Introduction to UNIX:  Exercise Sheet Three

1. Describe three different ways of setting the permissions on a file or directory to r--r--r--. Create a file and see if this works.
2. Team up with a partner. Copy /bin/sh to your home directory. Type "chmod +s sh". Check the permissions on sh in the directory listing. Now ask your partner to change into your home directory and run the program ./sh. Ask them to run the id command. What's happened?  Your partner can type exit to return to their shell.
3. What would happen if the system administrator created a sh file in this way? Why is it sometimes necessary for a system administrator to use this feature using programs other than sh?
4. Delete sh from your home directory (or at least to do a chmod -s sh).
5. Modify the permissions on your home directory to make it completely private. Check that your partner can't access your directory. Now put the permissions back to how they were.
6. Type umask 000 and then create a file called world.txt containing the words "hello world". Look at the permissions on the file. What's happened? Now type umask 022 and create a file called world2.txt. When might this feature be useful?
7. Create a file called "hello.txt" in your home directory using the command cat -u > hello.txt. Ask your partner to change into your home directory and run tail -f hello.txt. Now type several lines into hello.txt. What appears on your partner's screen?
8. Use find to display the names of all files in the /home subdirectory tree. Can you do this without displaying errors for files you can't read?
9. Use find to display the names of all files in the system that are bigger than 1MB.
10. Use find and file to display all files in the /home subdirectory tree, as well as a guess at what sort of a file they are. Do this in two different ways.
11. Use grep to isolate the line in /etc/passwd that contains your login details.
12. Use find and grep and sort to display a sorted list of all files in the /home subdirectory tree that contain the word hello somewhere inside them.
13. Use locate to find all filenames that contain the word emacs. Can you combine this with grep to avoid displaying all filenames containing the word lib?
14. Create a file containing some lines that you think would match the regular expression: (^[0-9]{1,5}[a-zA-z ]+$)|none and some lines that you think would not match. Use egrep to see if your intuition is correct.
15. Archive the contents of your home directory (including any subdirectories) using tar and cpio. Compress the tar archive with compress, and the cpio archive with gzip. Now extract their contents.
16. On Linux systems, the file /dev/urandom is a constantly generated random stream of characters. Can you use this file with od to printout a random decimal number?
17. Type mount (with no parameters) and try to interpret the output.

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