1. The Shell Chapter 7

2. Overview The Command Line Standard IO Redirection Pipes Running a Program in the Background Killing (a process!)

3. The Command Line The shell executes a program when you give it a command The line that contains the command, including any arguments, is called the command line

4. The Command Line Syntax Dictates the ordering and separation of the elements on a command line e.g. command [arg1] [arg2] … [argn] Not all commands require arguments Some commands do not allow arguments Some commands require specific arguments

5. The Command Line Syntax (cont.) Arguments A sequence of nonblank characters is called a token or word An argument is a token, such as a filename, that a command acts upon e.g. $ cp temp tempcpy temp is arg1 tempcpy is arg2

6. The Command Line Syntax (cont.) Options an argument that modifies the effects of a command more than one option typically be specified options are specific to and interpreted by the program (command), not the shell most utilities allow the grouping of options after a single hyphen (-) help option  Many utilities display a help message when the -help option is used  All GNU Project utilities accept --help

7. The Command Line Syntax (cont.) Options (cont.) Remember! The command must be in the search path, or a path must be supplied on the command line

8. The Command Line Executing the command line When a command is issued, the shell starts a new process The process is the execution of a command While the command is executing, the shell waits for the process to finish. At this point, the shell is in an inactive state called sleep

9. Standard Input and Output Output & Input Standard Output A place that a program can send output, such as text The program never “knows” where the output it sends to standard output is going  Could be a printer  Could be an ordinary file  Could be the screen (default) Standard Input A place that a program gets input from  Could be another program  Could be the keyboard (default)

10. Standard Input and Output Output & Input Standard Error A place that a program can send error messages to

11. Standard Input and Output The Screen as a File Besides ordinary files, directory files, hard links, and soft links, Linux has an additional type of file - device files Device files reside in the Linux file structure (usually under /dev) Represent a peripheral device

12. Standard Input and Output The Screen as a File (cont.) The device name that the who utility displays after your username is the filename of your screen e.g. /dev/pts/4 When working with multiple windows, each window will have its own device name You can read from and write to this device file as though it were a text file

13. Standard Input and Output The Screen as a File (cont.) e.g. Using the keyboard and screen as standard input and standard output

14. Standard Input and Output Redirection Allows you to alter where standard input comes from Allows you to alter where standard output goes to Redirecting Standard Output (>) redirect standard output - instructs the shell to redirect the output of a command to the specified file instead of the screen e.g. ls -l > dirlisting.txt

15. Standard Input and Output Redirection (cont.)

16. Standard Input and Output Redirection (cont.) Redirecting Standard Input (<) redirect standard input - instructs the shell to redirect a command’s input to come from the specified file instead of from the keyboard

17. Standard Input and Output Redirection (cont.) Appending standard output to a file (>>) - append output - causes the shell to add new information to the end of a file, leaving any existing information intact. e.g. $ cat orange this is orange $ cat pear >> orange $ cat orange this is orange this is pear

18. Standard Input and Output Redirection (cont.)

19. Standard Input and Output Pipes The shell uses a pipe to connect the standard output of one command directly to the standard input of another command The symbol for a pipe is a vertical bar (|) e.g. command_a [args] | command_b [args] is the same as: command_a > temp command_b < temp rm temp

20. Standard Input and Output Pipes (cont.) Filters A filter is a command that processes an input stream of data to produce an output stream of data e.g. sort

21. Running a program in the background So far, all commands and utilities used have been running in the foreground When a command is run in the foreground, the shell waits for it to finish before giving you another prompt When a command is run in the background, you do not have to wait for the command to finish before running another command

22. Running a program in the background JOBS A series of one or more commands that can be connected by pipes Only one foreground job allowed in a window or on a screen Many background jobs are allowed Running many jobs at a time utilizes multitasking

23. Running a program in the background JOBS (cont.) To run a job in the background, type an ampersand (&) just before [RETURN] The shell will assign a small number to the job (job number) and displays it between brackets Following the job number, the shell displays the process id (PID) number E.g. $ ls –l | lpr & [1] 22092 $ … [1]+ Donels –l | lpr

24. Running a program in the background Moving a job from the foreground to the background CONTROL-Z Suspends a job Shell stops the process and disconnects standard input from the keyboard bg Command to send a job to the background E.g. move job 1 to background $ bg 1

25. Running a program in the background Moving a job from the foreground to the background (cont.) fg Brings a job from the background to the foreground Only the foreground job can accept input from the keyboard E.g. $ fg 1

26. Running a program in the background Killing a job kill Aborts a background job Uses the PID or job number as an argument E.g. $tail –f outfile & [1] 18228 $ ps | grep tail 18228 pts/400:00:00 tail $ kill 18228 [1]+ Terminatedtail –f outfile

27. Running a program in the background Killing a job (cont.) E.g. $tail –f outfile & [1] 18236 $ bigjob & [2] 18237 $ jobs [1]- Runningtail –f outfile & [2]+ Runningbigjob & $ kill %1 $ RETURN [1]- Terminatedtail –f outfile

28. Filename generation/Pathname expansion When you give the shell abbreviated filenames that contain special characters (metacharacters or wildcards), the shell can generate filenames that match the names of existing files Filenames that contain these characters are called ambiguous file references The process the shell performs on these filenames is called pathname expansion or globbing

29. Filename generation/Pathname expansion The ? Special character Matches any single character in the name of an existing file E.g. $ lpr memo? E.g. $ ls mem memo12memo9memoalex newmemo5 memomemo5memoamemos $ ls memo? memo5 memo9 memoa memos

30. Filename generation/Pathname expansion The * special character Matches any number of characters, including zero characters, in a filename E.g. $ ls amemomemomemoalx.0620memosallyuser.memo memmemo.0612memoalx.keepsallymemo memalxmemoamemorandumtypescript $ echo memo* memomemo.0612memoamemoalx.0620 memoalx.keepmemorandummemosally $ echo *me amemomemosallymemouser.memo $ echo *alx* memalxmemoalx.0620memoalx.keep

31. Filename generation/Pathname expansion The [] special characters Causes the shell to match filenames containing the individual characters surrounded by the brackets or a range of characters E.g. $ lpr part0 part1 part2 part3 part5 $ lpr part[01235] $ lpr part[1-35] E.g. print 39 files $ lpr part[0-9] part[12][0-9] part3[0-8]

32. Hands On Time See ftp site for lab file: BAI517 – Ch 6-7 Utils and Shell Exercise.doc