Understanding Working In The Linux Shell

Understanding Working In The Linux Shell

Understanding Working In The Linux Shell

 

 

 

In Linux, the user communicates with the OS by using the CLI or the GUI. Linux often starts in the GUI by default. This hides the CLI from the user. One way to access the CLI from the GUI is through a terminal emulator application. These applications provide user access to the CLI and are often named as some variation of the word “terminal”. In Linux, popular terminal emulators are Terminator, eterm, xterm, konsole, and gnome-terminal.

 

Fabrice Bellard has created JSLinux which allows an emulated version of Linux to run in a browser. Search for it on the internet. Open a Linux console in JSLinux and type the ls command to list the current directory content. Keep the tab open if you would like to try out some of the other commands discussed in this article.

 

The figure shows gnome-terminal, a popular Linux terminal emulator.

Note: The terms shell, console, console window, CLI terminal, and terminal window are often used interchangeably.

The image is a screenshot of a Linux terminal. At the top of the terminal screen the logged-in username and directory are shown. In this case, it is rod@desktop:~. Displayed in the terminal window are commands and output. Command: rod@desktop:~$ uname -a, output: Linux desktop 3.13.0-32-generic #57-Ubuntu SMP Tue Jul 15 03:51:08 UTC 2014 x86_64x86_64 GNU/Linux. Command: rod@desktop:~$ ls -l Documents/ output: total 12 drwxrwxr-x 3 rod rod 4096 Dec 8 2013 alr drwxrwxr-x 3 rod rod 4096 Aug 13 13:24 backups -rw-rw-r-- 1 rod rod 0 Aug 13 13:27 configs -rw-rw-r-- 1 rod rod 0 Aug 13 13:27 notes drwxrwxr-x 2 rod rod 4096 Aug 13 13:26 OS_images Command:rod@desktop:$ ls -l Documents/ | grep OS output:drwxrwxr-x 2 rod rod 4096 Aug 13 13:26 OS_images.

Basic Commands

Linux commands are programs created to perform a specific task. Use the man command (short for manual) to obtain documentation about commands. As an example, man ls provides documentation about the ls command from the user manual.

Because commands are programs stored on the disk, when a user types a command, the shell must find it on the disk before it can be executed. The shell will look for user-typed commands in specific directories and attempt to execute them. The list of directories checked by the shell is called the path. The path contains many directories commonly used to store commands. If a command is not in the path, the user must specify its location, or the shell will not be able to find it. Users can easily add directories to the path, if necessary.

To invoke a command via the shell, simply type its name. The shell will try to find it in the system path and execute it.

The table lists basic Linux commands and their functions.

Command Description
mv Moves or renames files and directories
chmod Modifies file permissions
chown Changes the ownership of a file
dd Copies data from an input to an output
pwd Displays the name of the current directory
ps Lists the processes that are currently running in the system
su Simulates a login as another user or to become a superuser
sudo Runs a command as a super user, by default, or another named user
grep Used to search for specific strings of characters within a file or other command outputs. To search through the output of a previous command, grep must be piped at the end of the previous command.
ifconfig Used to display or configure network card related information. If issued without parameters, ifconfig will display the current network card(s) configuration. Note: While still widely in use, this command is deprecated. Use ip address instead.
apt-get Used to install, configure and remove packages on Debian and its derivatives. Note: apt-get is a user-friendly command line front-end for dpkg, Debian’s package manager. The combo dpkg and apt-get is the default package manager system in all Debian Linux derivatives, including Raspbian.
iwconfig Used to display or configure wireless network card related information. Similar to ifconfigiwconfig will display wireless information when issued without parameters.
shutdown Shuts down the system, shutdown can be instructed to perform a number of shut down related tasks, including restart, halt, put to sleep or kick out all currently connected users.
passwd Used to change the password. If no parameters are provided, passwd changes the password for the current user.
cat Used to list the contents of a file and expects the file name as the parameter. The cat command is usually used on text files.
man Used to display the documentation for a specific command.

Note: It is assumed that the user has the proper permissions to execute the command. File permissions in Linux are covered later in this chapter.

File and Directory Commands

Many command-line tools are included in Linux by default. To adjust the command operation, users can pass parameters and switches along with the command. The table lists a few of the most common commands related to files and directories.

