264 lines
13 KiB
Plaintext
264 lines
13 KiB
Plaintext
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H-NET H-NET H-NET H-NET H-NET H-NET H-NET H
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N N
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E ** H-Net Magazine ** E
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T T
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H Volume One, Issue 1, File #03 of 20 H
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N N
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E An Introduction to UNIX, by MINOTAUR. E
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T T
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H-NET H-NET H-NET H-NET H-NET H-NET H-NET H
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UNIX is widely-touted as 'the operating sustem of the future', though in fact
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it dates from 1969 (it was developed originally by Bell Laboratories, which
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runs most of the telephone systems in the USA). Since then it's been under
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continuous development; so it's not so much an old operating system as a fairly
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mature one, if you see what I mean. Hackernet BBS has many unix files for you.
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Unix gets brownie points straight off because it was conceived by a user of
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computers rather than a software specialist or some other sort of software
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designer. So it is relatively easy to use; which nearly all other software the
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prime goal often seems to be to make money, or to sell more hardware and
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software, or whatever.
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In brief, UNIX is a general-purpose, multi-user operarting system with a clever
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method of holding files. It's a complex system, which means both that it is
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rich in facilities and difficult to get to grip with - until now!.....
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BEGINNER'S GUIDE TO THE WONDERFUL WORLD OF UNIX
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The aim of this first textfile is to give an insight into the fundamentals of
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UNIX. Subsequent textfiles will delve into all areas of UNIX in greater depth.
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WHAT IS UNIX?
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~~~~~~~~~~~~~
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The UNIX operating system can be divided into a number of distinct parts. First
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there is the part of the system which performs all of the interface to the
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hardware, scheduling disks, managing memory, handling terminal I/O and
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generallu handling any requests to and from the devices on the system. This
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part of UNIX is called the UNIX Kernel. It is in fact one large, compiled 'C'
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program which is kept on the hard disk and loaded into memory when the system
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is booted up. The UNIX Kernel is always kept in memory because practically
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everything that is done on UNIX uses the Kernel. Buts on its own, it is of
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little use. There is no point in just having an interface to the hardware - an
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itnerface to the user is also required. This is provided by the UNIX shell.
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The shell is another name for a TIP (Terminal Interface Program) or a JCL (Job
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Control Language). Its job is to read input from the terminal and execute the
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right programs, depending on the input. A program which is executing in UNIX
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is called a process. In fact, the shell is also a 'C' program which is
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executing for each terminal which is logged on. It reads input from the
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terminal, interprets the input in various ways and starts the appropriate
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processes.
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As well as interpreting input and executing processes, the shell has its own
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control-flow constructs and it can therefore be used as a programming language.
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It also handles pipes and redirection of I/O which will be the subject of a
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later textfile.
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The way the UNIX shell works is much the same as other operating systems. It
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displays a prompt, normally '$' the user types in the command to execute and
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the shell reads the command, searches for the program which is the same name as
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the command typed in and it then executes this file, if it found it. When the
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program finishes executing, the shell redisplays the prompt to the terminal.
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OK, so we have a Kernel to handle the devices and a Shell to interpret input
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form the users terminal. The final and by far the largest part of UNIX is the
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Programs. There are many Programs in UNIX which form a part of the operating
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system. In UNIX jargon, these are called Utilities.
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There are Utilities to look at directories, edit files, semd files to the line
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printer, perform backups, look at files, delete files, the list is endless.
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These Utilities are, to the user, commands which are typed in at the terminal
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prompt. For example; 'ls' is the UNIX Utility to list the directory; 'cd' is
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the Utility to change directory. In some systems the Utilities form a part of
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the terminal interface program which interacts with the user. In UNIX, very
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few Utilities are a part of the Shell, the majority of them are external to the
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Shell and are called up whenever they are needed.
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This is because there are 200-300 UNIX Utilities, and including all of them in
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the shell would obviously increase the size of the Shell to an unreasonable
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level. It is much neater to keep them on the disk to be called as and when
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they are needed.
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On a typical UNIX system, as well as the operating system, there would also be
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some other packages, word processing, spreadsheets, etc. These are necessary
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because UNIX does not include any of these - UNIX is only an Operating System.
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THE UNIX FILE STRUCTURE
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~~~~~~~~~~~~~~~~~~~~~~~
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The above description of UNIX is very conceptual. You never actually se the
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UNIX Kernel, excpept perhaps as a process on the system. The Shell too, is
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just always there when you login. The file structure, however, has to be
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manipulated and managed by the user all the time. The UNIX file structure is
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hierarchical.
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Within the file structure there are files and there are directories - as shown
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in the diagram below :-
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/(root)
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_________________________________________________
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bin dev etc lib tmp unix usr
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----- ----- ----- ----- ----- ----- -----
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--- ---
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Files contain data and directories contain either files and/or directories. At
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the top of the hierarchy is the ROOT directory, this is sometimes referred to
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as /(slash). Below the root directory are the system files and directories.
