Introduction

Why C and UNIX?

• Common features - positive
  • very powerful and flexible
  • industry standards
  • available on a wide variety of computers
  • work well together
• Common features - negative
  • NOT user friendly - difficult to learn
  • minimal error checking and reporting
  • cryptic commands

UNIX Basics

UNIX is a multiuser operating system - it permits multiple users on the same computer at the same time.

Consequences:

• UNIX needs to know the identity of users
  • keeping track of file ownership
  • accounting of other resources
  • routing email
  • ...
• UNIX needs proof of identity. If no proof was required, users could:
  • view, delete or modify others' files
  • use others' resources
  • send threatening email to the president as someone else
  • ...

user id

a public identifier that uniquely identifies a user of a UNIX system

password

a secret string of characters used to prove that a user is who he/she claims to be

Thus, each user needs both a user id and password to gain access (login) to a UNIX system. Both a public and private identification mechanism are needed.

Your password should:

• be kept secret. Do not tell anyone else your password!
• be something you can remember
• be something no one else can guess
• be at least 6 characters/symbols long (only the first 8 count)
• contain at least one numeric digit or punctuation symbol.

We'll be using a group (cluster) of UNIX workstations at U.P.
Steps for logging in:

1. get an account (from me)
2. go to W208, W304 or W305 and pick a PC
3. turn on the PC and monitor if necessary (it will take several minutes to boot)
4. start Windows if necessary by entering

   w31
   

   at the C: prompt
5. using the left mouse button, double click on the Internet Apps icon
6. double click on the telnet icon
7. enter the name of the computer you want to use. In our case:

   farman.cac.psu.edu
   

8. enter your user id at the login prompt

   login:
   

9. enter your password at the password prompt

   password:
   

You're in!

The first thing to do is to change your password to one that satisfies our criteria.

1. Enter the command:

   passwd
   

   note the missing or
2. You will be prompted to enter your old password. Do so.
3. You will be prompted to enter your new password. Do so.
4. You will be prompted to confirm your new password. Enter it again.

It is vital to logout (end your user session) when you are finished. Otherwise, anyone can sit down and access your files, etc. Use the

logout

exit

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Creating and Editing Files with vi

vi is a text editor that is a standard part of UNIX. To allow full screen editing, vi needs to know what type of terminal you are using. This is done by setting the environment variable term. Environment variables are used to tell UNIX what customizations you would like in your account. We'll set term to vt100, which works well with PCs and is adequate for us. Either of the following commands will work:

set term = vt100

setenv TERM vt100

vi filename

vi has 3 modes:

input mode

used for entering text into a file

command mode

used for entering commands (not echoed on screen)

ex mode

another mode for entering another set of commands (shown at the bottom of the screen)

Initially, vi is in command mode. The file vi.txt summarizes the use of vi.

To avoid having to set term every time you log in, put the appropriate setting in your .cshrc file. This file is executed every time you log in, and so is the best place to set environment variables.

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Basic UNIX Commands

Many UNIX commands take parameters, which specify inputs for the command. e.g. vi takes a file name.

Many UNIX commands also use flags, which modify the meaning of the command slightly. Flags are usually one letter codes preceded by a dash. e.g. -a

Multiple flags can be combined - the following have the same meaning:

ls -a -l
ls -al

If a command takes both parameters and flags, the flags usually precede the parameters. e.g.

ls -l vi.txt

Some of the more important UNIX commands are summarized in the file unix.txt.

Hidden files have file names that start with a period. These files are not shown by ls unless the -a flag is used. Hidden files are usually used to hide system files that users rarely need to see or modify. e.g. the .cshrc file.

A text file containing UNIX commands (such as the .cshrc file) can be executed, causing all the UNIX commands it contains to be executed. This is done using the source command. E.g.:

source .cshrc

Do not try the lpr command as described in the text. The machines we are using are located at U.P., so using this command will cause something to print there. To print file filename, use:

lpr -Pcap1 filename

The -P flag lets you specify the printer to send the file to by name. Your print out will go to the laser printer behind the desk in W305 Olmsted. If the student operator on duty doesn't have your print out placed on the counter at the front of the room, ask him or her for it.

