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Top Forums Shell Programming and Scripting Using arrays in bash using strings to bash built-in true Post 302615141 by kristinu on Thursday 29th of March 2012 07:59:57 AM
Old 03-29-2012
Quote:
Originally Posted by Scrutinizer
While I agree with Corona688 that there are no booleans in shell, they can be emulated to some extent using the true and false built-ins if you put them in variables. This is something I found out myself at some point, and although I don't see this practice very often, I like to use it in my scripts, because I think it makes the code more mnemonic.
Code:
parameter_A_used=true parameter_B_used=false

if $parameter_A_used && $parameter_B_used; then
  echo "True"
else 
  echo "False"
fi

Code:
if $parameter_A_used && ! $parameter_B_used; then
  echo "True"
else
  echo "False"
fi

Code:
$ a=true b=false
$ if $parameter_A_used && $parameter_B_used; then   echo "True"; else    echo "False"; fi
False
$ if $parameter_A_used && ! $parameter_B_used; then   echo "True"; else    echo "False"; fi
True

What really happens here is that the shell built-ins inside these variables get executed and then produce a 1 or 0 return code.

You do need to be careful and disciplined that you only set these variables in your script itself and only to true or false for boolean purposes. Do not use variables that can contain user input or for example input from a file, because the content of these variables will get executed and there can be a security risk involved if you are careless with that.

If you are not comfortable with this then use 0 and 1 values and test for that in [ ... ] statements.
I fully understand what you mean. It was my purpose and was tending to your way of doing things. I originally was using the 0 and 1 thing, but when I was doing the checking I was using

Code:
 hasArgument=1
 if [ $hasArgument == 1 ]; then
   ...
 fi

The above is not a very good system if one wants to check if you have an argument or not,
as I always need to see if I had put a 0 or 1.

So you are saying that I can just use
Code:
hasArgument=1
if  $hasArgument; then
  ...
fi

The above is much better.
 

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ECHO(3) 								 1								   ECHO(3)

echo - Output one or more strings

SYNOPSIS
void echo (string $arg1, [string $...]) DESCRIPTION
Outputs all parameters. echo is not actually a function (it is a language construct), so you are not required to use parentheses with it. echo (unlike some other language constructs) does not behave like a function, so it cannot always be used in the context of a function. Additionally, if you want to pass more than one parameter to echo, the parameters must not be enclosed within parentheses. echo also has a shortcut syntax, where you can immediately follow the opening tag with an equals sign. Prior to PHP 5.4.0, this short syn- tax only works with the short_open_tag configuration setting enabled. I have <?=$foo?> foo. PARAMETERS
o $arg1 - The parameter to output. o $... - RETURN VALUES
No value is returned. EXAMPLES
Example #1 echo examples <?php echo "Hello World"; echo "This spans multiple lines. The newlines will be output as well"; echo "This spans multiple lines. The newlines will be output as well."; echo "Escaping characters is done "Like this"."; // You can use variables inside of an echo statement $foo = "foobar"; $bar = "barbaz"; echo "foo is $foo"; // foo is foobar // You can also use arrays $baz = array("value" => "foo"); echo "this is {$baz['value']} !"; // this is foo ! // Using single quotes will print the variable name, not the value echo 'foo is $foo'; // foo is $foo // If you are not using any other characters, you can just echo variables echo $foo; // foobar echo $foo,$bar; // foobarbarbaz // Some people prefer passing multiple parameters to echo over concatenation. echo 'This ', 'string ', 'was ', 'made ', 'with multiple parameters.', chr(10); echo 'This ' . 'string ' . 'was ' . 'made ' . 'with concatenation.' . " "; echo <<<END This uses the "here document" syntax to output multiple lines with $variable interpolation. Note that the here document terminator must appear on a line with just a semicolon. no extra whitespace! END; // Because echo does not behave like a function, the following code is invalid. ($some_var) ? echo 'true' : echo 'false'; // However, the following examples will work: ($some_var) ? print 'true' : print 'false'; // print is also a construct, but // it behaves like a function, so // it may be used in this context. echo $some_var ? 'true': 'false'; // changing the statement around ?> NOTES
Note Because this is a language construct and not a function, it cannot be called using variable functions. SEE ALSO
print(3), printf(3), flush(3), Heredoc syntax. PHP Documentation Group ECHO(3)
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