1. Operating Systems and Using Linux • Review Operating System. • Linux Overview. • Frequently Used Linux Commands. • Definition of an Algorithm. • Algorithm Examples.

2. Review Operating System • A computer program. • Performs many operations, such as: • Allows you to communicate with the computer (tell it what to do). • Controls access (login) to the computer. • Keeps track of all processes currently running. • At this point, your main concern is how to communicate with the computer using the OS.

3. Connecting to UMBC Linux Servers • See http://www.umbc.edu/oit/pc_ppp/index.html for dtails. • Setup your PC for dial-up connections. • Dial 410-719-1094 or 301-322-6199 with your myUMBC or gl account. (Helpdesk: 410-455-3838 or ECS 020). • Telnet to one of the following linux servers: • telnet linux1.gl.umbc.edu • telnet linux2.gl.umbc.edu • telnet linux3.gl.umbc.edu

4. Connecting to UMBC Linux Servers

5. Connecting to UMBC Linux Servers

6. Connecting to UMBC Linux Servers

7. How Do I Communicate With the Computer Using the OS? (con’t) • When you log in to the Linux system here, a user • prompt will be displayed: • linux#[1]% _ • where # is the number of the Linux server that you have connected to. You may use any of the Linux servers. • The number in the brackets will change as you work. It is • the “number” of the command that you are about to type. • If this prompt is not on the screen at any time, you are not communicating with the OS.

8. Linux Overview • Files and Filenames. • Directories and Subdirectories. • Frequently Used Commands.

9. Files • A file is a sequence of bytes. • It can be created by • a text editor (xemacs or pico), • a computer program (such as a C program). • It may contain a program, data, a document, or other information . • Files that contain other files are called directories (sometimes called folders).

10. Linux Filenames • Restrictions • May not contain blanks or other reserved characters. • Have a maximum length. • Are case sensitive. • It is best to stick with filenames that contain letters (uppercase or lowercase), numbers, and the underscore ( _ ) for now.

11. Directories • Directories contain files or other directories called subdirectories. They may also be empty. • Directories are organized in a hierarchical fashion. • They help us to keep our files organized.

12. Directories (con’t) /afs/umbc.edu/users/j/d/jdoe28 junk recipes cmsc104 pies cookies projects lectures apple peach choc_chip

13. Directories (con’t) • Your home directoryis where you are located when you log in (e.g., /afs/umbc.edu/users/j/d/jdoe28). • The current directory is where you are located at any time while you are using the system. • Files within the same directory must be given unique names. • Paths allow us to give the same name to different files located in different directories. • Each running program has a current directory and all filenames are implicitly assumed to start with the name of that directory unless they begin with a slash.

14. Moving in the Directory Tree • . (dot) is the current directory. • . . (dot-dot) is the parent directory. • Use the Linux command cd to change directories. • Use dot-dot to move up the tree, e.g., cd .. • Use the directory name to move down, e.g., cd cmsc104. • Use the complete directory name (path name) to move anywhere, e.g., cd cmsc104/proj1.

15. Wildcard Characters • You will find wildcard characters useful when manipulating files (e.g., listing or moving them). • The wildcard characters are * and ?. • ? is used to represent any single character. • * is used to represent 0 or more characters.

16. Frequently Used Linux Commands • pwd – present working directory to check where you are in the directory tree. • ls – list of files or subdirectories, e.g., ls proj* or ls –l. • cd – change directory to pathname, cd pathname. • cp – copy files from file1 to file1b, e.g., cp file1 file1b. • mv – rename file from file1 to file2, e.g., mv file1 file2. • rm – remove (delete) files, e.g., file1 file2 … • mkdir – make directory dir-name, e.g. mkdir dir-name. • rmdir – remove (delete) directory, e.g. rmdir dir-name. • man – look up how to use the command, man cp. • passwd – to change password. • ctl-c – to terminate command, process, … • Other commands: cat, more.

17. Frequently Used Linux Commands References: • Linux man page. • Links from the cmsc 104 homepage. http://www.cs.umbc.edu/courses/undergraduate/104/fall02/chen/ • Books and the Internet.

