Introduction to Linux Commands

Learn linux commands
u Linux is one of the popular version of UNIX operating System.
uIt is open source as its source code is freely available.
uIt is free to use.
u An operating system is a program that acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs.
u In technical terms, it is a software which manages hardware. An operating System controls the allocation of resources and services such as memory, processors, devices and information.

General Classification of Linux commands:

    File System Commands
    Process Management commands
    File Permission commands
    Secure Shell (SSH) commands
    Search commands
    System Info commands
    File Compression commands
    Network commands
    Installation commands

Basic Features:

u Following are some of the important features of Linux Operating System.
u Portable – Portability means software can work on different types of hardware.
u Open Source – Linux source code is freely available and it is a community based development project. Multiple team’s works in collaboration to enhance the capability of Linux operating system and it is continuously evolving.
u Multi User – Linux is a multiuser system means multiple users can access system resources like memory/ ram/ application programs at same time.
u Multiprogramming – Linux is a multiprogramming system means multiple applications can run at same time.
u Hierarchical File System – Linux provides a standard file structure in which system files/ user files are arranged.
u Shell – Linux provides a special interpreter program which can be used to execute commands of the operating system. It can be used to do various types of operations, call application programs etc.
u Security – Linux provides user security using authentication features like password protection/ controlled access to specific files/ encryption of data.
u Linux System Architecture is consists of following layers
u Hardware layer – Hardware consists of all peripheral devices (RAM/ HDD/ CPU etc).
u Kernel – a Core component of Operating System, interacts directly with hardware, provides low-level services to upper layer components.
u Shell – An interface to the kernel, hiding the complexity of kernel’s functions from users. Takes commands from the user and executes kernel’s functions.
u Utilities – Utility programs giving user most of the functionalities of operating systems.

Linux Basic Commands:

What is Command?
A command is an instruction given by a user telling a computer to do something or Command is nothing but Standard Input, Standard Output and Standard Error.
Basic Commands?
# Who– List users logged into the server
#pwd– Show the “present working directory”, or current directory.
#cd– Change current directory to your HOME directory.
#cd..– Change current directory to the parent directory of the current directory.
#cd.– Current directory
#ls– list a directory
#ls –l– list a directory in long (detailed) format
#ls –a– List the current directory including hidden files. Hidden files start with “. “
# mkdir <dir name>– Creates Directory
# rmdir <dir name>– Removes Directory
#more– Look at file, one page at a time
#touch <file name>– Creates empty text file with 0 bytes
# cat > <file name>– Create and type file
ctrl + d save & end the file
# cat <file name >– List the contents of the file
#rm –r <dir name>– Delete directory and its contents
# rm <file name>– Delete the file
#cp file1 file2– copy a file
# cp /tmp/test /var– Copy tes file to var directory
# cp /tmp/test /var/test– Copy test file to var dir with same name (test)
# mv <source> <destination>– Move file/directory
# mv /tmp/test /var– Move test file to var directory
# mv /tmp/test /var/test– Move test file to var dir with same name (test)
# mv /tmp/test /var/test5– Move test file to var dir and rename it as test5
File System Commands:
      $ ls                                                       Directory Listing
      $ls – al                                                 Formatted listing with hidden files
      $cd <<directory path>>                    Change directory
      $cd                                                       Change to home directory
      $pwd                                                    Show  current directory
      $Mkdir <<directory name>>             Create directory
      $rm file                                                 delete file
      $rm –r <<dir>>                                     delete directory 
      $rm –f <<file name>>                         force remove file
      $rm –rf  <<dir>>                                       Force remove directory
      $cp file1 file2                                            Copy file1 to file2
      $cp –r  <<dir1>>  <<dir2>>                     Copy dir1 to dir2. Create dir2                                                                             If It doesn’t exist
       $mv file1 file2                                          Remove or move file1 to file2.
                                                                       If file2 is an existing directory
                                                                       Move file1 into directory file2
      $touch file                                                 Create or update file
      $cat  >  file                                                  Places standard input into file
      $more file                                                   Output the contents of file
      $head file                                                    Output the first 10 lines of file
      $ nano file                                                   Open file in the Nano editor
Process Management Commands:
      $ps                                       Display your currently active processes
      $top                                     Display all running processes
      $kill pid                                kill process id pid
      $killall proc                          kill all processes named proc*
      $bg                                        Lists stopped or background jobs; resume a           
                                                 stopped job in the background
      $fg                                         brings the most recent job to the foreground
      $fg n                                      brings job n to the foreground
File Permission commands:
$chmod octal file             Change the permissions of file to octal, which can be found separately for user, group, and world by adding:
Ø   4 – Read (r)
Ø   2 – Write (w)
Ø   1 – Execute (x)
     7 — Total

$ chmod 777 sample.log          Read, write and execute for all for file sample.log
Search commands:
      $grep pattern files – search for pattern in files
      $grep -r pattern dir – search recursively for pattern in dir
      $command | grep pattern – search for pattern in the output of command
      $locate file – find all instances of file
System Info commands:
      $date    –   show the current date and time
      $cal   –  show this month’s calendar
      $uptime   show current uptime
      $w – display who is online
      $whoami – who you are logged in as
      $finger user – display information about user
      $uname -a – show kernel information
      $cat /proc/cpuinfo – cpu information
      $cat /proc/meminfo – memory information
      $man command – show the manual for command
      $df – show disk usage
      $du – show directory space usage
      $free – show memory and swap usage
      $whereis app show possible locations of app
      $which app – show which app will be run by default
File Compression commands:
      $tar cf file.tar files   create a tar named file.tar containing files
      $tar xf file.tar – extract the files from file.tar
      $tar czf file.tar.gz files – create a tar with Gzip compression
      $tar xzf file.tar.gz – extract a tar using Gzip
      $tar cjf file.tar.bz2 – create a tar with Bzip2 compression
      $tar xjf file.tar.bz2 – extract a tar using Bzip2
      $gzip file – compresses file and renames it to file.gz
      $gzip -d file.gz – decompresses file.gz back to file
Network commands:
      $ping host – ping host and output results
      $whois domain – get whois information for domain
      $dig domain – get DNS information for domain
      $dig -x host – reverse lookup host
      $wget file – download file
      $wget -c file – continue a stopped download
Installation commands:
      $sudo apt-get install <<package name>> – Installs the package in ubuntu.
      $sudo yum install <<package name>> – Installs the pakage in CentOS

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