Operating Systems and Linux

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Operating Systems and Linux
Partha Sarathi Dasgupta MIS Group Indian
Institute of Management Calcutta
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Partitions

• Major partitions on a Linux system
• data partition normal Linux system data,
  including the root partition containing all
• the data to start up and run the system
• swap partition expansion of the computer's
  physical memory, extra memory on hard
• disk.
• Usually, systems contain a root partition, one
  or more data partitions and one or more
• swap partitions.
• fdisk
• Swap space is only accessible for the system
  itself, and is hidden from view during
• normal operation. Any problem with using the
  swap partition?
• Typically, system data is separate from user
  data. Programs that offer services are kept
• in a different place than the data handled by
  this service. Different partitions created
• a partition with all data necessary to boot the
  machine
• a partition with configuration data and server
  programs
• one or more partitions containing the database
  tables, user mails, an ftp archive etc.
• a partition with user programs and application
• one or more partitions for the user specific
  files (home directories)
• one or more swap partitions

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How to check the mounted Partitions?
nettechgt df -h . Filesystem Size Used Avail
Use Mounted on /dev/hda7 980M 163M 767M 18
/ /dev/hda8 496M 183M 288M 39 /
/dev/hda1 124M 8.4M 109M 8 /boot
/dev/hda5 19G 15G 2.7G 85 /opt /dev/had6
7.0G 5.4G 1.2G 81 /usr /dev/had7 3.7G
2.7G 867M 77 /var
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Init process
The kernel, once it is loaded, finds init in sbin
and executes it. When init starts, it becomes
the parent or grandparent of all of the
processes that start up automatically on Linux
system. The first thing init does, is reading
its initialization file, /etc/inittab. This
instructs init to read an initial configuration
script for the environment, which sets the path,
starts swapping, checks the file systems, and so
on. Basically, this step takes care of
everything that system needs to have done at
system initialization setting the clock,
initializing serial ports and so forth. Then
init continues to read the /etc/inittab file,
which describes how the system should be set up
in each run level and sets the default run level.
A run level is a configuration of processes.
All UNIX-like systems can be run in different
process configurations, such as the single user
mode, which is referred to as run level 1 or run
level S (or s). In this mode, only the system
administrator can connect to the system. It is
used to perform maintenance tasks without risks
of damaging the system or user data.
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Init process
What is init 6?
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How login is initiated
The idea behind operating different services at
different run levels essentially revolves around
the fact that different systems can be used in
different ways. Some services cannot be used
until the system is in a particular state, or
mode, such as being ready for more than one user
or having networking available. There are times
in which you may want to operate the system in a
lower mode. Examples are fixing disk corruption
problems in run level 1 so no other users can
possibly be on the system, or leaving a server
in run level 3 without an X session running. In
these cases, running services that depend upon a
higher system mode to function does not make
sense because they will not work correctly
anyway. By already having each service assigned
to start when its particular run level is
reached, you ensure an orderly start up process,
and you can quickly change the mode of the
machine without worrying about which services to
manually start or stop. Shutdown UNIX was not
made to be shut down, but if you really must, use
the shutdown command. After completing the
shutdown procedure, the -h option will halt the
system, while -r will reboot it.
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Links
Hard links to a file Soft links to a file SUID
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Inodes in File systems
In a file system, a file is represented by an
inode, a serial number. Every partition has its
own set of inodes throughout a system with
multiple partitions, files with the same inode
number can exist. Each inode describes a data
structure on the hard disk. When a hard disk is
initialized to accept data storage, usually
during the initial system installation process or
when adding extra disks to an existing system, a
fixed number of inodes per partition is
created. This number will be 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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Inode Data Structure

• At the time a new file is created, it gets a free
  inode.
• Inode contains the following information
• Owner and group owner of the file.
• File type (regular, directory, ...)
• Permissions on the file
• Date and time of creation, last read and change.
• Date and time this information has been changed
  in the inode.
• Number of links to this file
• File size
• An address defining the actual location of the
  file data.

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Path names and Files
Absolute path names Relative path names Copy a
file or create a link to a file Deleting files
and directories
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Default files
Standard input Standard Output Standard error
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Linux Processes Birth and Death
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Linux Processes Some facts

• State The execution state of the process
  (executing, ready, suspended, stopped, zombie)
• Scheduling information Information needed by
  Linux to schedule processes. A process can be
  normal or real-time and has a priority. Real-time
  processes are scheduled before normal processes,
  and within each category, relative priorities can
  be used. A counter keeps track of the amount of
  time a process is allowed to execute.
• Identifiers Each process has a unique
  identifier and also has user and group
  identifiers.

