Unix how does it work

If, on the other hand, your machine is atlantis, albion, antioch, daedalus, vulcan, etc. Then follow the steps above. The machines nearer the front of the lab with the larger montiors are newer and faster - pick these first if you can. The older the machines are towards the back. If you do use the remote login option - look at the number of users on each machine, and try and opick an unloaded one, for obvious reasons.

When successfully logged in, if you are sitting at a workstation then you will usually have several areas to type in, or windows, started up for you along with such graphical utilities as a clock and a calendar after the Message of The Day or motd has been displayed.

You should always read the motd as this where important information relating to the computer system are placed, advising of, for instance, the availability of new services, new software, or system down-time for maintenance. If you are logging in remotely from a PC or Macintosh with access to the net work then you will be presented with the motd and underneath that the UNIX prompt.

When using the workstations, you should see this prompt in each of your windows, but the motd should have disappeared. Since workstations are relatively expensive and hence a limited resource, and they may be under varying degrees of demand throughout the year, you should only login at a workstation i. The UNIX prompt is a sequence of characters that the part of the operating system OS that you interact with, known as the shell , places at the start of each line when it is expecting you to enter a command.

For instance. The nature of the prompt will vary from system to system, but the Geography machines have a prompt as above. When you become experienced you can customise the prompt to your own liking. The cursor a small solid rectangle or underscore symbol placed after the prompt tells you where your current typing position is clicking in the window with the mouse doesn't mover the cursor - it always remians at the end of the current line you're on.

The current line with the prompt is therefore known as the command line. There a variety of shells available and as you become more experienced you may choose to use shells other than the C-shell we provide by default so named because of similarities of the command syntax parts of the C programming language , and many of these shells offer command line editing as in DOS.

When you have finished your session at the computer, you must log off. From within a windowing session at the machine itself, select the logout option from your root menu see the section on the windowing system for more details , and then confirm the logout by clicking on yes.

If you leave yourself logged on then other people may come along and use your account, access your email etc. Whilst in a University environment this is not necessarily a problem - the next user will probably come along and log you out so that they can log on themselves it is important to log out so that other people don't waste time wondering whether you have finished, or have just popped out for a few minutes.

Don't lock your workstation - this is anti-social behaviour as it prevents anyone else using it, and if we see it we'll log you out anyway, and probably take all your pocket money away. If you leave for more than a few minutes, log yourself out. One of the advantages of a UNIX system is that you can leave a job running in the 'background' don't worry, we'll get to that later which means it'll keep running for as long as you want even after you log out.

All information in UNIX, whether it is a program, data or text, is stored in files. These files are maintained by the operating system on hard disks usually , and read into the computer's memory when required. If what you want is for the variable to be available to every session, instead of just the current one, you will need to set it in your shell run control. Then add the set line or the setenv line shown above to automatically set the variable or environment variable for every session of csh.

Bash is a great all-rounder, with excellent documentation, while Zsh adds a few features on top of it to make it even better. Fish is amazing for newbies and helps them learn the command line. Ksh and Tcsh are better suited for advanced users, who need some of their more powerful scripting capabilities. A Bourne shell drawback is that it lacks features for interactive use, such as the ability to recall previous commands history.

The Bourne shell also lacks built-in arithmetic and logical expression handling. The shell provides you with an interface to the UNIX system. There are different flavors of shells, just as there are different flavors of operating systems.

Skip to content Android Windows Linux Apple. Home » Other. Although UNIX was developed by programmers for programmers, it provides an environment so powerful and flexible that it is found in businesses, sciences, academia, and industry. Many telecommunications switches and transmission systems also are controlled by administration and maintenance systems based on UNIX.

While initially designed for medium-sized minicomputers, the operating system was soon moved to larger, more powerful mainframe computers. As personal computers grew in popularity, versions of UNIX found their way into these boxes, and a number of companies produce UNIX-based machines for the scientific and programming communities. Many computers do just one thing at a time, as anyone who uses a PC or laptop can attest.

Try logging onto your company's network while opening your browser while opening a word processing program. Chances are the processor will freeze for a few seconds while it sorts out the multiple instructions. UNIX, on the other hand, lets a computer do several things at once, such as printing out one file while the user edits another file.

This is a major feature for users, since users don't have to wait for one application to end before starting another one. The same design that permits multitasking permits multiple users to use the computer. The computer can take the commands of a number of users -- determined by the design of the computer -- to run programs, access files, and print documents at the same time. This is where the pipe, , pipes in. It represents the system call to pipe and all it does is redirect stdin and stdout for processes.

Since things have been designed so well, this otherwise complex function reduces to just this. This image is a bit of a simplification to explain the pipe redirection.

You know how the shell works now, so you know that the top bash forks another bash connected to tty , which produces the output of ls. You also know that the top bash was forked from the lower one, which is why it inherited the file descriptors of the lower one. This pipeline figures out the largest file in the current directory and outputs its size.

Who knew this was built into ls already. Notice how stderr is always routed directly to tty? What if you wanted to redirect stderr instead of stdout to the pipe? You can switch streams before the pipe. Source: this beauty. Local variables are ones you can create in a shell. Environment variables env vars are like global variables. They are passed to children.

Try this: call bash from bash from bash. The first bash is waiting on the second bash to exit, while the second one is waiting for the third one. When you call exec , the exit happens automatically.

If not, you want to type exit yourself to send the exit code to the parent. Now, thought experiment: What happens when you type ls into a shell? You know the fork , exec and wait cycle that occurs, along with tty. But, even before this happens, ls is just another utility function, right? Remember how the file system tree is heirarchical?

All the base level directories have a specific function. Bin stands for binaries. This is enough knowledge for us. So, you can do this:. But how did the shell know to look for ls in bin? This is where the magical environment variable, PATH comes in. The PATH is a colon separated list of directories. It executes the first file it finds.

Nothing should work without an absolute path. The above syntax is used to set the environment variables for just one command. The old PATH value still exists in the shell. The is syntax for a new subshell. Have you heard that. Now you know. When you do. So, it makes sense that if the current directory were always on the PATH , your scripts would work by name. And then run just script. There are several problems -. The PATH is busted!

Its either installed to a location not on your PATH so you can only call it from the place you installed it to. To fix this, you now know that you can either add that directory to the PATH , or call the file via its absolute path. This post has already become much longer than I expected. Also, programming with shell has been covered online a lot. It works really well on your machine and now you want to sell it to other users. Wait, I mean, in the spirit of open source, you want to make it available for others to use.

You also want to save them the PATH headaches. Package managers solve exactly this problem. There are three main package managers I know of: dpkg , rpm , and homebrew. For example, pip is a popular Python tool installer. Unix was the first of its kind operating system that allowed multiple users to use it, and every user could run more than one program at the same time.

This might sound trivial now, since almost every operating system has this, but it was revolutionary when it first came out. The days of leasing time on a big mainframe were over. You could let your program run in the background while someone else did their work. Bell labs developed it in the s. These were a set of guidelines, and any system that followed them could be certified as a UNIX system. Around the same time, some people were unhappy with the proprietary nature of UNIX, and came up with another open source operating system kernel called Linux.

These systems are thus also called UNIX-like.