CS 497C - Introduction to UNIX
Lecture 20: - The Shell
Chin-Chih Chang chang@cs.twsu.edu

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
* The * 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.