Shells in Linux

A shell is a command-line interpreter or shell that provides a traditional user interface for the UNIX operating system and for Unix-like systems. Users direct the operation of the computer by entering commands as text for a command line interpreter to execute or by creating text scripts of one or more such commands. The versatility of the command-line shell is what really allows this, but what makes each shell different and why do people prefer one over another?

 Fig1: Shells in Linux

The most influential UNIX shells have been the Bourne shell and the C shell. The Bourne shell, sh, was written by Stephen Bourne at AT&T as the original UNIX command line interpreter; it introduced the basic features common to all the UNIX shells, including piping, here documents, command substitution, variables, control structures for condition-testing and looping and filename wildcarding. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68. The C shell, csh, was written by Bill Joy while a graduate student at University of California, Berkeley. The language, including the control structures and the expression grammar, was modeled on C. The C shell also introduced a large number of features for interactive work, including the history and editing mechanisms, aliases, directory stacks, tilde notation, cdpath, job control and path hashing.

Users typically interact with a modern UNIX shell using a terminal emulator. Common terminals include xterm and GNOME Terminal. Both shells have been used as coding base and model for many derivative and work-alike shells with extended feature sets.

What Do Shells Do?

The command-line is a very interesting thing. Once considered to be the most advanced user interface, it has gone the way of waistcoats and fountain pens: shunned to the periphery. While you still see much of the purpose and utility in them, they’re usually left aside primarily for enthusiasts to appreciate, mainly because they spend the time to learn the ins and outs of them. Indeed, the command-line in any given operating system will have a lot of quirks because easy OS interprets commands differently. Today, this is mainly an issue between Linux, OS X, and Windows, but before this was a problem with most computers.

 Fig2: Shell types

Shells entered the picture and allowed a more standard extension of the command-line in a way that was much more inconspicuous. Shells added a lot of functionality, such as command and file name completion and more advanced scripting abilities, and helped bring some performance enhancements. They also did a lot to cover some annoying problems. For example, in Unix, you couldn’t back up through symlinks to the directories before you followed them. All in all, they added some features that allowed users to get their jobs done more quickly and efficiently, just like Linux’s plethora of alternative Window Managers.

Why Are There So Many?

The most prominent progenitor of modern shells is the Bourne shell – known as ‘sh’ – which was named after its creator Stephen Bourne who worked at AT&T. It became the default UNIX command-interpreter because of its support for command substitution, piping, variables, condition-testing, and looping, along with other features. This was in an era that programming really went along with using the command-line, a practice that many argue has been diluted today. It did not offer users much leeway in customizations for users, such as aliases, command completion and shell functions.

Fig3: Shell varieties

C shell (‘csh’) was developed Bill Joy at UCB and really shook things up. It added a lot of interactive elements that users could use to control their systems, like aliases (shortcuts for long commands), job management abilities, command history, and more. It was modeled off of the C programming language, an interesting idea because UNIX was written in C. It also meant that users of the Bourne shell had to learn C so they could enter commands in it. In addition, it had tons of bugs which had to be hammered out by users and creators alike over a large period of time. People ended up using the Bourne shell for scripts because it handled non-interactive commands better, but stuck with the C shell for normal use.

Over time, lots of people fixed the bugs in and added features to the C shell, culminating in something called ‘tcsh’. The problem, then, was that in distributed Unix-based computers, csh was still the default, and had some added “non-standard” features added in, creating a very fragmented mess of things (in retrospect). Then, David Korn from AT&T worked on the Korn shell – ‘ksh’ – which tried to mitigate the situation by using the Bourne shell’s language as a basis, but added in all of the new features that everyone was accustomed to. Unfortunately for many, it wasn’t free.

 Fig4: tcsh

Another response to the hectic proprietary csh implementations was the Portable Operating System Interface for UNIX, or POSIX. It was a successful attempt at creating a standard for command interpretation and eventually mirrored a lot of the features that the Korn shell had. Simultaneously, the GNU project was going on and was an attempt at creating a free operating system that was completely Unix-compatible. It developed a shell for its own purpose: the Bourne Again shell, formed by bashing together features from sh, csh, and ksh. The result, as seen in retrospect, was pretty impressive.

Kenneth Almquist created a Bourne shell clone – ‘ash’ – that was POSIX-compatible and would become the default shell in BSD, a different branch/clone of UNIX. Its uniqueness is that it’s really lightweight, so it became extremely popular in embedded-Linux systems. If you have a rooted Android phone that has Busy Box installed, it’s using code from ash. Debian developed a clone based on ash called ‘dash’.

 

Fig5: Ash

One of the most prominent of “new” shells is ‘zsh’, developed by Paul Falstad in 1990. It’s a Bourne-style shell that takes features from bash and previous shells and adds even more features. It has spell-checking, the ability to watch for logins/logouts, some built-in programming features like bytecode, support for scientific notation in syntax, allows for floating-point arithmetic, and then some. Another is the Friendly Interactive Shell, ‘fish’, which focuses on command syntax that’s easy to remember and use.

On the whole, most shells were created as clones of previous shells that added functionality, fixed bugs, and bypassed licensing issues and fees. The notable exceptions are the original Bourne shell and the C shell, and both rc shell and ash which aren’t entirely original but definitely have some niche-utility.

Which Should I Use?

With so many out and about, you’d think that it’s hard to choose which shell to use, but it’s actually not very difficult. Since so many are based off of Bourne shell, basic things will be the same between most shells.

 Fig6: Shell choices

Bash is the most widely used shell out there and is the default for most Linux distributions. It’s really robust and has tons of features, most of which you probably won’t use unless you program, so it’s pretty safe to say you can stick to this one. Because it’s so common, it’s perfect for scripting things that will be used across different platforms. If you want to try something different that’s a little more user-friendly, you can try out fish instead.