18. Examples • For example, • make a subdirectory for cmsc104, e.g., • mkdir cmsc104, cd cmsc104, and • make subdirectories for projects, lectures, homeworks, exams, e.g., mkdir projects homeworks exams lectures . • Directory tree: cmsc104 exams homeworks lectures projects ex1 ex2 final hw1 … hwn L1 … Ln proj1 … projn

22. linux1[3]% ls -l • drwxr-xr-x 2 lichen 32 2048 Sep 9 15:49 Desktop • lrwxr-xr-x 1 lichen 32 7 Sep 2 00:38 Mail -> ../Mail • drwxr-xr-x 2 lichen 32 2048 Aug 13 1998 bin • drwx------ 3 lichen 32 2048 Sep 10 15:25 li • lrwxr-xr-x 1 lichen 32 10 Sep 2 00:38 www -> ../pub/www access # links owner group file size last modified date filename r: read w: write x: execute l: link d: directory

23. Algorithms, Part 1 of 3 Topics • Definition of an Algorithm. • Algorithm Examples. • Syntax versus Semantics. Reading • Sections 3.1 - 3.3

24. Problem Solving • Problem solving is the process of transforming the description of a problem into the solution of that problem. • We use our knowledge of the problem domain. • We rely on our ability to select and use appropriate problem-solving strategies, techniques, and tools.

25. Algorithms • An algorithm is a step by step solution to a problem. • Why bother writing an algorithm? • For your own use in the future. You won’t have to rethink the problem. • So others can use it, even if they know very little about the principles behind how the solution was derived.

26. Examples of Algorithms • Washing machine instructions. • Washing hair instructions (ambiguous algorithm). • A classic: finding the greatest common divisor (GCD) using Euclid’s Algorithm.

27. Washing Machine Instructions • Separate clothes into white clothes and colored clothes. • If white clothes: • Set water temperature knob to HOT. • Place white laundry in tub. • If colored clothes: • Set water temperature knob to COLD. • Place colored laundry in tub. • Add 1 cup of laundry detergent to tub. • Close lid and press the start button.

28. Washing Machine Instructions (con’t) Observe that • There are a finite number of steps. • We are capable of doing each of the instructions. • When we have followed all of the steps, the washing machine will wash the clothes and then will stop.

29. Refinement of Algorithm Definition • Our old definition: • An algorithm is a step by step solution to a problem. • Adding our observations: • An algorithm is a finite set of executable instructions that directs a terminating activity.

30. Ambiguous Algorithms Washing Hair Instructions: • Lather. • Rinse. • Repeat. How could these instructions be hard to follow?

31. Final Version of the Algorithm Definition • Our old definition: • An algorithm is a finite set of executable instructions that directs a terminating activity. • Final version: • An algorithm is a finite set of unambiguous, executable instructions that directs a terminating activity.

32. History of Algorithms • The study of algorithms began as a subject in mathematics. • The search for algorithms was a significant activity of early mathematicians. • Goal: To find a single set of instructions that can be used to solve any problem of a particular type (a general solution).

33. Euclid’s Algorithm Problem: Find the largest positive integer that divides evenly into two given positive integers, i.e., the Greatest Common Divisor (GCD). Algorithm: • Let M, N be two positive integers, where M > N. • Divide M by N and call the remainder R. • If R is not 0, then Let M = N, N=R. Go to step 2. • Else (R = 0) GCD = N.

34. Finding the GCD of 24 and 9 M N R 24 9 6 9 6 3 6 3 0 Done. So, 3 is the GCD of 24 and 9.

35. Euclid’s Algorithm (con’t) • Do we need to know the theory that Euclid used to come up with this algorithm in order to use it? • What intelligence is required to find the GCD using this algorithm?

36. The Idea Behind Algorithms • Once an algorithm behind a task has been discovered. • We don't need to understand the principles. • The intelligence is "encoded into the algorithm.“ • The task is reduced to following the instructions.

37. Algorithm Representation • Syntax and Semantics • Syntax refers to the representation itself. • Semantics refers to the concept represented (i.e., the logic).

38. Contrasting Syntax and Semantics • In the English language, we have both syntax and semantics. • Syntax is the grammar of the language. • Semantics is the meaning. • Given the following sentence, I walked to the corner grocery store. • Is this sentence syntactically correct? • Is it semantically correct?

39. Contrasting Syntax and Semantics (con’t) • Given the following sentence, I talked to the funny grocery store. • Is this sentence syntactically correct? • Is it semantically correct? • How about I grocery store walked corner the to.

40. Contrasting Syntax and Semantics (con’t) • Conclusion: An English sentence may be syntactically correct, yet semantically incorrect. • This is also true of algorithms. • And it is also true of computer code. • Note that it only makes sense to consider semantic if syntax is correct.

41. Next • More problem solving and algorithms. • Pseudocode. • Assignment: • Read 3.1 – 3.10 (skip the C code, just the pseudocode.) • Login to one of the linux servers, perform some of the unix commands from home or school. • linux1.gl.umbc.edu • linux2.gl.umbc.edu • linux3.gl.umbc.edu