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Midterm Review

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  1. Midterm Review IT244 - Introduction to Linux / Unix Instructor: Bo Sheng

  2. Basic Utilities • ls, cat, echo • ls –l, ls -a • cat file1 file2 file3 … • echo “hello world” > file • echo $VAR, echo hello*

  3. Basic Utilities • more, less, head, tail • head -5 file • tail -3 file • pwd, cd • cp, mv, rm, mkdir, rmdir • ‘-i’ option • ‘-r’ option: rm –r dir1 • rmdiran empty directory

  4. Basic Utilities • sort, wc, grep, |(pipe) • wc -l file, wc –c file • grep “keyword” file, grep -v • sort linux | head -5 • bzip2, gzip, tar (no compression)

  5. Filename and Path • Hierarchical tree structure • Absolute path name • ls /home/it244/it244 • Relative path name • cd /home/it244 • ls it244 • ‘.’ ‘..’ ‘~’ • ‘*’ ‘?’ • echo hello*, echo ./.?a*

  6. Access Permissions • r, w, x (chmod) • chmoda+rw file • chmod a=rx file • chmod 644 file • Execute permission of a directory • A directory is a table of files (filename:meta) • Executable means searchable

  7. Access Permissions • Operations on book/book1 • ls book/book1 • book ( x ) • cat book/book1 • book1 ( r ), book ( x ) • rm book/book1 • book ( xw ), book1 ( w )

  8. Links • Hard link and soft link • Hard link cannot refer to a directory • Soft link can point to a nonexistent file $ ln -s hello hello.sl $ rm hello $ echo “new” > hello $ cat hello.sl new

  9. Execute Commands • Foreground and background jobs • command &, CTRL+Z • difference between a suspended job and a background job, • jobs, ps, kill, bg, fg • Group commands • (cmd1; cmd2; cmd3)

  10. Processes • PID and PPID • sleep 10& • ps -f

  11. Redirection • Standard input, output, error output • cat file1 file2 > out 2>err • cat file1 file2 &> out • cat file1 file2 > out 2>&1 • Redirected output will not be displayed on the screen.

  12. Variables • System variables • $PATH, $PWD, $HOME • User defined variables • VAR=hello • export VAR=hello • declare; declare -x

  13. Expansion/ Quote • Types • echo hello*; echo ~ • echo $VAR • VAR=hello* • echo $VAR • Single quote suspends all expansions

  14. Questions Both hard links and symbolic links can refer to directories. True / False Utilities gzip, bzip2, and tar are used to compress files. True / False A shell script that needs user’s input cannot be running on the background. True / False

  15. Questions • Assume there are 3 files: file1, file2, and file3. After executing the following commands, how many files are there and what are the contents? $ cp file1 file2 $ mv file2 file3 $ rm file2 • Assume there are two directories dir1 and dir2, and dir1 contains a subdirectory dir3, what’s the result of $ cp -r dir1 dir2

  16. Questions We have learned that “grep -v” can exclude the lines containing the specified keywords. Assume there exist the following files, $ls abcbabbcdabcdbcdeabcde What is the output of “ls | grep –v ab | wc -l” : ( “wc -l” outputs the number of lines of the input text ) a. 4 b. 3 c. 2 d. 1

  17. Questions Assume your permissions allow you to write to ‘file1’, but not to delete it. Which of the following operations will certainly be denied: • ls -l file1 • cat file1 • cp file1 file2 • echo “hello world” >> file1

  18. Questions • How to determine if a given filename is a regular file or directory? • To execute ‘cat dir1/file1’, what permissions are needed and why

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