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Linux shell is a user interface used for executing the commands. Shell is a program the user uses for executing the commands. In UNIX, any program can be the users shell. Shell categories in Linux are:
Bourne shell compatible, C shell compatible, nontraditional, and historical.
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A shell script, as the name suggests, is a script written for the shell. Script here means a programming language used to control the application. The shell script allows different commands entered in the shell to be executed. Shell script is easy to debug, quicker as compared to writing big programs. However the execution speed is slow because it launches a new process for every shell command executed. Examples of commands are cp, cn, cd.
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Kernel is used in UNIX like systems and is considered to be the heart of the operating system. It is responsible for communication between hardware and software components. It is primarily used for managing the systems resources as well.
Kernel Activities:
The Kernel task manager allows tasks to run concurrently.
Managing the computer resources: Kernel allows the other programs to run and use the resources.
Resources include i/o devices, CPU, memory.
Kernel is responsible for Process management. It allows multiple processes to run simultaneously allowing user to multitask.
Kernel has an access to the systems memory and allows the processes to access the memory when required.
Processes may also need to access the devices attached to the system. Kernel assists the processes in doing so.
For the processes to access and make use of these services, system calls are used.
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A pipe is a chain of processes so that output of one process (stdout) is fed an input (stdin) to another. UNIX shell has a special syntax for creation of pipelines. The commands are written in sequence separated by |. Different filters are used for Pipes like AWK, GREP.
e.g. sort file | lpr ( sort the file and send it to printer)
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Trap command: controls the action to be taken by the shell when a signal is received.
Trap [OPTIONS] [ [arg] signspec..]
Arg is the action to be taken or executed on receiving a signal specified in signspec.
e.g. trap “rm $FILE; exit” // exit (signal) and remove file (action).
Shift Command: Using shift command, command line arguments can be accessed. The command causes the positional parameters shift to the left. Shift [n] where n defaults to 1. It is useful when several parameters need to be tested.
Getopts command: this command is used to parse arguments passed. It examines the next command line argument and determines whether it is a valid option.
Getopts {optstring} {variable1}. Here, optsring contains letters to be recognized if a letter is followed by a colon, an argument should be specified. E.g (whether the argument begins with a minus sign and is followed by any single letter contained inside options ) If not, diagnostic messages are shown. It is usually executed inside a loop.
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A stateless Linux server is a centralized server in which no state exists on the single workstations. There may be scenarios when a state of a partilcuar system is meaningful (A snap shot is taken then) and the user wants all the other machines to be in that state. This is where the stateless Linux server comes into picture.
Features:
It stores the prototypes of every machine.
It stores snapshots taken for those systems.
It stores home directories for those system.
Uses LDAP containing information of all systems to assist in finding out which snapshot (of state) should be running on which system.
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Nslookup is used to find details related to a Domain name server. Details like IP addresses of a machine, MX records, servers etc. It sends a domain name query packet to the corresponding DNS.
Nslookup has two modes. Interactive and non interactive. Interactive mode allows the user to interact by querying information about different hosts and domains.
Non interactive mode is used to fetch information about the specified host or domain.
Interactive mode:
Nslookup [options] [server]
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Bash is a free shell for UNIX. It is the default shell for most UNIX systems. It has a combination of the C and Korn shell features. Bash shell is not portable. any Bash-specific feature will not function on a system using the Bourne shell or one of its replacements, unless bash is installed as a secondary shell and the script begins with #!/bin/bash. It supports regular and expressions. When bash script starts, it executes commands of different scripts.
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Process states in Linux:
Running: Process is either running or ready to run.
Interruptible: a Blocked state of a process and waiting for an event or signal from another process.
Uninterruptible:- a blocked state. Process waits for a hardware condition and cannot handle any signal.
Stopped: Process is stopped or halted and can be restarted by some other process.
Zombie: process terminated, but information is still there in the process table.
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Zombie is a process state when the child dies before the parent process. In this case the structural information of the process is still in the process table. Since this process is not alive, it cannot react to signals. Zombie state can finish when the parent dies. All resources of the zombie state process are cleared by the kernel.