You can also use ftp from a PC to download your file to that PC for printing.

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The UNIX File System

UNIX uses a directory structure similar to that of the DOS or MacIntosh operating system.

• files are contained in directories
• directories form a tree structure. This happens because directories can contain other directories.
• the root of the tree is a directory called /. (In DOS, the root is \.)

Each user has a directory, called their home directory. When you log in, you are automatically placed in your home directory. Usually, your home directory has the same name as your user id.

When one directory contains another:

• the containing directory is called the parent directory
• any contained directories are called subdirectories

Files and directories in the working directory can be referred to just by name (as we've been doing). Files and directories in other directories can be referred to in 2 different ways:

• relative to the working directory
  • specify a path (sequence of directory names) that leads to the file or directory
  • in the path, directory names are separated by /
  • use .. to refer to the parent directory
  • e.g. to edit file vi.txt in the pub subdirectory of the parent directory:

    vi ../pub/vi.txt
    

• absolute (from the root directory)

  No matter what my working directory is, I can edit file vi.txt as follows:

  vi /tmp_mnt/home/users4/taw2/pub/vi.txt
  

The tilde (~) can be used as a shorthand for your home directory. For me,

vi ~/pub/vi.txt

Relative and absolute paths can be used when referring to files or directories for any UNIX command. E.g.:

cd ../pub
cd ~/pub
mv foo ~/pub

To avoid having to use paths when referring to UNIX commands and any locally installed software, the environment variable path is used. This specifies all paths to be searched for executable commands. E.g.:

setenv PATH /usr/bin:/usr/ucb:/usr/local/bin

Similarly, the environment variable manpath can be used to specify all directories where manual pages (used by the man command) are located. Unless manual pages are placed in nonstandard locations, this variable need not be set.

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Protection Modes

In UNIX, access to files and directories is controlled through three protection modes.

What UNIX protection modes mean for files and directories.

mode	file	directory
read permission	ability to see contents and copy the file	ability to list files via ls
write permission	ability to delete file or modify contents	ability to create, modify and delete files in the directory
execute permission	ability to run the file (e.g. a.out)	ability to cd into the directory or access files in the directory

If you own a file or directory, you can set the protection mode for three different groups of users:

user group	definition of that group
user	just you
group	those in your group use the id command to see what groups you are in
others	everyone else

The protection modes in force for a file or directory are displayed by the ls -l command.

Example - my file vi.txt and my pub directory:

-rw-r--r--  1 taw2         3484 Aug 27 14:36 vi.txt
drwxr-xr-x  3 taw2         1536 Apr 29 17:30 pub

• first column:
  • d if the entry is a directory
  • - if the entry is a file
  • l if the entry is a link to a file or directory that resides somewhere else
• columns 2, 3 and 4:
  • indicate permissions for user
  • column 2 is r if the user has read permission,
    - otherwise
  • column 3 is w if the user has write permission,
    - otherwise
  • column 4 is x if the user has execute permission,
    - otherwise
• columns 5, 6 and 7: indicate permissions for the group
• columns 8, 9 and 10: indicate permissions for others

The owner of a file or directory can change permissions of that file or directory using the chmod command. This command takes two parameters: protection mode changes and a file or directory name (or path).

Example:

chmod o+rwx unix.txt

In the protection mode changes parameter:

• the first column specifies which group of users the change will be made for:
  • u user
  • g group
  • o others
• the second column specifies whether the permission is being granted or denied:
  • + grant the permission
  • - deny the permission
• the rest specifies which permissions are being granted or denied, and so is any or all of r, w or x

To keep others from copying or deleting your homework in file assign1.c:

chmod o-rwx assign1.c
chmod g-rwx assign1.c

If you put all your assignments in a directory called assignments:

chmod o-x assignments
chmod g-x assignments

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