Command Description
Is Displays the files inside a directory
cd Changes the current directory
mkdir Creates a directory under the current directory
cp Copies files from source to destination
mv Moves files to a different directory
rm Removes files
grep Searches for specific strings of characters within a file or other commands outputs
cat Lists the contents of a file and expects the file name as the parameter

Working with Text Files

Linux has many different text editors, with various features and functions. Some text editors include graphical interfaces while others are command-line only tools. Each text editor includes a feature set designed to support a specific type of task. Some text editors focus on the programmer and include features such as syntax highlighting, brackets and parenthesis check, and other programming-focused features.

While graphical text editors are convenient and easy to use, command line-based text editors are very important for Linux users. The main benefit of command-line-based text editors is that they allow for text file editing from a remote computer.

Consider the following scenario: a user must perform administrative tasks on a Linux computer but is not sitting in front of that computer. Using SSH, the user starts a remote shell to the remote computer. Under the text-based remote shell, the graphical interface is not available, which makes it impossible to rely on tools such as graphical text editors. In this type of situation, text-based programs are crucial.

The figure shows nano, a popular command-line text editor. The administrator is editing firewall rules. Text editors are often used for system configuration and maintenance in Linux.

The image is a screenshot showing a terminal running GNU nano 4.9.2 text editor to edit a script to modify firewall rules named fw_rules. At the bottom of the nano screen is a text menu of the options: ^G Get Help, ^O Write Out, ^W Where Is, ^K Cut Text, ^J Justify, ^C Cur Pos, ^X Exit, ^R Read File ,^\ Replace, ^U Paste Text, ^T To Spell, ^_ Go to Line, M-U Undo, M-E Redo, M-A Mark Text, M-6 Copy Text.

Due to the lack of graphical support, nano (or GNU nano) can only be controlled with the keyboard. For example, CTRL+O saves the current file; CTRL+W opens the search menu. GNU nano uses a two-line shortcut bar at the bottom of the screen, where commands for the current context are listed. Press CTRL+G for the help screen and a complete list of commands.

4.2.5

The Importance of Text Files in Linux

In Linux, everything is treated as a file. This includes the memory, the disks, the monitor, and the directories. For example, from the operating system standpoint, showing information on the display means to write to the file that represents the display device. It should be no surprise that the computer itself is configured through files. Known as configuration files, they are usually text files used to store adjustments and settings for specific applications or services. Practically everything in Linux relies on configuration files to work. Some services have not one, but several configuration files.

Users with proper permission levels can use text editors to change the contents of configuration files. After the changes are made, the file is saved and can be used by the related service or application. Users are able to specify exactly how they want any given application or service to behave. When launched, services and applications check the contents of specific configuration files to adjust their behaviour accordingly.

In the figure, the administrator opened the host configuration file in nano for editing. The host file contains static mappings of host IP addresses to names. The names serve as shortcuts that allow connecting to other devices by using a name instead of an IP address. Only the superuser can change the host file.

Note: The administrator used the command sudo nano /etc/hosts to open the file. The command sudo (short for “superuser do”) invokes the superuser privilege to use the nano text editor to open the host file.

The image is a screenshot showing a terminal running GNU nano 4.9.2 text editor to edit the /etc/hosts file. The contents of the file is shown: #Static table lookup for hostnames. #See hosts(5) for details. 127.0.0.1 localhost ::1 localhost 127.0.0.1 secOps.localdomain secOps At the bottom of the nano screen is a text menu of the options: ^G Get Help, ^O Write Out, ^W Where Is, ^K Cut Text, ^J Justify, ^C Cur Pos, ^X Exit, ^R Read File ,^\ Replace, ^U Paste Text, ^T To Spell, ^_ Go to Line, M-U Undo, M-E Redo, M-A Mark Text, M-6 Copy Text.

Action Point

I know you might agree with some of the points that I have raised in this article. You might not agree with some of the issues raised. Let me know your views about the topic discussed. We will appreciate it if you can drop your comment. Thanks in anticipation.

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About Adeniyi Salau 889 Articles
I am an IT enthusiast and a man of many parts. I am a Certified Digital Marketer, Project Manager and a Real Estate Consultant. I love writing because that's what keeps me going. I am running this blog to share what I know with others. I am also a Superlife Stem Cell Distributor. Our Stem Cell Products can cure many ailments.

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