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System calls used for Process management:
Fork () :- Used to create a new process
Exec() :- Execute a new program
Wait():- wait until the process finishes execution
Exit():- Exit from the process
Getpid():- get the unique process id of the process
Getppid():- get the parent process unique id
Nice():- to bias the existing property of process
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ls - list directory contents
ls -l (-l use a long listing format)
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ls -a (-a, do not hide entries starting with .)
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ps -f (-f does full-format listing.)
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ps -au Myname (-u by effective user ID (supports names)) (a - all users)
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kill -9 8 (process_id 8) or kill -9 %7 (job number 7)
kill -9 -1 (Kill all processes you can kill.)
killall - kill processes by name most (useful - killall java)
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vi screen editor or jedit, nedit or ex line editor
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tail -10 file_name ( last 10 rows)
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Using telnet server_name or ssh -l ( ssh - OpenSSH SSH client (remote login program))
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man command_name
info command_name (more information)
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ls -lac
-a all entries
-c by time
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Sum of all directories called file system.
A file system is the primary means of file storage in UNIX. File systems are made of inodes and superblocks.
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By storing and manipulate files.
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ps -fu pat (-f -full_format u -user_name )
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Grep, Kde( works on Linux and Unix)
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cp /tmp/file_name . (dot mean in the current directory)
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rm -rf directory_name
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Internal commands are stored in the; same level as the operating system while external commands are stored on the hard disk among the other utility programs.
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The three main parts are the command, options and arguments.
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An argument is what the command should act on: it could be a filename, directory or name. An option is specified when you want to request additional information over and above the basic information each command supplies.
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Online help provides information on each operating system command, the syntax, the options, the arguments with descriptive information.
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Two forms of security are Passwords and File Security with permissions specified.
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Home directory is the directory you begin at when you log into the system. Working directory can be anywhere on the system and it is where you are currently working.
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The parent directory is above the current directory, so it is closer to the root or top of the file system.
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With the more command you display another screenful by pressing the spacebar, with pg you press the return key.
The more command returns you automatically to the UNIX shell when completed, while pg waits until you press return.
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It is sometimes better to use the more command when you are viewing a file that will display over one screen.
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The mv command moves files and can also be used to rename a file or directory.
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Two methods that could be used:
a. use the mv command
b. copy the file and give it a new name and then remove the original file if no longer needed.
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To list all the files beginning with A command: ls A*
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The double quote (") allows the dollar sign ($) to retain its special meaning. Both the backslash (\) and single quote (') would remove the special meaning of the dollar sign.
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A shortcut method would be: mv file?.txt newdir
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a.Copy a file to make a new file.
b. Use the output operator e.g. ls -l > newfile.txt
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The operator > either overwrites the existing file (WITHOUT WARNING) or creates a new file.
The operator >> either adds the new contents to the end of an existing file or creates a new file.
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locate file_name (locate - list files in databases that match a pattern)
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Ctrl-Alt-F1 Exit to command prompt
Ctrl-Alt-F7 or F8 Takes you back to KDE desktop from command prompt
Crtl-Alt-Backspace Restart XWindows
Ctrl-Alt-D Show desktop
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The tar program is an immensely useful archiving utility. It can combine an entire directory tree into one large file suitable for transferring or compression.
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Files come in eight flavors:
Normal files
Directories
Hard links
Symbolic links
Sockets
Named pipes
Character devices
Block devices
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Use tar command (The GNU version of the tar archiving utility):
tar -zxvf file_name.tar.gz
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General Regular Expression Parser.
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Top provides an ongoing look at processor activity in real time.It displays a listing of the most CPU-intensive tasks on the system, and can provide an interactive inter¬ face for manipulating processes. (q is to quit)
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Use diff command that is compare files line by line
diff -u /usr/home/my_project1/etc/ABC.conf /usr/home/my_project2/etc/ABC.conf
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Use tail command - output the last part of files
tail -n file_name ( the last N lines, instead of the last 10 as default)
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for i in *log*; do rm $i; done
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Three modes -
Command mode: letters or sequence of letters interactively command vi.
Insert mode: Text is inserted.
Command line mode: enter this mode by typing ":" and entry command line at the foot of the screen.
