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History of Unix

At the same time a team from the University of California at Berkeley was working to improve UNIX. In 1977 it released the first Berkeley Software Distribution, which became known as BSD. Over time this won favour through innovations such as the C shell.

Meanwhile the AT&T version was developing in different ways. The 1978 release of Version 7 included the Bourne Shell for the first time. By 1983 commercial interest was growing and Sun Microsystems produced a UNIX workstation. System V appeared, directly descended from the original AT&T UNIX and the prototype of the more widely used variant today.

Unix Today

From a user's perspective they are very similar and you are unlikely to have difficulty unless you use more than one type of system. In this case you might notice differences in the structure of the file system or in how certain commands behave. The on-line manual should be helpful if you have problems.

Although UNIXhelp is now based on a System V variant of UNIX, you should see only minor differences in the example input and output if your system is a BSD one.

The OS

Although these are mostly similar, there are small differences which can cause problems. The most obvious examples are the layout of the file system and the exact format of certain commands.

Processes

When the operating system is started after a boot, a single process is started. This process is the parent of all subsequent processes. Each process created on the system has a unique number, known as its PID, associated with it.

When you login to the system a process is started to run your shell program. Any processes that are started from within your shell - such as entering a command - are the children of this process. A process can have many children, but only one parent.

Command Lines

For example the wc command counts the number of words, characters and lines in a file. By using a different option you can choose what is counted.

wc -w file1 counts the words
wc -c file1 counts the characters
wc -l file1 counts the lines

As well as using the shell to run commands you can use its built-in programming language to write your own commands or programs. You can put commands into a file - known as a shell script - and then execute that file as you would a command or program.

The use of the Bourne shell (sh) is illustrated as this is available on all UNIX systems. Bourne shell scripts will also work under the Bourne Again SHell. The C and TC shells use a programming language which is similar to the C programming language.

These are the contents of a shell script called display:
cat display
# This script displays the date, time, username and
# current directory.
echo "Date and time is:"
date
echo
echo "Your username is: `whoami` \\n"
echo "Your current directory is: \\c"
pwd

The first two lines beginning with a hash (#) are comments and are not interpreted by the shell. Use comments to document your shell script; you will be surprised how easy it is to forget what your own programs do!

The backquotes (`) around the command whoami illustrate the use of command substitution.

The \\n is an option of the echo command that tells the shell to add an extra carriage return at the end of the line. The \\c tells the shell to stay on the same line. See the man page for details of other options.

The argument to the echo command is quoted to prevent the shell interpreting these commands as though they had been escaped with the \\ (backslash) character.Before using a file as a shell script you must change its access permissions so that you have execute permission on the file, otherwise the error message Permission deniedis displayed.

To run the shell script, simply type its name at the prompt. The commands in the script will then execute one at a time as though you were typing them in at the terminal.

To give yourself execute permission for the file containing the script use the command:
chmod u+rwx filename

The +rwx after the u allows you to read, write to and execute the script: no one else has permission to read, write or execute.

To give other users permission to read and execute but not alter the shell script use:
chmod go+rx filename

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