettech:> df -h .Filesystem Size Used Avail Use% Mounted on/dev/hda7 9!M "#$M7#7M "% /

/dev/hda 9#M "$M &M $9% //dev/hda" "&M .M "!9M % /'/dev/hda( "9) "() &.7) (% /o*t/dev/had# 7.!) (.) ".&) "% /us+/dev/had7 $.7) &.7) #7M 77% /va+


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/nit process

(he kernel, once it is loaded, finds initin s$in and executes it 0hen initstarts,

it $ecomes the parent or grandparent of all of the processes that start upautomatically on Linux system

(he first thing initdoes, is reading its initialiation file, etcinitta$ (his

instructs initto read an initial configuration script for the en%ironment, "hich

sets the path, starts s"apping, checks the file systems, and so on 3asically, this

step takes care of e%erything that system needs to ha%e done at system

initialiation: setting the clock, initialiing serial ports and so forth

(hen initcontinues to read the etcinitta$ file, "hich descri$es ho" the system

should $e set up in each run le%el and sets the default run level

& run le%el is a configuration of processes &ll !4/56like systems can $e run

in different process configurations, such as the single user mode, "hich is

referred to as run le%el 1 or run le%el # *or s+ /n this mode, only the system

administrator can connect to the system /t is used to perform maintenance

tasks "ithout risks of damaging the system or user data


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/nit process

0hat is init 8


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-o" login is initiated

(he idea $ehind operating different ser%ices at different run le%els essentially re%ol%es

around the fact that different systems can $e used in different "ays #ome ser%ices cannot

$e used until the system is in a particular state, or mode, such as $eing ready for more

than one user or ha%ing net"orking a%aila$le

(here are times in "hich you may "ant to operate the system in a lo"er mode 9xamples

are fixing disk corruption pro$lems in run le%el 1 so no other users can possi$ly $e on

the system, or lea%ing a ser%er in run le%el "ithout an 5 session running /n these cases,running ser%ices that depend upon a higher system mode to function does not make sense

$ecause they "ill not "ork correctly any"ay 3y already ha%ing each ser%ice assigned

to start "hen its particular run le%el is reached, you ensure an orderly start up process,

and you can uickly change the mode of the machine "ithout "orrying a$out "hich

ser%ices to manually start or stop

Shutdown

!4/5 "as not made to $e shut do"n, $ut if you really must, use the shutdowncommand

&fter completing the shutdo"n procedure, the 6h option "ill halt the system, "hile 6r "ill

re$oot it


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Links

-ard links to a file

#oft links to a file

#!/)


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/nodes in =ile systems

/n a file system, a file is represented $y an inode, a serial num$er

9%ery partition has its o"n set of inodes> throughout a system "ith

multiple partitions, files "ith the same inode num$er can exist

9ach inode descri$es a data structure on the hard disk

0hen a hard disk is initialied to accept data storage, usually during the

initial system installation process or "hen adding extra disks to an existing

system, a fixed num$er of inodes per partition is created

(his num$er "ill $e the maximum amount of files, of all types *including

directories, special files, links etc+ that can exist at the same time on the

partition


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/node )ata #tructure

At the time a ne, le is c+eated it ets a f+ee inode.

0node contains the follo,in info+mation:

12,ne+ and +ou* o,ne+ of the le.

1File ty*e 3+eula+ di+ecto+y ...4

15e+missions on the le

16ate and time of c+eation last +ead and chane.

16ate and time this info+mation has 'een chaned in the

inode.

1um'e+ of lin8s to this le

1File size

1An add+ess denin the actual location of the le data.


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Path names and =iles

&$solute path names

Aelati%e path names

Bopy a file or create a link to a file

)eleting files and directories


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)efault files

#tandard input

#tandard Cutput

#tandard error


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Linux Processes: 3irth and )eath


