Shell Programming and Scripting

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 sometimes 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.

Also, since true and false are built-ins, they can be overridden by shell functions, so you need to be aware of that also, like with any other utility of built-in that you use your script..

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.

No, that will not work. I meant:

Code:

hasArgument=true
if  $hasArgument; then
  ...
fi

And you do need to be disciplined in setting all these variables to true or false first, because of this:

Code:

$ some_unset_parameter=
$ if $some_unset_parameter; then
>   echo hello
> fi
hello

So there are some drawbacks to this approach...

I see, so when I am using the 0 and 1 thing, I have always have to check if it is a 1 or a 0.

What do you mean with the "0 and 1 thing" ?

Code:

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

This is how it actually works:

Code:

$ COMMAND=echo
$ $COMMAND a b c d
a b c d

$

So you can put the name of a command in a variable, and call it with $VARIABLE.

It just so happens there are commands named true and false!

Code:

$ whereis true false
true: /bin/true /usr/share/man/man1/true.1.bz2 /usr/share/man/man1p/true.1p.bz2
false: /bin/false /usr/share/man/man1/false.1.bz2 /usr/share/man/man1p/false.1p.bz2

$

And they do what they say they do. true always returns success, false always returns failure, which means when you join them together with && and ||, they do what they look like they should do.

They're often shell builtins, too.