UNIX – The Shell

1. UNIX – The Shell • The Shell • The agency that sits between the user and the UNIX system • All the wonderful things that we can do with UNIX are possible because the Shell does a lot of work on our behalf • Shell looks for special symbols in the command line, perform the tasks associated with them, and then executes the command. • for example, it opens a file to save command ouput whenever it sees the > symbol. © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

2. UNIX – The Shell • The Shell • Unique and Multifaceted program • It is a command interpreter and a programming language rolled into one • It is a process that creates an environment to work in • focus is on the shell’s basic interpretive activities • rm * or ls | more © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

3. UNIX – The Shell • The Shell as a Command Processor • When you login to a UNIX machine, you first see a prompt • A UNIX command starts running at the terminal when you login • It starts functioning when we log in • It withers away when we log out • This command is the UNIX shell • run the ps command (shows processes); you will see it running; • $ ps © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

4. UNIX – The Shell • The Shell as a Command Processor • When we key in a command, it goes as input to the shell • The shell first scans the command line for metacharacters • Metacharacters are special characters that mean nothing to the command, but means something special to the shell • echo date > date.sh • rm * • ls | more © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

5. UNIX – The Shell • The Shell as a Command Processor • When the shell sees the metacharacters like the >, |, *, etc. in its input, it translates these symbols to their respective actions before the command is executed. • It replaces the * with all filenames in the current directory so that rm ultimately runs with these names as arguments. • When the shell sees >, it opens the file date.sh and connects echo’s output to it © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

6. UNIX – The Shell • The Shell as a Command Processor • Basically the shell recreates the command line by removing all metacharacters and finally passes on the command to the kernel for execution © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

7. UNIX – The Shell • The Shell as a Command Processor • Following activities are typically performed • Shell issues a prompt and waits for you to enter a command • It scans the command line for metacharacters and expands abbreviations to create a simplified command line • It then passes on the command line to the kernel for execution • The shell waits for the command to complete and normally can’t do any work while the command is running • After the command execution is complete, the prompt reappears and the shell returns to its waiting role to start the next cycle. © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

8. UNIX – The Shell • Shell offerings • UNIX system offers a variety of shells for you to choose from. The shells we consider can be grouped into two categories: • The Bourne family comprising the Bourne shell (/bin/sh) and its derivatives – the Korn shell (/bin/ksh) and Bash (/bin/bash) • The C Shell (/bin/csh) and its derivatives, Tcsh (/bin/tcsh) • To know the shell you are using invoke $SHELL © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

9. UNIX – The Shell • Pattern Matching – The Wild Cards • The * and ? • The * metacharacter is one of the characters of the shell’s special set. It matches any number of characters including none. When it is appended to the string chap, the chap* matches filenames beginning with the string chap – including the file chap • ls -l chap* • echo * • rm * -- bad news © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

10. UNIX – The Shell • Pattern Matching – The Wild Cards • The * and ? • The ? Matches a single character. • ls chap? • chapx chapy chapz • ls chap?? • chap01 chap02 chap03 chap04 chap15 chap16 chap17 © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

11. UNIX – The Shell • Pattern Matching – The Wild Cards • The Character Class • It comprise a set of characters enclosed by the brackets [ and ], but it matches a single character in the class • ls chap0[124] • chap01 chap02 chap04 • ls chap0[1-4] range 1,2,3,4 • ls chap[x-z] © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

12. UNIX – The Shell • Pattern Matching – The Wild Cards • The Mystery of the find Command • find / -name “*.[hH][tT][mM][lL]” –print • will display html and HTML files • find . –name “note??” –print • wild cards in the feature of find and not the shell • quotes around the pattern, ensures that the shell can’t even interpret this pattern. © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

13. UNIX – The Shell • Pattern Matching – The Wild Cards • Matching the Dot • The * and ? Don’t match all filenames beginning with a . (dot) • to matche hidden files in your directory having at least 3 characters after the dot, • ls .???* • to match files with the dot anywhere but the beginning • ls *c © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

14. UNIX – The Shell • Redirection • What is a Terminal? • a generic name that represents the screen, display or keyboard (or even an X window that emulates a terminal) • We see command output and error messages on the terminal (display), and we sometimes provide command input through the terminal (keyboard) • The shell associates three files with the terminal • two for display and one for keyboard © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

15. UNIX – The Shell • Redirection • What is a Terminal? • even though our terminal is also represented by a specific device name, commands don’t usually read from or write to this file. • All terminal related activity is performed with the three files that the shell makes available to every command • These files are actually streams of characters which many commands see as input and ouput © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

16. UNIX – The Shell • Redirection • What is a Terminal? • stream is simply a sequence of bytes • when a user logs in, shell makes available 3 files representing three streams • each stream is associated with a default device, and generally speaking the device is the terminal © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

17. UNIX – The Shell • Redirection • Standard Input • The file (or stream) representing input, which is connected to the keyboard • Standard Output • The file (or stream) representing output, which is connected to the display • Standard Error • The file (or stream) representing error messages that emanate from the command or shell.This is also connected to the display © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

18. UNIX – The Shell • Redirection • A group of UNIX commands read and write to these files • A command is designed to send output to this file not to the terminal • similarly, it is not designed to accept input from the keyboard, but from the standard file which it sees as a stream • All commands using streams will always find these files open and available © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

19. UNIX – The Shell • Redirection • Even though the shell associates each of the files with a default physical device, can unhook a stream from its default device • when it see some special characters in the command line • we as users of UNIX have to instruct the shell to do that by using symbols like > and < in the command line © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

20. UNIX – The Shell • Redirection • Standard Input • This file can represent three input sources • The keyboard, the default source • A file using redirection with the < symbol ( metacharacter) • Another program using a pipeline © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

21. UNIX – The Shell • Redirection • Standard Input • wc without an argument and have no special symbols like < and | in the command line; wc obtains its input from the default source • apache[kkhan:301] wc • Standard input can be redirected • It can come from a file • or a pipeline • 3 14 71 © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

22. UNIX – The Shell • Redirection • Standard Input • wc with the filename as argument • apache[kkhan:302] wc /etc/passwd • 18 44 732 /etc/passwd © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

23. UNIX – The Shell • Redirection • Standard Input • wc with a character • apache[kkhan:302] wc < /etc/passwd • 18 44 732 • On seeing the <, the shell open disk file. /etc/passwd, for reading • It unplugs the standard input file from its default source and assigns it to /etc/passwd © 2006 The McGraw-Hill Companies, Inc. All rights reserved.

24. UNIX – The Shell • Redirection • Standard Input • wc reads from standard input which has earlier been reassigned by the shell to /etc/passwd • wc has no idea where stream came from; it is not even aware that shell had to open the file on its behalf! • Input from both file and Standard Input • cat – foo • cat foo – bar © 2006 The McGraw-Hill Companies, Inc. All rights reserved.