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UNIX
Objective of UNIX
The objectives of this module is to
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introduce you to managing a UNIX system |
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write shell scripts |
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process text files and generate reports (program in awk) |
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use system tools and utilities |
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develop programs based on sockets and process communication |
1.0 History
This section introduces you to a brief overview of the history of UNIX.
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written by Ken Thompson and Dennis Ritchie |
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developed at Bell Laboratories, USA, derived from multics (1969) |
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Thompson developed a new programming language 'B' |
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Ritchie enhanced 'B' to 'C' and helped develop 'UNIX' |
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Ran on PDP-7 |
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Two flavours, SYSTEM V (Commercial, run by AT&T) and BSD (Educational, run by Bell Labs) |
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Recent developments are graphical interfaces, MOTIF, X Windows, Open View |
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Major
suppliers today Sun Microsystems (SPARC) Data General (AVION) IBM (RS6000 AIX) Hewlett Packard Santa Cruz Organisation (SCO) DEC |
A free UNIX system called LINUX is also available for download from the Internet or available on CD-ROM.
2.0 Main Features of UNIX
This section contains a brief overview of the main features of UNIX. At the time
UNIX was introduced, some of these features set the UNIX operating system apart
from other systems available at that time. Today, many of these features are
common place.
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multi-user more than one user can use the machine at a time supported via terminals (serial or network connection) |
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multi-tasking, more than one program can be run at a time |
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hierarchical directory structure, to support the organisation and maintenance of files |
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portability only the kernel ( <10%) written in assembler. This meant the operating system could be easily converted to run on different hardware |
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tools for program development, a wide range of support tools (debuggers, compilers) |
3.0 The UNIX Operating System
This section outlines the basic structure of the UNIX operating system, as a
division of three parts.
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kernel schedules programs manages data/file access and storage enforces security mechanisms performs all hardware access |
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shell presents each user with a prompt interprets commands types by a user executes user commands supports a custom environment for each user |
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utilities file management (rm, cat, ls, rmdir, mkdir) user management (passwd, chmod, chgrp) process management (kill, ps) printing (lp, troff, pr) program development tools |
The switching between user programs is done by part of the kernel. To switch from one program to another requires,
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a regular timed interrupt event |
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saving the interrupted programs state and data |
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restoring the next programs state and data |
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running that program till the next timed interrupt occurs |
The timed event is usually about 1 to 10 milliseconds apart. It is generated by a real-time clock.
4.1 Handling Programs
Each computer has a maximum amount of memory (RAM) that is installed in the
computer. Some of this memory is required by the operating system. The remainder
is available to user programs.
The more memory that can be provided in total the better. Where there is insufficient main memory to run a users program, some other users program residing in main memory must be written out to the disk unit to create some free memory space.
This process is called swapping. When the system becomes overloaded (where there are more users than the system can handle), the operating system spends most of its time shuttling users programs between main memory and the disk unit, and users response time degrades. This is called disk thrashing and is overcome by installing more main memory.
4.2 Processes
Each program running on a UNIX system is called a process. When a user types a
command, UNIX constructs a Process Control Block (PCB) for the process that
process. Each process has a PCB that holds its priority, the process state,
register information and additional details.
UNIX provides a set of utilities for managing processes.
| ps | list processes |
| kill | kill a process |
| & | run a process in the background |
If the system administrator found that a particular user was performing an operation that was consuming too much computing time or dominating a system resource such as a printer, they could use the ps command to identify the offending users process and then use the kill command to terminate that process.
Each program is assigned a priority level. Higher priority tasks (like reading and writing to the disk) are performed more regularly. User programs may have their priority adjusted dynamically, upwards or downwards, depending upon their activity and available system resources.
Multi-tasking systems support foreground and background tasks. A foreground task is one that the user interacts directly with using the keyboard and screen. A background task is one that runs in the background (it does not have access to the screen or keyboard). Background tasks are usually used for printing.
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lets the user carry on with more important tasks |
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examples are printing and formatting documents |
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the ampersand symbol (&) is appended to the command |
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the shell assigns a process number (pid) to the command |
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background jobs can be deleted using the kill command |
4.3 When a user logs on
When the UNIX system starts up, it also starts a system process (getty) which
monitors the state of each terminal input line. When getty detects a user has
turned their terminal on, it presents the logon prompt and once the password is
validated, the UNIX system associates the shell program (/bin/sh) with that
terminal.
