Blank a space or tab. Word- a sequence of characters considered as a single unit by the shell. Also known as a token. Name

Week 4

In week 4 we were thoroughly introduced to The Shell.

First we started with some definitions:

blank - A space or tab.

Word- A sequence of characters considered as a single unit by the shell. Also known as a token.

Name- A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.

Metacharacter -A character that, when unquoted, separates words. One of the following: | & ; ( ) < > space tab

control operator - A token that performs a control function. It is one of the following symbols: || & && ; ;; ( ) |

Reserved words are words that have a special meaning to the shell.

A shell script is a file that contains commands to be executed by the shell. Making a File Executable – use chmod: $ chmod +x scriptname

Read (command) = is used to accept the user input and store the input in variable.

If a word begins with a tilde character (`~'), all of the characters preceding the first slash (or all characters, if there is no slash) are treated as a possible login name.

Command substitution allows the output of a command to replace the command name. There are two forms:

$(command) or `command

Built-in Shell Variables are very useful and important:

$#	Number of command-line arguments.
$-	Options currently in effect (arguments supplied to sh or to set).
$?	Exit value of last executed command.
$$	Process number of current process.
$!	Process number of last background command.
$0	First word; that is, command name.
$n	Individual arguments on command line (positional parameters). The Bourne shell allows only nine parameters to be referenced directly (n = 1-9); the Korn shell allows n to be greater than 9 if specified as ${n}.
$*	All arguments on command line ("$1 $2...").
"$@"	All arguments on command line, individually quoted ("$1" "$2" ...).

The shift command – promotes each of the command line arguments. $2 becomes $1, the third becomes the second, etc.

The set command - sets the values of the command line arguments

Control Flow Commands – they include control structures:

• 
  Simple two-way branching If statement
  
• 
  Multiple branching Case statement
  
• 
  For, While and Until statements