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When a user logs into the system, he logs into his HOME directory. Every user
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of the system has a HOME directory where he can create and delete files and
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directories. This part of the system belongs to him. User directories are
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usually kept in the directory 'usr' which is directly below the root directory.
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For example, say the user Fred has the directory structure as shown below :-
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/(root)
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usr
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fred
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----------------------------------
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cprogs memos wpfles sprdsht pending
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----- ----- ----- ----- -----
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1990
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When Fred logs on he will automatically go to the directory 'fred'. If
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however, Fred wants to examine his spreadsheets, he may want to go to the
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spreadsheet directory. He would do this by using the 'cd' command to change
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directory, i.e:
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$cd sprdsht
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This command would make his CURRENT DIRECTORY the spreadsheets directory but
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his HOME directory wouls remain the same. If he wanted to move to his 1988
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directory from his home directory the command would be :
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$cd sprdsht/1990
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The '/' between the two directory names is the delimiter.
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The construct 'sprdsht/1990' is called a Pathname. Infact this particular type
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of Pathname is called a Relative Pathname because it is relative to the
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directory the user is currently in. That is, the path 'sprdsht/1990' would be
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meaningless if the users current directory was 'wpfls'.
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Another type of pathname is an Absolute Pathname. This, as the name suggests,
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is a path from the root directory and is therefore independent of the user's
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current directory. Absolute pathnames are represented by using a '/' as the
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first character of a path. For example, the Absolute Pathname of the directory
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mentioned would be '/usr/fred/sprdsht/1990'.
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The command :-
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$cd /usr/fred/sprdsht/1990
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would take Fred right to the directory regardless of his current directory. the
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command to find the Absolute Directory is :-
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$pwd
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The stands for 'print working directory'. As mentioned above, the command to
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look at the contents of a directory is 'ls'. By simply typing :-
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$ls
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a list of filenames is output. This looks like :-
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cprogs
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memos
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wpfls
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sprdsht
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pending
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This is fine if just the names is required but, it is impossible to tell from
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this information which are files and which are directories. As with a lot of
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UNIX commands, the 'ls' command performs a mimimum function.
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UNIX commands, generally speaking, have a number of options which can be used
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with them, depending on exactly what the user wants. For example the 'ls'
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command just lists directories, if you want extra information, you have to ask
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for it. UNIX must have either been written by a mimimalist or someone who
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didn't like typing!!! It does no more and no less than it is asked to, this is
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something that users find quite difficult to understand.
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The 'ls' command is the perfect example of this. In most other systems it
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would be called 'dir' or 'list'. The most commonly used option with 'ls' is
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'ls -l'. In this case the 'l' stands for 'long'. The command therefore
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produces a long listing of the current directory. The output looks soemthing
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like that shown below :-
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drwxr_xr_x 4 Fred 96 Jun 1 10:00 cprogs
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drwxr_xr_x 4 Fred 195 Jul 25 09:34 memos
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drwxr-xr_x 2 Fred 167 Aug 14 17:29 wpfls
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drwxr_xr_x 2 Fred 84 May 7 07:56 sprdsht
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drwxr_xr_x 2 Fred 952 Jun 16 13:51 pending
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This will tell you whether the entry is a file or a directory. The permissions
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(r is for read, w is for write, x is for execute), the number of links to the
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file, the owner, the size (in bytes), the date it was last modified, and the
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name of the entry. Options for the commands are sometimes (not always)
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preceeded with a minus sign and they usually consist of a letter which usually
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stands for something. The reason for this vagueness in the format of UNIX is
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that UNIX has evolved through time and commands have been written by a variety
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of people from a variety of places so there are no hard and fast rules - this
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is another thing that newcomers to UNIX find frustrating. It would be true to
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say that for each one of the 'rules' mentioned above, there are exceptions.
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There are also special commands for adding and removing directories from the
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directory structure. They are 'rmdir' for removing directories and 'mkdir' for
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making directories. They both expect a Pathname as a parameter which is the
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name of the directory to be created or removed. For example, to create the
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directory 'letters' below the directory 'wpfls' from Fred's HOME directory :-
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$mkdir wpfls/letters
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To remove this directory :-
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$rmdir wpfls/letters
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The command 'rmdir' will only remove empty directories, that is directories
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which do not contain files or sub-directories.
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Every directory in UNIX has two hidden entries, these are '.'(dot) and '..'
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(dot-dot). These are created when the directory is created. '.' references
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the current directory and '..' references the directory above the current
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directory (called the Parent directory). These two directories are in fact the
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key to the way the whole file system is put together. They can be seen by
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using the '-a' option in the 'ls' command (i.e. 'ls -al') and they can also be
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used in relative pathnames. For example :-
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$cd ../wpfls
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from the directory 'sprdsht' in Fred's HOME directory is a perfectly acceptable
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command.
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With these commands many generations of UNIX users have had their first
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foot-hold on UNIX. If you have any UNIX problems or comments to make then
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please leave me (MINOTAUR) a message on the Hackernet BBS.
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Minotaur.
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===============================================================================
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[Hackernet BBS,LEEDS,UK(0532)557739, 24hrs. Home of H-Net Hacking magazine]
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