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user is presented with a shell. This is a program which displays the
users prompt, handles user input and displays output on the terminal.
The shell program provides a mechanism for customising each users setup requirements, and storing this information for re-use (in the file .profile). The user interacts with /bin/sh, which interprets each command typed. Internal commands are handled within the shell (set, unset), external commands are invoked as programs (ls, grep, sort, ps). |
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There are a number of different command line shells (user interfaces).
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Bourne (sh) |
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Korn (krn) |
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C shell (csh) |
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Bash (an improved Bourne shell with history and aliases) |
The shell is often called a command line shel, since it presents a single prompt for the user. The user types a command, the shell invokes that command, then presents the prompt again when the command has finished. This is done on a line by line basis, hence the term 'command line'.
Recent enhancements turn the command line interface into a graphical one (X, MOTIF, OPENVIEW), where programs are represented as objects or icons on a screen. By use of a mouse these icons are selected and run. This is similar to operating systems like Windows or OS/2.
5.0 Unix Command Syntax
This section outlines the standard format for all UNIX commands.
| Commands are short two or three character names, and accept additional options that control their actions. |  |
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all commands have a similar format |
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commands are generally two or three characters long |
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commands are case sensitive (use lowercase) |
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options always preceed filenames |
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options are prefixed using a - symbol |
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the man command can be used to display the correct syntax |
6.0 The UNIX File System
This section discusses UNIX files and directories. The file system refers to the
way in which UNIX implements files and directories. In UNIX, a file system has
the following features,
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hierarchical structure (support for directories) |
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files are expandable (may grow as required) |
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files are treated as byte streams (can contain any characters) |
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security rights are associated with files and directories (read/write/execute privledge for owner/group/others) |
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files may be shared (concurrent access) |
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hardware devices are treated just like files |
6.1 What is a file?
A file is a collection of information, which can be data, an application,
documents; in fact, under UNIX, a file can contain anything. When a file is
created, UNIX assigns the file a unique internal number (called an inode).
6.2 What is a file Link?
A file link is a directory entry which points to an original file somewhere
else. A link is made to an existing file using the ln command. This creates a
directory entry which points to the existing file (it does not make a copy of
the existing file). This allows more than one reference to an exisiting file.
For instance, a person can give another access to a file and let them create a link to it. In this way they both can access and work with the same file, ensuring that the information they enter into the file is up-to-date. Only the original owner of the file may delete the file.
6.3 What are the security rights associated with files and
directories?
UNIX provides three sets of security rights for each file and directory entry.
These sets are for the owner, the group to the owner belongs, and any other
users on the system.
Each user belongs to a group (only one at a time). Group membership facilitates the sharing of common files. A user can change their membership to another group by using the newgrp command.
The security rights are
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read (read, display, copy the file contents) |
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write (modify and append to the file contents) |
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execute (run the file as a program) |
| The security bits are grouped as a series of three bits for each of the owner, group and other access rights. |  |
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The
group to which the file belongs is changed using the chgrp
command.
The owner of the file is changed using the chown command. Security rights for a file or directory are modified by using the chmod utility.
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6.4 Listing files
| Files
are listed using the ls command.
This picture shows the security rights associated with the file, as well as the owner (joe), the group that the owner belongs to, the size of the file and other information. |
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6.5 Wild Card Characters
Wild card characters are used when working with a number of files at once.
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6.6 Mountable File Systems
All UNIX systems have at least one permanent non-removable hard disk system. The
root directory and the directories below it are stored on this disk. Its
structure is known as the root file system.
| If an additional hard disk is added, UNIX creates a separate new filesystem on this disk. Before it can be used, it must be attached to the root file system. This is called mounting an additional filesystem. |  |
An empty directory on the root file system is selected as the mount point, and using the UNIX mount command, the new file system will appear under this directory.
