You described yourself as "done some C++" in your original thread and i suppose it is the difference between shell scripting and high level languages, which makes this hard to understand for you:

A single line in a HLL is read by a parser, then compiled to object code and finally this object code is executed. All the interpretation of the code is done at compile time and all the execution is done at run time. This is fundamentally different with shell scripting.

The big difference is that the interpretation part and the execution part takes place at the same time and in fact is a tightly interwoven process. The following, for instance, has no counterpart in any high-level language:

Code:

cmd="ls -l"
$cmd

In line one a variable is created and filled with a certain content. In the second line this variable is executed as command! Well, in fact not the variable is executed, but the command is: when the shell has to execute this line the first step is to "expand" all the variables, which means to literally replace them with their values. Only then the resulting line is executed.

This makes shell code very versatile (it was created with the purpose of "being able to quickly whip together something" in mind).

This evaluation process does not stop at variables. You sure have done something like this already:

Code:

ls -l *txt

The asterisk is also interpreted by the shell. It is replaced ("expanded") to the list of files matching the pattern (in this case "anything ending in 'txt'").

Consider the following line:

Code:

cmd="ls -l *"
$cmd

This is what the shell will do:

Code:

cmd="ls -l *"
$cmd                           # first, replace the variable with its content
ls -l *                        # now replace the wildcard with the list of files
ls -l a.file b.file c.file     # finally execute this

(i supposed to have the three files "a.file", "b.file" and "c.file" in this directory)

The only way to prevent the shell from doing this expansion is to quote: use double quotes to prevent it from expanding wildcards (which is why it hasn't expanded the asterisk in line 1 already) and single quotes to prevent expansion of variables:

Code:

var="ls *"

echo $var          # results in "ls a.file b.file c.file"
echo "$var"        # results in "ls *"
echo '$var'        # results in a literal "$var"

I hope this helps.

bakunin