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Linux/Scripting Language Assignment Help | Scripting Language Assignment Help | Realcode4you


  • A Linux shell provides a command line environment through which a user can interact with the kernel

  • The user provides the kernel with instructions using text-based commands

  • These commands are translated by the shell into something the kernel can comprehend, which are then executed by the kernel.


Built-in System Utilities

  • Linux built-in system utilities are programs that allow a user to perform tasks which involve complex actions

  • Utilities provide user interface functions that are basic to an operating system, but which are too complex to be built into the shell

  • Examples of utilities are programs that let us see the contents of a directory, create, edit, move, copy files and delete files and check the status of connected devices


Application Software and Utilities

  • These are not part of the operating system, but additional programs that are bundled with a Linux distribution, or available separately

  • These can range from additional or different versions of basic utilities, to full scale commercial applications

  • These include office productivity applications, web browsers, media players, text editors and third-party programming environments just to name a few


Linux CLI Shells

  • A shell is the interface between a Linux system and a user

  • It is used to invoke commands as required to perform specific tasks

  • It acts as an interpreter between the user and the protected Linux kernel

  • There are several shells available for Linux including bsh, ksh, tcsh, bash and others

  • In this unit, we will be using the bash shell


Command Line Interface

When accessing a Linux shell, you can either do this locally or remotely

Remote Terminal: Access the shell terminal of a Linux machine (server) across the internet.

Local Terminal: Access the shell terminal of the Linux machine (workstation) you are using.


The Terminal



Linux Command Basics

- To execute a command, enter its name, followed by required flag(s) and arguments into the command line

- Command flags allow you to change a command’s default operation

- Flag Conventions:

  • A single hyphen followed by one or more letters, e.g (“ls -l”), (“ls -la”)

  • Sometimes have double dashes followed by a keyword (“--help”)

  • All of a command’s flags can be found by consulting its man pages entry, i.e. man ls


The List (ls) Command

ls command example:

  • ls list all user files in the current working directory

  • ls -a list all files including hidden file beginning with a period “.”

  • ls -ld * List details about a directory and not its contents

  • ls -F Put an indicator character at the end of each name

  • ls –l Simple long listing

  • ls –lh Give human readable file sizes

  • ls –lS Sort files by file size

  • ls –lt Sort files by modification time


Linux File Systems



  • The Linux file system follows a hierarchical structure and is based on the Filesystem Hierarchy Standard (FHS), which defines the organisation and layout of directories in a Linux system

  • Data files are stored in directories (folders) that can be nested as deeply asrequired

  • There are many core directories that form an essential part of any Linux system, each of which Linux admins and developers need to be aware of


/bin (Binary):

  • The /bin directory contains essential executable files that are required for basic system functionality

  • These files are accessible to all users and are crucial for booting the system and performing essential system operations, including in single-user mode


/etc (Configuration):

  • The /etc directory stores system-wide configuration files for services, applications, and the overall system settings

  • Examples of files in /etc include passwd (user account information), hosts (network hostnames), and fstab (file system table)


/sbin (System Binary):

  • The /sbin directory holds system binaries that are essential for system administration and maintenance

  • These executables are typically used by the system administrator or root user for tasks like managing the network, configuring hardware, and performing system recovery operations

  • Regular users typically don't have direct access to these files


/var (Variable):

  • The /var directory stores variable data files that change during system operation and expected to grow over time, such as log files, spool files, temporary files, and databases

  • Examples of directories within /var include /var/log for system log files, /var/spool for print queues and mail spools, and /var/www for web server content.

/usr

  • The /usr directory contains various subdirectories that store user-related data and applications

  • It’s the largest directory in the Linux file system • The main subdirectories in /usr include:


/usr/bin contains non-essential command-line binaries for all users

/usr/sbin Similar to /sbin, it holds system administration binaries, but these are not crucial for the system's basic functionality

/usr/lib contains libraries that support the binaries in /usr/bin and /usr/sbin

/usr/local used for installing software manually by the system administrator, separate from the operating system's package manager

/usr/share contains architecture-independent data such as documentation, images, and common files shared across different applications


/dev (Devices):

  • The /dev directory contains device files that represent and provide access to physical and virtual devices connected to the system.

  • These device files allow user programs to interact with hardware devices and peripherals

  • For example, /dev/sda represents the first SCSI or SATA disk, and /dev/tty1 represents the first virtual console.


/proc:

  • /proc is a special directory in Linux providing a virtual file system interface to the kernel and its running processes

  • It does not represent physical files on disk, but rather, provides a dynamic view of the system's processes and kernel-related info

  • Each running process in Linux has a corresponding directory inside /proc, e.g. /proc/1234 represents the process with PID (Process Identifier) 1234.


Linux files are stored in a single rooted, hierarchical file system

  • Data files are stored in directories (folders)

  • Directories may be nested as deep as needed

Naming Files

  • Files are named by chaining directories from the root downwards to the containing directory and the file name itself

  • This is known as the pathname

Current Working Directory

One directory is designated the current working directory

  • If you omit the leading / then path name is relative to the current working directory

  • Use pwd to find out where you are



Special File Names

Some file names are special:


/ Represents the root directory

- this is not to be confused with the root user

. Represents the current directory

.. Represents the parent (previous) directory ~ represents the user’s home directory


Linux environment and path variables

Environment Variables:

Environment variables are global settings that control the function of the shell and other Linux programs. They are sometimes referred to global shell variables.


- Setting:

VAR=/home/fred/doc  
export TERM=ansi 
SYSTEMNAME=`uname -n` 

Using Environment Variables:

echo $VAR 
	cd $VAR  
	cd $HOME 
	echo “You are running on $SYSTEMNAME” 

  • Displaying - use the following commands:

set (displays local & env. Vars)

Export

  • Variables can be retrieved by a script or a program


PATH Environment Variable

Controls where commands are found

PATH is a list of directory pathnames separated by colons. For  example: PATH=/bin:/usr/bin:/usr/X11R6/bin:/usr/local/bin:/home/scully/bin 
  • If a command does not contain a slash, the shell tries finding the command in each directory in PATH. The first match is the command that will run


Linux File Permissions

- Linux provides three kinds of permissions:

  • Read (r) - users with read permission may read the file or list the directory

  • Write (w) - users with write permission may write to the file or new files to the directory

  • Execute (x) - users with execute permission may execute the file or lookup a specific file within a directory


- In Linux, file and directory permissions are managed through the concepts of owner, group, and others. These entities determine who can perform certain actions on a file or directory, such as reading, writing, or executing.

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