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Use command: ZZ that is save changes and quit.
Use command line: ":wq" that is write changes and quit.
Use command line: ":q!" to ignore changes and quit.
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yy - copy a single line defined by current cursor position
3yy - copy 3 lines. Current line and two lines below it.
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Yes, these are internet terminals that were built in India, based on the [x86 system-on-chip] STPC chip, I am told; also, there evidently is a Turkish-built digital TV that runs Linux BIOS. I have also heard that there are routers and many other embedded devices running Linux BIOS. I think at this point that 1 million is a low number. I am in contact with other set-top box vendors that are talking about numbers in the lOs of millions for their products. These numbers actually make the OLPC numbers seem small, which is in it amazing.
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I’d like to communicate the basic ideas — that Linux is a good BIOS, and why; why Linux BIOS is built the way it is; where we are going; and how people can help. Most importantly, why it all matters — and it really matters a lot. We’re on the verge of losing control of the systems we buy, and we need to make a conscious effort, as a community, to ensure this loss of control does not happen. That effort will not be without some sacrifice, but if we are to maintain our ability to use and program our machines, and have fun with them, we have to act now. Because, if the computing business stops being fun, what’s the point$
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A set of instructions/queries that should be executed or rolled back as a single atomic unit.
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Each row has two additional columns associated with it - creation time and deletion time, but instead of storing timestamps, MySQL stores version numbers.
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Table-level locks allow the user to lock the entire table, page-level locks allow locking of certain portions of the tables (those portions are referred to as tables), row-level locks are the most granular and allow locking of specific rows.
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The front layer takes care of network connections and security authentications, the middle layer does the SQL query parsing, and then the query is handled off to the storage engine. A storage engine could be either a default one supplied with MySQL (MyISAM) or a commercial one supplied by a third-party vendor (ScaleDB, InnoDB, etc.)
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8 bit microprocessors are used in a variety of applications such as appliances automobiles ,industrial process and control applications.
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Nonvolatile Read Write Memory, also called Flash memory. It is also know as shadow RAM.
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ROM cannot be used as stack because it is not possible to write to ROM.
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Stack is a portion of RAM used for saving the content of Program Counter and general purpose registers.
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Flag is a flip-flop used to store the information about the status of a processor and the status of the instruction executed most recently .
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All x86 processors have pipelined structure.
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Compiler is used to translate the high-level language program into machine code at a time. It doesn’t require special instruction to store in a memory, it stores automatically. The Execution time is less compared to Interpreter.
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RAM: Read / Write memory, High Speed, Volatile Memory. ROM: Read only memory, Low Speed, Non Voliate Memory.
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Floating .gate Avalanche Injection MOS (FAMOS) transistor is used in each cell of EPROM.
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Cache Memory is scratch pad of computer.
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Cache memory is a small high-speed memory. It is used for temporary storage of data & information between the main memory and the CPU (center processing unit). The cache memory is only in RAM.
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Interrupt is a signal send by external device to the processor so as to request the processor to perform a particular work.
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Static RAM: No refreshing, 6 to 8 MOS transistors are required to form one memory cell, Information stored as voltage level in a flip flop.
Dynamic RAM: Refreshed periodically, 3 to 4 transistors are required to form one memory cell, Information is stored as a charge in the gate to substrate capacitance.
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In primary storage device the storage capacity is limited. It has a volatile memory. In secondary storage device the storage capacity is larger. It is a nonvolatile memory. Primary devices are: RAM / ROM. Secondary devices are: Floppy disc I Hard disk.
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Microprocessor contain ROM chip because it contain instructions to execute data.
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Latch is a D- type flip-flop used as a temporary storage device controlled by a timing signal, which can store 0 or 1. The primary function of a Latch is data storage. It is used in output devices such as LED, to hold the data for display.
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In Microprocessor more op-codes, few bit handling instructions. But in Microcontroller: fewer op-codes, more bit handling Instructions, and also it is defined as a device that includes micro processor, memory, & input / output signal lines on a single chip.
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It has limitations on the size of data. Most Microprocessor does not support floating-point operations.
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The data bus is Bi-directional because the same bus is used for transfer of data between Micro Processor and memory or input / output devices in both the direction.
