Chapter 2 - UNIX Utilities for Nonprogrammers

Quiz

A hacker might try to break security by trying to obtain your password or get a copy of an password file that is not shadowed and write a program to compare encrypted results to the contents of the password file.

Passwords should be long, at least 6 characters, and made up of nonsense letters and numbers, not words out of the dictionary. However, there should also be some pattern that you can recognize because if you wind up having to write down your password in order to remember it, you have defeated the purpose. One trick is to pick a phrase and then use the first letter from each word in the phrase (and still add a digit or two somewhere). For example, ìi0icluî is a good password and you can remember it by remembering: ìif only I could learn UNIXî (using a zero for the ìoî in ìonlyî).

Use the mv command to move (or rename) a file.

This can be a confusing issue. UNIX itself differentiates between uppercase and lowercase characters. In the file system, for example, the file ìabcî and ìABCî are not the same file. So you would say that UNIX is case sensitive. However, applications may or may not make this distinction. E-mail addresses processed by sendmail and most other mail programs are not case sensitive because the e-mail message standard specifies that addresses should not be.

Bourne shell (sh)

C shell (csh)

Korn shell (ksh)

Bourne Again shell (bash)

Shell scripts can usually be written faster because there is less overhead and you can take advantage of existing UNIX commands to perform the task. Scripts do not require compilation the way a C program does. For small tasks where the power and speed advantages of a C program are not a requirement, a shell script is probably easier and faster to write.

You can terminate a process by sending it an INTR signal (either with the kill command or by typing the corresponding interrupt character, usually Control-C, if the program is running at the terminal or in a terminal window).

You can indicate the end-of-input when entering text by typing the character set as the ìeofî character by stty, usually Control-D.

Terminate a shell by typing the exit command or the ìeofî character (as in #8). The C shell can be set where the eof character wonít work and you must type exit or logout.

The location of a process is where its current working directory points in the directory hierarchy.

type

access permissions

owner

group membership

hard link count

modification date and time

Groups provide another level of file access specification than just the owner and all other users. Users in the same group can, for example, set read access on a file that applies only to the members of that group while still denying access to the file to all other users.

The read and write flags allow access to the directory just as regular files. The execute flag allows access to the files and subdirectories contained in the directory (based on their own access permission, of course).

Only the owner of the file and the superuser (root) may change a fileís owner.

Exercises

At the time of the printing of the first edition of this book, many network connections were via UUCP instead of direct Internet connections. In this scenario, email was often  ìstored and forwardedî and might take several hops, requiring hours or even days to reach its destination. Today, most sites are connected to Internet Service Providers and access to the Internet is more direct, so email will likely arrive within a few minutes.

The ìcurrent groupî of a process is its effective group ID. This may differ from its real group ID as it can be changed with the newgrp command. So at any given moment, a process can belong to a different group than its userís default group, but only one. The real group ID is still used for accounting purposes, but the effective group ID determines file access or modification permission.

Some versions of UNIX honor access permissions for multiple groups (without requiring you change your effective group ID with the newgrp command) but even in these cases, the process still has only one group to which it really belongs at any moment. This group is the one that will be used when the process creates files. A file may only belong to a single group, so the effective group ID of the process is used. If a process could belong to multiple groups at once, there would be no way to determine which one should be used when creating a file.

There is no real answer here, the exercise should lead to a discussion of issues. One might use an Access Control List (found in other operating systems) to solve the problem.

There is no correct answer here, but it should lead to a lively discussion.

Project

Set and change group access for files and see how access is allowed or denied.