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Linux Processes Some facts

• IPC Processes can interact with each other.
• Links Each process includes a link to its
  parent process, links to its siblings, and links
  to all of its children.
• Times and timers includes process creation time
  and the amount of process time so far consumed by
  a process.
• File system pointers to any file opened by this
  process, as well as pointers to the current and
  root directories for this process.
• Address space Virtual address space assigned to
  the process.
• Processor-specific context Registers and stack
  info constituting the context of the process.

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Linux Process States

• Running Process is either running or Ready to
  run
• Interruptible A blocked state and process is
  waiting for an event, such as
• end of an I/O operation, availability of a
  resource, or a signal from another
• process.
• Uninterruptible A blocked state. Process is
  waiting directly on hardware
• conditions and will not handle any signals.
• Stopped Process halted and can be restarted
  only by some other process.
• E.g., a process that is being debugged can be
  stopped.
• Zombie Process has been terminated, but its
  structural information is still
• in the process table.

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Linux Process Cycle
Stopped
Signal
Signal
Running state
Termination
Creation
Ready
Executing
Zombie
Scheduling
Event
Uninterruptible
Signal or Event
Interruptible
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Input-Output Redirection

• Output redirection with gt and
• Redirecting "nothing" to an existing file is
  equal to emptying
• the file
• same redirection to an nonexistent file will
  create a new empty file with the given name
• To find a word within some text, display all
  lines matching pattern1, or exclude lines also
  matching pattern2 from being displayed
• grep pattern1 file grep -v pattern2

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Input-Output Redirection

• To display output of a directory listing one
  page at a time
• ls -la less
• To find a file in a directory
• ls -l grep part_of_file_name
• who wc l
• aspell lt text.txt gt error.log
• cat afile gtgt bfile
• File descriptors
• ls gt dirlist 2gt1
• set -o noclobber

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Automatic Processes Daemons

• Daemons are server processes that run
  continuously.
• Most of the time, they are initialized at system
  startup and
• then wait in the background until their service
  is required.
• A typical example is the networking daemon,
  xinetd, which is
• started in almost every boot procedure. After
  the system is
• booted, the network daemon just sits and waits
  until a client
• program, such as an FTP client, needs to
  connect.

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Processes
at options time Execute commands at a specified
time and optional date. The commands are taken
from standard input or from a file. Example at
100 am tomorrow lt scriptfile By default, only a
privileged user can execute the command. Role of
script files, e.g., /etc/profile
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Cron and Crontab

• The cron system is managed by the cron daemon.
• It gets information about which programs and
  when to run them from the
• system's and users' crontab entries.
• Only the root user has access to the system
  crontabs, while each user
• should only have access to his own crontabs. On
  some systems (some)
• users may not have access to the cron facility.
• At system startup the cron daemon searches
  /var/spool/cron/ for crontab
• entries which are named after accounts in
  /etc/passwd, it searches
• /etc/cron.d/, /etc/crontab, then uses this
  information every minute to check
• if there is something to be done. It executes
  commands as the user who
• owns the crontab file and mails any command
  output to the owner.

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Cron and Crontab
Users are supposed to edit their crontabs in a
safe way using the crontab e command. The
default editor is vi This crontab entry
reminds bob to go to his sports club every
Thursday night bobgt crontab -l
(/tmp/crontab.18185 installed on Wed Sep 19
163708 2001) 38 16 3 mail -s "sports
evening" bob
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TCP/IP Testing and Troubleshooting
ping The IP protocol includes control messages
called (Internet Control Message Protocol) ICMP
packets. One type of ICMP packet is called an
echo request,'' and the IP rules require its
recipient to send back an echo reply.'' These
are incredibly useful because you can determine
(1) whether the remote host is up and talking to
the network, (2) the time required for a packet
to make a round-trip to the host, and (3) (by
sending a few dozen echo requests) what fraction
of the packets sent between the hosts get lost
somewhere along the way. The ping command (named
after the sound of an active sonar system) sends
echo requests to the host one specifies on the
command line, and lists the responses received
their round trip time. When terminated ping
(probably by hitting control-C) it summarizes the
results, giving the average round trip time and
the percent packet loss. This command is used
constantly to determine whether there is a
problem with the network connection between two
hosts.
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TCP/IP Testing and Troubleshooting
ping options host ping a makes response
audible every time it is received. ping i wait
wait for wait seconds between sending
packets ping w n exit ping after n seconds.
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TCP/IP Testing and Troubleshooting
traceroute options host packetsize TCP/IP
command. While ping gives information about the
performance of the network path between two
hosts, traceroute will actually show the route.
It attempts to list the series of hosts through
which senders packets travel on their way to a
given destination. By observing the output of
this command, and especially by following it up
with pings of specific hosts on the route, the
exact location of a bad (high error or latency)
link can be discovered. Attempts tracing by
launching UDP probe packets with a small TTL,
then listening for an ICMP time exceeded
gateway. Host is the dest hostname/IP address.
Packetsize is the bytes of probe datagrams.