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The address bus is unidirectional because the address information is always given by the Micro Processor to address a memory location of an input I output devices.
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High-density n- type Complimentary Metal Oxide Silicon field effect transistor.
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The processor made of PMOS I NMOS / HMOS I HCMOS technology is called 1st / 2nd / 3rd / 4th generation processor, and it is made up of 4 / 8 / 16 / 32 bits.
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Because 8085 processor has 8 bit ALU (Arithmetic Logic Review). Similarly 8086 processor has 16 bit ALU.
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8-bit Processor - 8085 I Z80 / 6800; 16-bit Processor - 8086 / 68000 / Z8000; 32-bit Processor - 80386 I 80486.
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Microprocessor is a program-controlled device, which fetches the instructions from memory, decodes and executes the instructions. Most Micro Processor is single- chip devices.
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You use the touch command to create an empty file without needing to open it. Answers a and e point to invalid commands, though either of these might actually be aliased to point to a real command. Answers b and c utilize editors, and so do not satisfy the requirements of the question. actually touch is used to change the timestamps of a file if its exits, otherwise a new file with current timestamps will be created.
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The kill command by itself tries to allow a process to exit cleanly. You type kill -9 PID, on the other hand, to abruptly stop a process that will not quit by any other means. Also, pressing CtrI+C works for many programs. Answers b and d are only valid in some contexts, and even in those contexts will not work on a hung process.
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You can use rmdir or rm -rf to delete a directory. Answer a is incorrect, because the rm command without any specific flags will not delete a directory, it will only delete files. Answers d and e point to a non-existent command.
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Cron command is used to schedule the task daily at the same time repeatedly ,“at” command is used to schedule the task only once i.e. to run only one time.
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The touch command is usually used to modify either a file’s access or modification time. It can also be used to create a new file.
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The -r option tells the cp command to recurs the directories. The -P option retains the original permissions.
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The dd command is a special copy command often used for floppy disks and tapes. The if= option specifies the source; the bs= is the block size; and the of= option is the output.
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In order to delete a file, you must have write rights to the directory containing the file.
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Use the asterick (*) to match to zero or more characters. The ‘$‘ matches to any one character so sale$ would not find sale.
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Using the -c option with the grep command will show the total number of lines containing the specified pattern rather than displaying the lines containing the pattern.
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The -n option when used with sed prints only the lines containing the pattern. In this case, the pattern is ‘Bob’ and the file to be searched is kickoff.
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The .Z extension indicates that this is a file that has been compressed using the compress utility. The zcat utility provides the ability to display the contents of a compressed file.
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If you do not unmount the floppy before removing it, the files on the floppy may become corrupted.
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Quotas are set on a partition by partition basis. If your users have home directories on different partitions, you will need to configure quotas for each partition.
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The repquota command is used to get a report on the status of the quotas you have set including the amount of allocated space and amount of used space.
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By creating a link to the file in each user’s home directory, each user is able to easily open and edit the spreadsheet. Also, any changes that are made are seen by all the users with access.
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Because the link in bob’s directory is a symbolic link, if the file sales data in the /data directory is deleted, the symbolic link will no longer work.
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When the sticky bit is set on a world writable directory, only the owner can delete any file contained in that directory.
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The digits of your umask represent owner, group and others in that order.
The 0 gives read and write for files and the 2 gives read only for files.
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When using sed to do a search and replace, its default action is to only replace the first occurrence in each line. Adding the ‘g’ makes sed replace all occurrences of the search term even when it occurs multiple times on the same line.
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By default, expand converts tabs to eight spaces. Use the -t option to change this behavior.
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If the number of lines to display is not specified, the first ten lines of the specified file are displayed. The asterick tells head to display the content of each file in the present working directory.
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The od text filter will dumpt the contents of a file and display it in 2-byte octal numbers.
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The paste text filter usually joins two files separating the corresponding lines with a tab. The -s option, however, will cause paste to display the first file, dog, then a new line character, and then the file cat.
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The default page length when using pr is 66 lines. The -l option is used to specify a different length.
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The tac text filter is a reverse cat. It displays a file starting with the last line and ending with the first line.
