UNIX for Dummies Questions & Answers

Code:

#!/bin/bash

if [ true ] && [ ! false ]
then
        echo "True"
else
        echo "False"
fi

Hi everyone, I am new to UNIX, here I have a if statement elevating two boolean conditions. I thought the output should be True because there are [true] + [true] in the statement. But it turns out to be False.

Can anyone explain to me how to properly use the boolean condition here? Thanks a lot!

There are utilities named true and false on UNIX and Linux systems, but you aren't using them in your if statement. If you wanted to use them, the if statement would be:

Code:

if true && ! false
then    echo true
else    echo false
fi

The if statement you had was using the test utility to evaluate whether or not "true" was <not an empty string> which evaluates to true and the second test utility invocation to evaluate whether or not NOT ("false" was <not an empty string>) which evaluates to false. Since both did not evaluate to true, the AND of the two tests evaluates to false and you executed the "else" branch of your if statement.

Thank you, Don!

I just changed my code and it is giving out error

Code:

line 46: no: command not found
line 46: -d: command not found

but my code are different from what I typed above.

Code:

 elif ! $recursive && -d $originalPath/$filename
 then
          echo ".................."

for the first condition, recursive takes on the boolean value, and the second condition is to check if the file exists in that directory.

Would you suggest a better approach?

First look at the test man page on your system (or the man page for the shell you're using and look at its description of its built-in test utility). With what you're testing here, you do want to use the test utility; with what you were doing in the first posting in this thread, you didn't want to use the test utility.

After looking at the man page, it should be obvious that the test utility takes two forms:

Code:

test expression

and

Code:

[ expression ]

and some shell also provide other forms. But, these two ways of writing a test command produce exactly the same results; the 2nd form is much more common in shell scripts. Note that the expression must be separated from the test or the [ and ] by one or more spaces or tabs.

From the error messages you're getting, I will make the assumption that the shell variable recursive is expected to either have the value "no" or "yes". Assuming that is correct, you probably want an if statement similar to:

Code:

if false
then    echo "1st <if> true"
elif [ "no" != "$recursive" ] && [ -d "$originalPath/$filename" ]
then    echo "2nd <if> recursive AND $originalPath/$filename is a directory"
else    echo "2nd <if> not recursive OR $originalPath/$filename is not a directory"
fi

I picked up the value "no" for recursive from the errors you got. If my guess that "yes" is the other valid value, I would rewrite the elif clause to be:

Code:

elif [ "yes" = "$recursive" ] && [ -d "$originalPath/$filename" ]

to get rid of the double negative logic ("no" is not the value of recursive) because it is easier for humans to parse "yes" is the value of recursive.