If you tinker with embedded-Linux systems a lot, like to put Linux on ridiculous things like your Nintendo DS, or you really like Debian, then ash/dash is probably best suited for you. Again, it works mostly like the others, but since it’s sort of bare-bones and light-weight, you’ll find some more complicated functionality missing.

Fig7: Zsh

If you plan on programming or learning to develop on the command-line, then you’ll have to be a little choosier. Bash is a fine choice, but I know enough people who have switched to Zsh for its extras. I guess it really depends on how complicated your projects will get and what type of functionality you prefer from your shell. Some people still stick to tcsh because they know and use C regularly and it’s easier for them. Odds are that if you’re not sure which shell to choose, you probably don’t program much, so try to choose something that will line up with what you want to learn and do some research on what others in that field use.

You can easily install and remove different shells using the Software Center on Ubuntu or your preferred package manager. Shells are located in the /bin/ directory, and as long as you’re running a modern Linux distribution, it’s easy to change what your default is. Just enter the following command:

• chsh

You’ll be prompted for your password, and then you can change to a different shell by entering its path.

 

Fig8: Chsh 

In square brackets, you’ll see your current default, and if you want to leave it as-is, just hit Enter.

So guys, which shells you use or prefer? Do comment.

Getting System informantion under Linux

Hello firends in this blog I am writing about how to extract information of system. Information like architecture, kernel etc. For an adminstrator it is important to know there own system configuration. Yes it is part of system administration. There are various commands available to check system configuration such as df, du,free etc. But here we are discussing some of them.

There is most popular command UNAME (In small) is available in our system. You can get information about it by:

root@ravikumar:~$man uname

1. Ok, now we will discuss how to use uname to get system information. Look at figure 1 first command . It shows that the name of the kernel which our system using i.e. Linux.
2. To check that which version of kernel system is using look at figure 1 second command, shows kernel version.
3. To check which type of operating system it is use fourth command in figure 1.
4. If you want to see all information at once then refer command 4 in figure 1. There are other options are also available please check it by self.                   
   
   
   
5. To check that disk space usage use command “ df ”. shown in figure 2 first command.
6. To check used and free space memory use second command in figure 2.
   
   
   
7. To check disk space usage of each file on system use third command in figure 2. command is "du -h".        
   
   
   
   
   

XAMPP: A web server package

XAMPP is a cross platform web server package. Cross platform means it works on every possible operating system without any compatibility issues. XAMPP is the acronym for: X-cross platform, A- Apache HTTP server, M- MySQL, P-PHP, P- Perl. The main advantage of XAMPP is that it requires very less configuration for its components, which make it quite user friendly. The default FTP user for XAMPP is "newuser", and default FTP password is "wampp". Similarly default MySQL user is "root", and there is no default password for MySQL.

XAMPP contains its own XAMPP Control Panel, where all the modules and their current status is given. We can very easily start or stop any module which we want, by clicking on "Action" button beside modules. We can even configure the modules. A small pop-up screen appears when we click on "config" button.

There is one main config button on top right side.  When we click on that main config button on top right corner then the following window will appear.

Below all this modules there is a terminal which displays the actions and generate a log file which we can view by clicking on logs. There are several different XAMPP distributions, 1. XAMPP for Linux, 2. XAMPP for Windows, 3. XAMPP for Mac OS X, 4. XAMPP for SOLARIS. Installing XAMPP is also very simple task in windows 8 just download the zip file and extract it and double click on .exe file. The folder name xampp will be created in C drive. So basically XAMPP is very user friendly and easy to use cross platform web server.

XAMPP is developed by Apache Friends and the Official website for XAMPP is  www.apachefriends.org/en/xampp.html

Resource utilization in Unix like operating system

Uptime:

The uptime shell command is used to get the system overall performence. It will display report in one line that how long system has been started, how many users are currently logged in and system load avarage during last 1, 5, 15 minutes.

Figure 1.Output of uptime

SAR(System Activity Report):

The sar command displays more information of CPU utilization. It displays CPU utilization for specified time. In Ubuntu sar package is not installed. We can install it by using: “root:\sudo apt-get install sar”. The sar command basically looks like “root:\sar seconds time”. Here seconds is used to display CPU utilization in specified time and time is for display report specified number of time. There are some other options available to display other system statistics, are as A, B, b, C, d, e, f, H, h, I, i, m, n, o, P, p, q, r, R, S, s, t, u, V, v, w, y.

Figure 2 Output of sar

PS:

ps command is used to examine the the running the processes. The most useful options is ps -el. Here e specifies every running processes and l options is for generating long list. The output of ps -el looks like as:

meaning of fields in output of ps -el given bellow:

• S: Shows the process’s current state (running, sleeping, and so forth) . S field takes one of the following values:

1. O— The process is currently executing on a CPU.

2.   S — The process is sleeping, waiting for another event to complete.

3. R — The process is in the run queue (runnable) and is waiting to run.

4. Z — The process is a zombie (it terminated, but its parent has not reaped its exit status).

5.  T — The process is stopped.

• UID: Lists the numeric user ID (UID) of the user who owns the process
• PID:  Lists the process’s process ID (PID) .
• PPID:Lists the PID of the parent process.       
• C: Shows the CPU utilization of the process.
• PRI:States the process’s priority (higher numbers mean higher priority) .
• NI:Shows the process’s nice value (higher numbers mean lower priority) .
• SZ:Lists how much swap space (virtual memory) the process requires .
• TTY:Names the terminal on which the process started (also known as the controlling terminal).
• TIME:Summarizes the total CPU time (in hours and minutes) that the
• process has consumed .
• CMD:Displays the command that initiated the process .

 There are some other commands available to check resource utilization.

Figure 3. Output of ps -el