Introduction to Unix – CS 21

1. Introduction to Unix – CS 21 Lecture 4

2. Lecture Overview • * • cp, mv, and rm • Looking into files • The file command • head and tail • cat and more • What we’ve seen so far

3. Homework and Quiz • Homework #1 now available at http://www.cs.ucr.edu/~villarre/cs21/homework1.html • Due next Thursday (January 20th) at the beginning of class • Quiz #1 next Thursday (January 20th) • Everything covered up through today will be fair game • Homework assignments, lab assignments, reading assignments, and lecture material

4. The First Wildcard • * (asterisk) • Wildcards are just like wild cards in poker • They can be anything (within reason) • In Unix, this means that * can be replaced by anything in the current directory • * actually gets replaced by everything in the current directory

5. Example Of * Usage

6. Why Is This Useful? • At first glance, this doesn’t seem to mean much • After all, ls without the * will list all the files anyway • You can specify a little more than every file • *.txt will match all files that end in .txt

7. Example Of Advanced * Usage

8. Moving Files Around • There are a couple of essential commands to move files around • cp • mv • Remember, you can use touch to create an empty file to play around with

9. cp Usage • The cp command makes a copy of a file • Usage: cp OLDFILE NEWFILE

10. Examples Of cp

11. Problems With cp?

12. Commonly Used Flags With cp • -i flag • Asks if you want to write over a file that already exists • -r flag • Recursively will copy all files and subdirectories

13. What Does Recursion Mean? • The same program gets called over and over again • In this context, cp gets called on all files in all subdirectories • Parent directories and other files “above” the current directory are not affected

14. /usr ~/temp /usr/bin /usr/misc /usr/zzz ~/temp/usr A Graphical Representation Of Recursion cp –r /usr ~/temp / /home

15. Keep In Mind All Of The Permissions • You will only be able to copy a file that you have read permission on • You will only be able to create a file in a directory that you have write permission in

16. mv Usage • The mv command will move a file from one location to a new location • Usage: mv OldFile NewFile • You can also think of this as a rename command

17. Examples Of mv

18. How Does This Compare To Windows? • Windows will let you drag and drop files from one location to another • Right click if you would like to copy • A little progress bar shows up animating the file being moved over to the new folder • Other than that, it is exactly the same

19. rm Usage • The rm command will remove a file • This doesn’t normally include directories • Usage: rm filename

20. Examples Of rm

21. Commonly Used Flags • -i • Verify the delete • -f • Force the removal of a file • If you have permission, the file is gone, regardless of any warnings that might pop up • “Yeah, yeah, just do it” • Overrides the –i flag • -r • Recursively delete files

22. Dangers Of rm • Unix Is missing something you are probably used to • rm is probably one of the most dangerous commands in Unix

23. Once It’s Gone, It’s Gone… • There is no way to get a file that has been removed back • Only run rm if you are absolutely sure you want to remove a file • The –i flag provides a little protection • Prompts the user if they are really sure they want to delete the file

24. Extreme Dangers Of rm • rm –rf ~ • Say “Bye, bye” to your home directory • rm –rf / • You won’t have permission to delete much, but… • If you are root, say goodbye to the entire system! • rm –rf . • rm –rf * • Don’t look as dangerous, but you have to be absolutely certain you know where you’re at

25. So Why Use rm –rf Then? • The most destructive command in Unix, why would you ever want to use it? • Just so happens that you WILL want to remove large portions of your files at some time (most likely many times) • Much easier to run “rm – rf” than delete each file individually

26. Avoiding The Dangers Of rm • The best way is to make sure you are always using the –i flag and only use –f when you are certain • Always check where your current directory is so you don’t delete the wrong file • Make copies of important files just in case

27. Unix, The Multichoice OS • What’s the difference? • rm –rf subdirectory/ • rmdir subdirectory/

28. Multichoice Again • What’s the difference? • mv fileA fileB • cp fileA fileB rm fileA

29. Examining Files Closer • As previously stated, everything in Unix is a file • But different files have different uses • How do you tell what type a file is? • Example: In Windows, a *.doc file is a Word Document • No such restriction is enforced in Unix • A *.doc file might even be an executable!

30. The file Command • A helpful command to get you started in your quest for knowledge: file • Checks the first few bytes of the file in question and takes its best guess as to what type of file it is • Sometimes file gets it wrong, but most of the time it is pretty good

31. Example Different Types Of Files

32. What If I Want To Actually Look Inside The File Myself? • If the file is a text file, several options exist • head • tail • cat • more • If the file is a binary (executable), you don’t want to read it! (trust me)

33. The head Command • Print out the first few lines of a text file • 10 by default • Provides a quick way to see if this is the file you’re looking for • Doesn’t bombard you with a million line file scrolling off the screen • Usage: head FILE

34. Common Flags For The head Command • -6 • Only print out the first 6 lines • Actually, any number works here the same way • What counts as a line? • Everything up until the new line terminator

35. Examples Of head Usage

36. The tail Command • Pretty much the opposite of head • Prints out the last few lines of a file • 10 by default • Usage: tail FILE • Just like head, -NUM • Prints out the last NUM lines

37. Examples Of tail Usage

38. The cat Command • The cat command will print out an entire file to the screen • Usage: cat FILE • Cat? • Short for concatenate • This command can be used to print out multiple files one right after the other • cat FILE1 FILE2

39. Problem With cat • If the file is very large, it will scroll off the screen too fast to read • No way to read a scrolling file without stopping the program • Cntrl-C will kill a running program

40. The more Command • Works exactly like cat, but doesn’t automatically scroll the screen • Usage: more FILE • This is the first truly interactive program we’ve seen • You can control how the program runs while it is running

41. More Example • How do you scroll to the next screen? • Hit the space bar

42. Advanced more usage • Return will move you one line • NUM followed by return will move you NUM lines • Example: 5 [Return] displays the next 5 lines • q will quit more without finishing

43. less Is more • A better version of more exists: less • Allows all of the same options as more • Allows easier moving through the file • Arrow keys and page up, page down will move you both forward and backward • Which you choose to use is, of course, up to you

44. What About Binary Files?

45. You Now Should Have Enough Info To Cause Some Damage • You now have the ability to: • Wander about the system • Create simple files and change all the properties of these files • Copy and move files around • Check out what type of files you are looking at and read the interesting ones • Delete files you no longer want

46. The Class So Far… • History of Unix and the hacker connection • Logging on and getting help • man • Environment variables • The Unix Directory Structure • ls • cd • pushd and popd • Relative and absolute pathnames • . And ..

47. Class Summary Continued • Disk usage • du • Compressing files • Symbolic Links • Ownership and permissions • chmod • umask

48. The Class So Far Continued • Moving files • cp • mv • Checking the contents and type of files • file • head and tail • cat and more

49. In Lab Today • You will practice setting permissions and the effect they have • chmod and umask • Start creating simple files and moving them around the system • Read files using head, tail, cat, and more

50. Next Week • Things really start to get interesting as we start getting programs to work together and tie everything together • We’ll look at more uses of wildcards and start making Unix “sentences” with pipes and filters • We’ll look at the oddly named but strangely powerful command: grep