Linux Shell and Command

 

What Is the Shell?

When we speak of the command line, we are really referring to the shell. The shell is a program that takes keyboard commands and passes them to the operating system  to carry out. Almost all Linux distributions supply a shell program from the GNU  roject called bash. The name is an acronym for bourne-again shell, a reference to the fact that bash is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.

Terminal Emulators

When using a graphical user interface (GUI), we need another program called a terminal emulator to interact with the shell. If we look through our desktop menus, we will probably find one. KDE uses konsole, and GNOME uses gnome-terminal, though it’s likely called simply Terminal on your menu. A number of other terminal emulators are available for Linux, but they all basically do the same thing: give us access to the shell. You will probably develop a preference for one or another terminal emulator based on the number of bells and whistles it has.

Understanding the File System Tree

Like Windows, a Unix-like operating system such as Linux organizes its files in what is called a hierarchical directory structure. This means they are organized in a tree-like pattern of directories (sometimes called folders in other systems), which may contain files and other directories. The first directory in the file system is called the root directory. The root directory contains files and subdirectories, which contain more files and subdirectories, and so on. 

Note that unlike Windows, which has a separate file system tree for each storage device, Unix-like systems such as Linux always have a single file system tree, regardless of how many drives or storage devices are attached to the computer. Storage devices are attached (or more correctly, mounted) at various points on the tree according to the whims of the system administrator, the person (or people) responsible for the maintenance of the system. 

The Current Working Directory

Most of us are probably familiar with a graphical file manager that represents the file system tree, Notice that the tree is usually shown upended, that is, with the root at the top and the various branches descending below. However, the command line has no pictures, so to navigate the file system tree, we need to think of it in a different way. Imagine that the file system is a maze shaped like an upside-down tree and we are able to stand in the middle of it. At any given time, we are inside a single directory, and we can see the files contained in the directory and the pathway to the directory above us (called the parent directory) and any subdirectories below us. The directory we are standing in is called the current working directory. To display the current working directory, we use the pwd (print working directory) command. 

Listing the Contents of a Directory

To list the files and directories in the current working directory, we use the ls command. 

[me@linux~]$ ls

Desktop Documents Music Pictures Public Templates Videos

Actually, we can use the ls command to list the contents of any directory, not just the current working directory, and there are many other fun things it can do as well. 

Changing the Current Working Directory

To change our working directory (where we are standing in the tree-shaped maze), we use the cd command. To do this, type cd followed by the pathname of the desired working directory. A pathname is the route we take along the branches of the tree to get to the directory we want. We can specify pathnames in one of two different ways: as absolute pathnames or as relative pathnames. Let’s deal with absolute pathnames first. 

Absolute Pathnames

 An absolute pathname begins with the root directory and follows the tree branch by branch until the path to the desired directory or file is completed. For example, there is a directory on your system in which most of the system’s programs are installed. The directory’s pathname is /usr/bin. This means from the root directory (represented by the leading slash in the pathname) there is a directory called usr that contains a directory called bin. 

[me@linux~]$ cd /usr/bin

[me@linux bin]$ pwd

/usr/bin

[me@linux bin]$ ls

...Listing of many, many files ...

Now we can see that we have changed the current working directory to /usr/bin and that it is full of files. Notice how the shell prompt has changed? 

As a convenience, it is usually set up to automatically display the name of the working directory.

Relative Pathnames

Where an absolute pathname starts from the root directory and leads to its destination, a relative pathname starts from the working directory. To do this, it uses a couple of special notations to represent relative positions in the file system tree. These special notations are . (dot) and .. (dot dot).

The . notation refers to the working directory, and the .. notation refers to the working directory’s parent directory. Here is how it works. Let’s change the working directory to /usr/bin again.

Options and Arguments

This brings us to a very important point about how most commands work. Commands are often followed by one or more options that modify their behavior and, further, by one or more arguments, the items upon which the command acts. So, most commands look kind of like this:

command -options arguments

Most commands use options, which consist of a single character preceded by a dash, for example, -l. Many commands, however, including those from the GNU Project, also support long options, consisting of a word preceded by two dashes. Also, many commands allow multiple short options to be strung together. In the following example, the ls command is given two options, which are the l option to produce long format output, and the t option to sort the result by the file’s modification time.

[me@linux~]$ ls -lt

We’ll add the long option --reverse to reverse the order of the sort.

[me@linux~]$ ls -lt --reverse  

Note Command options, like filenames in Linux, are case sensitive. The ls command has a large number of possible options