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Linux Processes: #ome facts

#tate: (he execution state of the process *executing, ready,

suspended, stopped, om$ie+

#cheduling information: /nformation needed $y Linux to

schedule processes & process can $e normalor real-timeand

has a priority Aeal6time processes are scheduled $efore normal

processes, and "ithin each category, relati%e priorities can $e

used & counter keeps track of the amount of time a process is

allo"ed to execute

/dentifiers: 9ach process has a uniue identifier and also has

user and group identifiers


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Linux Processes: #ome facts

/PB: Processes can interact "ith each other

Links: 9ach process includes a link to its parent process, links

to its si$lings, and links to all of its children

(imes and timers: includes process creation time and the

amount of process time so far consumed $y a process

=ile system: pointers to any file opened $y this process, as "ell

as pointers to the current and root directories for this process

&ddress space: Dirtual address space assigned to the process

Processor6specific context: Aegisters and stack info

constituting the context of the process


15/25

Linux Process #tates

Running: Process is either running or Aeady to run

Interruptible: & $locked state and process is "aiting for an e%ent, such as

end of an /C operation, a%aila$ility of a resource, or asignal from another

process

Uninterruptible: & $locked state Process is "aiting directly on hard"are

conditions and "ill not handle anysignals

Stopped: Process halted and can $e restarted only $y some other process

9g, a process that is $eing de$ugged can $e stopped

Zombie: Process has $een terminated, $ut its structural information is still

in the process ta$le


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Linux Process Bycle

#topped

Aeady 9xecuting Eom$ie

!ninterrupti$le

/nterrupti$le

#cheduling

Aunning state

#ignal#ignal

9%ent

#ignal or 9%ent

(erminationBreation


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/nput6Cutput Aedirection

Cutput redirection "ith F and G

Aedirecting HnothingH to an existing file is eual to emptying

the file

same redirection to an nonexistent file "ill create a ne" empty

file "ith the gi%en name

(o find a "ord "ithin some text, display all lines matching

pattern1, or exclude lines also matching pattern2 from $eing

displayed:

greppattern1file | grep -vpattern2


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/nput6Cutput Aedirection

(o display output of a directory listing one page at a time:

ls 6la G less

(o find a file in a directory:

ls 6l G greppart_of_file_name

"ho G "c Il

aspell J texttxt F errorlog

cat afile FF $file

=ile descriptors

ls F dirlist 2FK1

set 6o noclo$$er


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&utomatic Processes: )aemons

)aemons are ser%er processes that run continuously

Most of the time, they are initialied at system startup and

then "ait in the $ackground until their ser%ice is reuired

& typical example is the net"orking daemon,xinetd, "hich is

started in almost e%ery $oot procedure &fter the system is

$ooted, the net"ork daemon just sits and "aits until a client program, such as an =(P client, needs to connect


20/25

Processes

at [options] time

9xecute commands at a specified timeand optional date (he commands are taken

from standard input or from a file

9xample: at 1:@@ am tomorro" J scriptfile

3y default, only a pri%ileged user can execute the command

Aole of script files, eg, etcprofile


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Bron and Bronta$

(he cron system is managed $y the cron daemon /t gets information a$out "hich programs and "hen to run them from the

system's and users' cronta$ entries

Cnly the root user has access to the system cronta$s, "hile each user

should only ha%e access to his o"n cronta$s Cn some systems *some+users may not ha%e access to the cron facility

&t system startup the cron daemonsearches %arspoolcron for cronta$

entries "hich are named after accounts in etcpass"d, it searches

etccrond, etccronta$, then uses this information e%ery minute to check if there is something to $e done /t executes commands as the user "ho

o"ns the cronta$ file and mails any command output to the o"ner


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Bron and Bronta$

!sers are supposed to edit their cronta$s in a safe "ay using the crontab e

command (he default editor is vi

(his cronta$ entry reminds $o$ to go to his sports clu$ e%ery (hursdaynight:

$o$:F cronta$ 6l

N *tmpcronta$1


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(BP/P (esting and (rou$leshooting

ping

(he /P protocol includes control messages called */nternet Bontrol Message

Protocol+ /BMP packets Cne type of /BMP packet is called an echo reuest,''

and the /P rules reuire its recipient to send $ack an echo reply'' (hese are

incredi$ly useful $ecause you can determine *1+ "hether the remote host is up

and talking to the net"ork, *2+ the time reuired for a packet to make a round6trip to the host, and *+ *$y sending a fe" doen echo reuests+ "hat fraction of

the packets sent $et"een the hosts get lost some"here along the "ay (heping

command *named after the sound of an acti%e sonar system+ sends echo

reuests to the host one specifies on the command line, and lists the responses

recei%ed their round trip time 0hen terminatedping*pro$a$ly $y hitting

control6B+ it summaries the results, gi%ing the a%erage round trip time and the

percent packet loss (his command is used constantly to determine "hether

there is a pro$lem "ith the net"ork connection $et"een t"o hosts


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ping [options host

ping !a: makes response audi$le e%ery time it is recei%ed

ping !i wait:"ait for waitseconds $et"een sending packets

ping !w n: exit ping after nseconds


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traceroute QoptionsR host QpacketsieR

(BP/P command 0hilepinggi%es information a$out the performance

of the net"ork path $et"een t"o hosts, traceroute"ill actually sho"

the route /t attempts to list the series of hosts through "hich senderSs

packets tra%el on their "ay to a gi%en destination 3y o$ser%ing theoutput of this command, and especially $y follo"ing it up "ithpings of

specific hosts on the route, the exact location of a $ad *high error or

latency+ link can $e disco%ered

&ttempts tracing $y launching !)P pro$e packets "ith a small ((L,

then listening for an /BMP Ttime exceededU gate"ay -ost is the dest

hostname/P address Packetsie is the N$ytes of pro$e datagrams

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Operating Systems and Linux II

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