CS 497C – Introduction to UNIX Lecture 20: - The Shell

1. CS 497C – Introduction to UNIXLecture 20: - The Shell Chin-Chih Changchang@cs.twsu.edu

2. The Shell • The shell is the agency between the user and the UNIX system. • The shell is a command processor; it processes the instructions you issue to the machine. • The Bourne Shell named after its founder Steve Bourne is the earliest shell that came with UNIX system. • Use echo $SHELL to know your shell.

3. The Shell as Command Processor • When you log on to a UNIX machine, you see a prompt. A UNIX command is running once you log in. This command is the shell. It withers away when you log out. • The shell accepts and interprets users requests. It examines and rebuilds the command lien and then leaves the execution work to the kernel. • The kernel handles the hardware on behalf of these commands and all processes.

4. The Shell as Command Processor • The shell is generally sleeping. It wakes up when input is keyed in at the prompt. • Sleeping, waiting, and waking are the cycle of the shell. • The following activities are performed by the shell: • It issues the prompt and sleeps till you enter a command.

5. The Shell as Command Processor • After a command is entered, the shell scans the command line for some special characters (metacharacters) for expansion. • It then passes the command to the kernel in a form which the kernel can understand. • After the job is complete, the prompt reappears and the shell returns to its sleeping role to start the next cycle. • You can use metacharacters to devise a generalized pattern or model that can often match a group of similar filenames.

6. Pattern Matching – The Wild Cards • * matches any number of characters including none. • Use ls *.lst to list all files with extension .lst. • ? matches a single character. • Use cp ???.c progs to copy files whose filename has 3 characters and the .c extension.

7. Pattern Matching – The Wild Cards • [cut] matches a single character – either a c, u, or t. • [!cut] matches a single character that is not a, c, or t. • Use cat *.[!o] to display all files having one extension character except object files. • [x-z] matches a single character from x to z. • Use rm note[01][0-9] to remove files note00, note01, …., note18, note19.

8. Pattern Matching – The Wild Cards • [!q-z] matches a single character that is not within q to z. • These metacharacters lose their meaning when placed in the wrong place. • The * doesn’t match all filenames beginning with a . (dot), or the / of a pathname. • To list all the hidden files having at least three characters after the dot, use ls .???*

9. Escaping – The Backslash (\) • It’s generally accepted principle that filenames shouldn’t contain the shell metacharacters. • Image a file named chap* created with the > symbol: $ echo > chap* • If you use rm chap*, you will remove all files beginning with chap.

10. Escaping – The Backslash (\) • The way to remove the file chap* is to use the backslash (\) as follows: $ rm chap\* • The use of the \ in removing any special character is called escaping or despecializing. • For example, to remove a file chap0[1-3], use: $ rm chap0\[1-3\]

11. Escaping – The Backslash (\) • Apart from the wild cards, some characters are also considered special by the shell: | <> ‘ “ • The new line character is also special to the shell. • When you enter a long chain of commands or a command with numerous arguments, you can split the command line by hitting [Enter] after the \.

12. Quoting • When a command argument is enclosed in quotes, the meanings of all enclosed special characters are turned off: $ echo ‘*?[8-9]’ • The argument above is said to be quoted. • The space is another character that has a special meaning to the shell. • To remove a file with the space in the filename, you can use quotes: $ rm ‘good morning”

13. Escaping and Quoting in echo • The \ is also used to emphasize a character. The \ combines with some character to represent an escape sequence. • These are escape sequence accepted by echo: • \t – A tab • \f – A formfeed (page skip) • \n – A newline

14. Redirection • The UNIX commands are designed to accept a character stream without knowing its source and destination. • A stream is just a sequence of bytes that many commands see as input and output. • UNIX treats these streams as files, and a group of UNIX commands reads from and writes to these files.

15. Redirection • The shell sets up three standard files (for input, output, and error), attaches them to a user’s terminal at the time of logging in, and close them when the user logs out. • The standard file for input is known as standard input and that for output is known as standard output. The error stream is known as standard error.

16. Redirection • The shell has set some physical devices as defaults for them: • Standard input – The default source is the keyboard. • Standard output – The default source is the terminal. • Standard error – The default destination is the terminal. • These default sources can be redirected to come from or go to any disk or some other device by some special characters.