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The Shell

The Shell. Chapter 7. Overview. The Command Line Standard IO Redirection Pipes Running a Program in the Background Killing (a process!). The Command Line. The shell executes a program when you give it a command

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The Shell

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  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 linee.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 upone.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 orangethis is orange$ cat pear >> orange$ cat orangethis is orangethis 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 > tempcommand_b < temprm 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]+ Done ls –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/4 00:00:00 tail $ kill 18228 [1]+ Terminated tail –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]- Running tail –f outfile & [2]+ Running bigjob & $ kill %1 $ RETURN [1]- Terminated tail –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.$ lsmem memo12 memo9 memoalex newmemo5memo memo5 memoa memos$ 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.$ lsamemo memo memoalx.0620 memosally user.memomem memo.0612 memoalx.keep sallymemomemalx memoa memorandum typescript$ echo memo*memo memo.0612 memoa memoalx.0620memoalx.keep memorandum memosally$ echo *meamemo memo sallymemo user.memo$ echo *alx*memalx memoalx.0620 memoalx.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

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