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Multiprogramming system
UNIX permits multiple programs (that can be run by different users) to remain in memory and compete for the attention of the CPU.
Multiuser system
UNIX allows multiple users to access the single system with one operating system (UNIX) on it.
Use who command to view the users working on the system.
Time Sharing
Method of operation in which multiple users with different programs interact nearly simultaneously with the CPU.
Kernel uses the principle of time-sharing for cycling through multiple jobs.
It breaks up a unit of time into several slices, and a user's job runs for the duration of a slice.
The moment the allocated time expires, the previous job is kept in abeyance and the next job is taken up.
This process goes on until the clock has turned full circle and the first job is taken up once again.
This switching happens so quick that every user feels as if he/she is the only one using the system.
Even though a program may run for the duration of the time slice allocated for it, it may prematurely leave the CPU during a blocking operation (like reading a file) that keeps the CPU idle.
Multitasking system
UNIX allows a single user run multiple tasks concurrently (switching between tasks very rapidly).
A user sees one job running in the foreground; the rest run in the background.
A user can switch jobs between background and foreground, suspend, or even terminate them.
Machines with multiple CPUs can even dedicate an entire processor for a single program (in a single-user, single-tasking situation).
UNIX applications:
General-purpose tools
Text manipulation utilities (called filters)
Compilers and interpreters
Networked applications
System administration tools
and many more ...
The shell and a set of essential applications form part of the POSIX specification.
"Small is beautiful."
UNIX keeps small commands for every kind of work, each of which performs one simple job only.
UNIX lets the user develop complex command routines by connecting these small commands.
Pipe (|) allows the output of one command to serve as input to another, thus connecting more than 2 commands.
These types of connected commands that can filter/manipulate data in other ways are called filters.
Pattern Matching
UNIX features very sophisticated pattern mathing features.
*(known as a metacharacter) is used by the shell to match a number of characters.
Some advanced tools like grep, sed and awk also use a different matecharacter set for matching strings contained in files.
e.g., printf.*name matches all lines that contain both printf and name. (This pattern is called a regular expression.)
Shell is also a programming language designed for programmers.
It has all the necessary elements such as control structures, loops, variables, etc.
Shell scripts are programs that run UNIX commands in a batch.
Many system's functions can be controlled and automated by using these shell scripts.
man command (program)
The help facility retains the most important reference for commands and their configuration files.