6.7 UNIX Standard Devices
| There are THREE standard devices supported by the UNIX shell, stdin, stdout and stderr. Each program that runs has allocated to it a stdin, stdout and stderr device. |  |
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these devices may be redirected to another device or file |
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standard output (stdout) is associated with the users terminal display |
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standard input (stdin) is associated with the users terminal keyboard |
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standard error (stderr) is associated with the users terminal display |
| When a program is executed, it has associated with it each of three standard UNIX devices. |  |
6.8 Device Redirection
| The input or output devices associated with a program may be redirected to another device. |  |
For example, a program that normally reads from stdin (the keyboard) can be redirected to read from a file.
| In a similar manner, a program that normally writes to stdout (the screen) can redirect its output to a file or printer. In this example, the ls command redirects stdout to files. |  |
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the standard devices may be redirected to a device or file |
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other devices can be printers, terminals |
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devices appear as files in the /dev subdirectory |
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the symbol > changes the standard output device, creating a new file |
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the symbol < changes the standard input device |
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>> redirects stdout, appending to an existing file |
6.8.1 Shell Device Redirection
It is also possible to redirect any device using the file descriptor number
assigned to that device.
| By default, the c compiler sends the error output to the screen. To redirect the error output to a file, it is necessary to use the file descriptor. |  |
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stdin has a file descriptor of 0 |
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stdout has a file descriptor of 1 |
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stderr has a file descriptor of 2 |
6.9 Pipes
Sometimes, the use of intermediatary files is undesirable. Consider an example
where you are required to generate a report, but this involves a number of
steps. First you have to concatenate all the log files. The next step after that
is strip out comments, and then to sort the information. Once it is sorted, the
results must be printed.
A typical approach is to do each operation as a separate step, writing the results to a file that then becomes an input to the next step.
| Pipelining allows you to connect two programs together so that the output of one program becomes the input to the next program. |  |
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allow commands to be combined in a sequential order |
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connects stdout of one program to the stdin of the next program |
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the symbol | (vertical bar) represents a pipe |
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any number of commands can be connected in sequence, forming a pipeline |
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all programs in a pipeline execute at the same time |
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complex operations are easily perfomed by piping commands |
7.0 Shell Scripts
To automate many routine tasks, a system administrator will create a file that
contains the commands to be executed. This is known as a shell script file.
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is an example of a shell script file.
It is a file called testlogin that accepts a username and then tests to see if that user is logged on. |
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Shell scripts
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contain command sequences |
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have execute permission (using chmod) |
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simplify repetitious command sequences |
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are run by the shell as if it has been typed at the terminal |
7.1 Create A Shell Script
Shell scripts must be in ASCII text format, and can be created using a simple
editor such as vi.
| This example uses the cat command (and redirect output) to create the script file that echos the current date when executed. |  |
7.2 Set 'execute' And Run A Shell Script
Once an administrator creates a shell script, its rights must be changed to
executable before it can be run.
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administrator used the chmod command to alter
permission settings for files.
Once the script file has execute rights, the shell script is run by typing its name. |
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7.3 Shell Variables
| When working with shell scripts, it is often desirable to use variables. These can be either user-defined or in-built variables that the shell supports. |  |
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the shell supports internal variables |
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application and user defined variables are also supported |
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the set command displays all currently defined shell variables |
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variables are used to find programs (path) or set terminal types |
7.4 Defining Shell Variables
| Shell
variables are created by assigning a string (sequence of characters) to
a variable name. Variables can also contain valid UNIX commands.
To access the value of a shell variable, the variable is prefixed with the $ symbol. |
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7.5 Shell Programming Language
| Commands can be combined in shell scripts using a mini programming language supported by the shell. |  |
Support for repetitive tasks (for and while statements) and conditions (if and select statements) allow system administrators to write scripts that are reasonably powerful.
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simple statements which when combined, create powerful scripts |
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support for conditional execution and conditional repetition |
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command substitution (the output of a command can be fed back into the script for execution) |
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support for shell variables |
7.6 Shell Arguments and Quotes
When using arguments to shell scripts or variables, the system administrator
must be careful to ensure that the resultant action is valid. Arguments are
often interpreted by the shell in a manner that is unintentional.
| This is due to the shell applying certain rules about special characters, so system administrators should exercise care when using these in arguments to scripts and variables. |  |
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the shell supports pattern matching and recognition |
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the asterisk (*) symbol matches all strings |
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the question mark (?) symbol matches a single character |
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the special symbols < > * ? [ ] have special meaning |
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to use special symbols, they are enclosed in single or double quotes, else the shell will try to execute them as commands or use them as parameters to commands |
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within double quotes, the symbols $ \ ` " retain their special meaning. To use these characters inside a double quoted string, preceed these symbols with a backslash (\) symbol |
