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Full Discussion: How can I write nested command substitutions?
Top Forums Shell Programming and Scripting How can I write nested command substitutions? Post 302802481 by alister on Friday 3rd of May 2013 02:02:01 PM
Old 05-03-2013
Should you actually ever need nested command substitutions, they're trivial if you use the modern syntax. Unless you are forced to use an ancient shell which does not support $(cmd), never use `cmd`.

One level of nesting using the deprecated, paleo-backtick command substitution syntax:
Code:
$ echo top `echo 0 \`echo 1\``
top 0 1

The simpler, modern syntax:
Code:
$ echo top $(echo 0 $(echo 1))
top 0 1

It may not seem like a big difference, but there are other subtleties involved. The backtick syntax has a more complicated and intrusive quoting mechanism whereby the top-level interferes with the code of nested subshells. If you don't know how it works, it will cause you pain. If, however, you do know how it works, it will also cause you pain.

With the modern syntax, anything in a substitution is written just as it is outside of a substitution, regardless of how deeply it's nested.

A contrived, comparitive demonstration of three levels of nesting. First, the modern syntax:
Code:
$ echo top $(echo 0 $(echo 1 $(echo 2 $(echo 3))))
top 0 1 2 3

Now the syntax of yore:
Code:
$ echo top `echo 0 \`echo 1 \\\`echo 2 \\\\\\\`echo 3\\\\\\\`\\\`\``
top 0 1 2 3

Regards,
Alister
These 3 Users Gave Thanks to alister For This Post:
drl Scott Yoda
 

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LEARN ABOUT PHP

boolval

BOOLVAL(3)								 1								BOOLVAL(3)

boolval - Get the boolean value of a variable

SYNOPSIS

       boolean boolval (mixed  $var)

DESCRIPTION

	Returns the boolean value of $var.

PARAMETERS

	      o $var
		- The scalar value being converted to a boolean.

RETURN VALUES

	The boolean value of $var.

EXAMPLES

       Example #1

	      boolval(3) examples

	      <?php
	      echo '0:	      '.(boolval(0) ? 'true' : 'false')."
";
	      echo '42:       '.(boolval(42) ? 'true' : 'false')."
";
	      echo '0.0:      '.(boolval(0.0) ? 'true' : 'false')."
";
	      echo '4.2:      '.(boolval(4.2) ? 'true' : 'false')."
";
	      echo '"":       '.(boolval("") ? 'true' : 'false')."
";
	      echo '"string": '.(boolval("string") ? 'true' : 'false')."
";
	      echo '"0":      '.(boolval("0") ? 'true' : 'false')."
";
	      echo '"1":      '.(boolval("1") ? 'true' : 'false')."
";
	      echo '[1, 2]:   '.(boolval([1, 2]) ? 'true' : 'false')."
";
	      echo '[]:       '.(boolval([]) ? 'true' : 'false')."
";
	      echo 'stdClass: '.(boolval(new stdClass) ? 'true' : 'false')."
";
	      ?>

	      The above example will output:

	      0:	false
	      42:	true
	      0.0:	false
	      4.2:	true
	      "":	false
	      "string": true
	      "0":	false
	      "1":	true
	      [1, 2]:	true
	      []:	false
	      stdClass: true

SEE ALSO

       floatval(3), intval(3), strval(3), settype(3), is_bool(3), Type juggling.

PHP Documentation Group 														BOOLVAL(3)
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