Investigacion De Sistemas De Computo

Shell Variables
Like other programming languages the csh language has variables. Some variables are used to control the
operation of the shell, such as $path and $history, which we discussed earlier.Other variables can be created
and used to control the operation of a shell script file.
3.1 Setting Variables
Values of shell variable are all character-based: A value is formally defined to be alist of zero or more
elements, and an element is formally defined to be a character string. In other words, a shell variable
consists of an array of strings.
For example,
set X
will set the variable $Xto have an empty list as its value. The command
set V = abc
will set V to have the string ‘abc’ as its value. The command
set V = (123 def ghi)
will set V to a list of three elements, which are thestrings ‘123’, ‘def’ and ‘ghi’.
The several elements of a list can be treated like array elements. Thus for V in the last example above, $V[2]
is the string ‘def’. We could change it, say to ‘abc’, bythe command
set V[2] = abc
3.2 Referencing and Testing Shell Variables
The value of a shell variable can be referenced by placing a $ before the name of the variable. The command
echo $path
will outputthe value of the variable $path. Or you can access the variable by enclosing the variable name in
curly brace characters, and then prefixing it with a $. The command
echo ${path}
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would have thesame result as the last example. The second method is used when something is to be
appended to the contents of the variable. For example, consider the commands
set fname = prog1
rm ${fname}.c
These woulddelete the file ‘prog1.c’.
To see how many elements are in a variable’s list, we prefix with a # then a $. The command
echo $#V
above would print 3 to the screen, while
echo $#path
would reveal thenumber of directories in your search path.
The @ command can be used for computations. For example, if you have shell variables $X and $Y, you
can set a third variable $Z to their sum by
@Z = $X +...