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The tail utility displays the end of a file. The -15 tells tail to display the last fifteen lines of each specified file.
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Dynamic addressing simplifies network administration because the s/w keeps track of IP addresses rather than requiring an administrator to manage the task. That means new computer can be added to the network without any risk of manually assigning unique IP address.
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NFS stands for Network File System. NFS enables filesystems physically residing on one computer system to be used by other computers in the network, appearing to users on the remote host as just another local disk.
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The file server is a machine that shares its disk storage and files with other machines on the network.
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We can use two major techniques to handle the collisions. They are open addressing and separate chaining. In open addressing, data items that hash to a full array cell are placed in another cell in the array. In separate chaining, each array element consists of a linked list. All data items hashing to a given array index are inserted in that list.
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The major advantage of a hash table is its speed. Because the hash function is to take a range of key values and transform them into index values in such a way that the key values are distributed randomly across all the indices of a hash table.
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The write command enables you to write an actual message on the other terminal online. You have to issue the write command with the login ID of the user with whom you want to communicate. The write command informs the user at the other end that there is a message from another user. write pastes that message onto the other user’s terminal if their terminal’s write permissions are set. Even if they are in the middle of an edit session, write overwrites whatever is on the screen. The edit session contents are not corrupted; you can restore the original screen on most editors with Ctrl-L. write is mostly used for one-way communication, but you can have an actual conversation as well.
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The root login does not restrict you in any way. When you log in as root, you become the system. The root login is also sometimes called the super user login. With one simple command, issued either on purpose or by accident, you can destroy your entire Linux installation. For this reason, use the root login only when necessary. Avoid experimenting with commands when you do log in as root.
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Swap space is used as an extension of physical RAM, the more RAM you have, the less swap space is required. You can add the amount of swap space and the amount of RAM together to get the amount of RAM Linux will use. For example, if you have 8MB of RAM on your machine’s motherboard, and a 16MB swap-space partition, Linux will behave as though you had 24MB of total RAM.
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command-The last field, called either command or login command, is used to specify what shell the user will use when he logs in.
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cylinders-When creating a new partition you must first specify its starting cylinder. You can then either specify its size or the ending cylinder.
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echo $SHELL-The name and path to the shell you are using is saved to the SHELL environment variable. You can then use the echo command to print out the value of any variable by preceding the variable’s name with $. Therefore, typing echo $SHELL will display the name of your shell.
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fc -5-The fc command can be used to edit or rerun commands you have previously entered. To specify the number of commands to list, use -n.
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fsck-The fsck command is used to check the integrity of the file system on your disk.
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kernel.h-To determine the various levels of messages that are defined on your system, examine the kernel.h file.
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root-Whenever you install Linux, only one user account is created. This is the super user account also known as root.
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Syslogd-The syslogd daemon is responsible for tracking system information and saving it to specified log flies.
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To the screen or display-By default, your shell directs standard output to your screen or display.
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Top-The top utility shows a listing of all running processes that is dynamically updated.
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The SYS user owns the data dictionary. The SYS and SYSTEM users are created when the database is created.
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dmesg
The dmesg command displays the system messages contained in the kernel ring buffer.
By using this command immediately after booting your computer, you will see the boot messages.
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2
Linux can be installed on two partitions, one as / which will contain all files and a swap partition.
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/var/log/messages
By default, the main system log is /var/log/messages.
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logrotate
The logrotate command can be used to automate the rotation of various logs.
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ctrl-z
Using ctrl-z will suspend a job and put it in the background.
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gpasswd -r
The gpasswd command is used to change the password assigned to a group. Use the -r option to remove the password from the group.
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The pwconv command creates the file /etc/shadow and changes all passwords to 'x' in the /etc/passwd file.
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The repquota command is used to get a report on the status of the quotas you have set including the amount of allocated space and amount of used space.
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The split text filter will divide files into equally sized pieces. The default length of each piece is 1,000 lines.
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You can use the z modifier with tar to compress your archive at the same time as creating it.
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This command uses the x switch to extract a file. Here the file memo.ben will be restored from the tarfile MyBackup.tar.
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By default, your shell directs standard output to your screen or display.
