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

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.

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.

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.

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.

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.

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.

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.???*

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.

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\[1-3\]

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 \.

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”

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

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.

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.

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.