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Full Discussion: Understanding ${parameter}
Top Forums Shell Programming and Scripting Understanding ${parameter} Post 302301330 by victorbrca on Thursday 26th of March 2009 02:05:21 PM
Old 03-26-2009
Understanding ${parameter}

I was reading Bash Cookbook and it had a line of command that assigned a value from var2 to var1 if var1 was empty. Someone told me I could do the same with ${parameter}.

I'm just trying to understand this. Would the following be correct?

For:

Code:
answer=
default=1

Should I assume that the two lines bellow will do the same?

Code:
[ -z "$answer" ] && answer="$default"
# or
${answer:=$default} 2> /dev/null

Thanks,

Vic.
 

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LIST(3) 								 1								   LIST(3)

list - Assign variables as if they were an array

SYNOPSIS
array list (mixed $var1, [mixed $...]) DESCRIPTION
Like array(3), this is not really a function, but a language construct. list(3) is used to assign a list of variables in one operation. PARAMETERS
o $var1 - A variable. RETURN VALUES
Returns the assigned array. EXAMPLES
Example #1 list(3) examples <?php $info = array('coffee', 'brown', 'caffeine'); // Listing all the variables list($drink, $color, $power) = $info; echo "$drink is $color and $power makes it special. "; // Listing some of them list($drink, , $power) = $info; echo "$drink has $power. "; // Or let's skip to only the third one list( , , $power) = $info; echo "I need $power! "; // list() doesn't work with strings list($bar) = "abcde"; var_dump($bar); // NULL ?> Example #2 An example use of list(3) <table> <tr> <th>Employee name</th> <th>Salary</th> </tr> <?php $result = $pdo->query("SELECT id, name, salary FROM employees"); while (list($id, $name, $salary) = $result->fetch(PDO::FETCH_NUM)) { echo " <tr> " . " <td><a href="info.php?id=$id">$name</a></td> " . " <td>$salary</td> " . " </tr> "; } ?> </table> Example #3 Using nested list(3) <?php list($a, list($b, $c)) = array(1, array(2, 3)); var_dump($a, $b, $c); ?> int(1) int(2) int(3) Example #4 Using list(3) with array indices <?php $info = array('coffee', 'brown', 'caffeine'); list($a[0], $a[1], $a[2]) = $info; var_dump($a); ?> Gives the following output (note the order of the elements compared in which order they were written in the list(3) syntax): array(3) { [2]=> string(8) "caffeine" [1]=> string(5) "brown" [0]=> string(6) "coffee" } NOTES
Warning list(3) assigns the values starting with the right-most parameter. If you are using plain variables, you don't have to worry about this. But if you are using arrays with indices you usually expect the order of the indices in the array the same you wrote in the list(3) from left to right; which it isn't. It's assigned in the reverse order. Warning Modification of the array during list(3) execution (e.g. using list($a, $b) = $b) results in undefined behavior. Note list(3) only works on numerical arrays and assumes the numerical indices start at 0. SEE ALSO
each(3), array(3), extract(3). PHP Documentation Group LIST(3)
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