Transcript Lecture slides

Operating Systems and Linux
Partha Sarathi Dasgupta
MIS Group
Indian Institute of Management Calcutta
1
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
2
How to check the mounted Partitions?
nettech:> df -h .
Filesystem
/dev/hda7
/dev/hda8
/dev/hda1
/dev/hda5
/dev/had6
/dev/had7
Size Used
980M 163M
496M 183M
124M 8.4M
19G 15G
7.0G 5.4G
3.7G 2.7G
Avail Use%
767M 18%
288M 39%
109M 8%
2.7G 85%
1.2G 81%
867M 77%
Mounted on
/
/
/boot
/opt
/usr
/var
3
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.
4
Init process
What is init 6?
5
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.
6
Links
Hard links to a file
Soft links to a file
SUID
7
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.
8
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.
9
Path names and Files
Absolute path names
Relative path names
Copy a file or create a link to a file
Deleting files and directories
10
Default files
Standard input
Standard Output
Standard error
11
Linux Processes: Birth and Death
12
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.
13
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.
14
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.
15
Linux Process Cycle
Stopped
Signal
Signal
Running state
Creation
Ready
Scheduling
Executing
Termination
Zombie
Event
Signal or Event
Uninterruptible
Interruptible
16
Input-Output Redirection
• Output redirection with > 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
17
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 < text.txt > error.log
• cat afile >> bfile
• File descriptors
• ls > dirlist 2>&1
• set -o noclobber
18
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.
19
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 1:00 am tomorrow < scriptfile
By default, only a privileged user can execute the command.
Role of script files, e.g., /etc/profile
20
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.
21
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:
bob:~> crontab -l
# (/tmp/crontab.18185 installed on Wed Sep 19 16:37:08 2001)
38 16 * * 3 mail -s "sports evening" bob
22
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.
23
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.
24
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 sender’s
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.
25