A Quick Look at the Unix Operating System

What is an Operating System?
An operating system is a program that provides an interface between:

• hardware and users,
• software and users,
• hardware and software,

Hardware Examples

• CPU
• Main memory
• Secondary storage (e.g., hard disk, floppy disk, CDROM)
• I/O devices (e.g., keyboard, mouse, printer)

Examples of Operating Systems

• Windows
• MacOS
• Phone and Tablet OS's, for example:
  • iOS
  • Android
• Unix: comes in lots of flavors
  • Linux (runs on PCs and many other kinds of computers)
  • FreeBSD (runs on PCs and many other kinds of computers; underpinning of MacOS X)
  • AIX (made by IBM)
  • HP-UX (made by Hewlett-Packard)
  • Solaris (made by Oracle)

What is Unix?

• Developed in the late 1960s/early 1970s at Bell Labs.
• Cryptic: many command names are 2 - 5 characters long.
• Text based like MS-DOS (because it's so old), not window based like Windows or MacOS.
• Very efficient, powerful, reliable.
• Common in academic and research environments.
• Different flavors (e.g., Linux, FreeBSD, AIX) aren't fully compatible but often are mostly compatible.
• Designed for people who already know everything about it.

Files & Directories
Unix files and directories are very much like files and directories (folders) in Windows and MacOS.

Basic Commands
All commands must be typed exactly right, or they will fail (or do something other than what you want).
Unix is case sensitive: it matters very much whether a letter is upper case (capital) or lower case (small).

• man command
  "Manual"
  Display the online documentation for a command. For example,
  man pwd displays the online documentation for the command pwd.
  man -k word gives a listing of all the commands related to word.
• date
  Today's date and time.
• pwd
  "Print Working Directory"
  Print the name of the current (working) directory.
  In MS-DOS, by contrast, the current working directory is included in the prompt:
  C:PROGS\MYPROGS>
• cd direc
  "Change Directory"
  Change the current working directory to a directory named direc.
  Like cd in MS-DOS.
• ls
  "List"
  List the files and subdirectories in the current working directory.
  Like dir in MS-DOS.
  You can get a more detailed listing of the current working directory's contents: ls -l
• mv srcfile dstfile
  "Move"
  Move the file named srcfile to a file named dstfile (i.e., change its name).
  You can also use mv to move a file from one directory to another:
  mv srcfile dstdirec
  Like move in MS-DOS.
• cp srcfile dstfile
  "Copy"
  Copy a file named srcfile to a new file named dstfile (or, if dstfile already exists, replace its contents with the contents of srcfile).
  Like copy in MS-DOS.
• rm thisfile
  "Remove"
  Remove (delete) a file named thisfile.
  Like del in MS-DOS.
• mkdir newdirec
  "Make Directory"
  Make a new directory called newdirec as a subdirectory of the current working directory.
• rmdir olddirec
  "Remove Directory"
  Remove an empty subdirectory named olddirec.
• cat thisfile
  "Concatenate"
  Output the contents of the file thisfile to the terminal.
  cat srcfile1 srcfile2 > dstfile
  Concatenate the contents of srcfile1 and srcfile2 (dump them out one after the other) and put the concatenated output into dstfile.
• more thisfile
  Output thisfile to the terminal in a controlled way.
• time command
  Determine how long it takes to run a command.

Standard I/O

Full Name	Abbrev	Meaning	Comment
Standard Input	stdin	A user typing at the keyboard
Standard Output	stdout	Outputting to the terminal screen	Buffered: Regardless of when output statements occur in the program, outputting doesn't happen until a bunch of text has been saved up.
Standard Error	stderr	Outputting to the terminal screen	Unbuffered: Outputting happens as soon as an output statement is executed.

Redirecting I/O
Instead of input coming from a live user at the keyboard, or output going to the terminal screen, they can be redirected from or to a file.

• program > outputfile
  The output of executing program goes into outputfile, instead of appearing on the terminal screen.
• program < inputfile
  The input needed for executing program come from inputfile, instead of being typed live by the user at the keyboard.
• program < inputfile > outputfile
  The input needed for executing program come from inputfile, and the output goes into outputfile.
• program >> outputfile
  The output of executing program is appended to the end of outputfile.

Root Directory
The root directory (which is denoted by a slash /) is the topmost directory; all other directories are subdirectories of it.
It's like the "My Computer" icon in Windows, or like the desktop in MacOS.

Directory Paths
The full name of a directory, starting from the root, is called a path. The full name of a file is its path, a slash and its filename. So, if a user neem1883 has a home directory
/coe/fsr/facst0/neem1883
in which there's a subdirectory CS1313, and in the CS1313 subdirectory is a file named my_number.c, then the path of my_number.c is
/coe/fsr/facst0/neem1883/CS1313
and the file's full name is
/coe/fsr/facst0/neem1883/CS1313/my_number.c

Special Directories

• The root directory is denoted with a slash:
  /
• The home directory of a user named neem1883 is denoted
  ~neem1883
• You can denote your own home directory as
  ~
• The current working directory is denoted with a single period:
  .
• The parent directory of the current working directory is denoted with two periods:
  ..
• The grantparent directory of the current working directory is denoted:
  ../..
  and so on.

Wildcards
Asterisks indicate parts of a filename that could be anything.

• *.c
  Every file whose filename ends in .c
• *number*
  Every file whose filename contains number somewhere in the name
• [a-z]*
  Every file whose filename starts with a lower case letter

File Permissions
The person who owns a file or directory gets to decide who can read it, who can write to it, and, if it's an executable, who can execute (run) it.

• User permission: what you as the owner are allowed to do with it.
• Group permission: what people like you (e.g., other students) are allowed to do with it.
• Other permission: what people who aren't like you (e.g., faculty) are allowed to do with it.
• Superusers: some people (typically the people in charge of administering the computer you're on) have a special status called superuser that allows them to read, write and execute files even if they're not supposed to have permission.
• Executing a directory: if a directory has execute permission for you, then you can go into it.
• chmod
  "Change Mode"
  Change the permissions on a particular file or directory.
  See the man page for details.

References
Paul DuBois, Using csh & tcsh. O'Reilly & Associates, 1995.
Daniel Gilly, Unix in a Nutshell, System V edition. O'Reilly & Associates, 1992.
Rebecca Thomas & Jean Yates, A User Guide to the Unix System. Osborne/McGraw-Hill, 1982.