logical condition

nano2 · 06-13-2011, 10:16 AM

Hi

Using the following in bash doesn't produce the correct result.
can any one spot why ?

Code:

 test= "A"
 if [ "$test" != "AB" ] ||  [ "$test" != "BC"] 
then
    echo "No Match"
else
    echo "Match"
fi

druuna · 06-13-2011, 10:20 AM

Hi,

What output do you expect, 'cause it does produce the correct result (No Match).

Logic:
[ "$test" != "AB" ] || [ "$test" != "BC" ]
is
[ "A" != "AB" ] || [ "A" != "BC" ]
is
true || true
is
true
and thus the then part is executed.

Hope this helps.

MensaWater · 06-13-2011, 10:26 AM

No - Since you don't tell us what YOUR expected result is and what result you are getting that you think is wrong.

However, a couple of things that I notice:
1) You have a space between test= and the variable - that is not correct - you need to remove the space.
2) You have quotes around the variable ("A") which are unnecessary and might be treated literally in some contexts.
3) You do not have a space before your final right bracket which means the test is not recognized as such.
4) You not need quotes around $test in your test statements.

nano2 · 06-13-2011, 02:01 PM

I am getting False result no match I have now tried to pass it in as command line

like so

Code:

test="$1"
if [  ! -z $test ]
then 
    if [ "$test" != "AB"] ||  [ "$test" != "BC"] 
    then
         echo "No Match"
    else
         echo "Exact Match"
    fi
fi

so when i pass in "AB" i still get the wrong match -can any one spot whats going on ?

Why can i not get the exact match

druuna · 06-13-2011, 02:16 PM

Hi,

That output is also correct:

Logic:

[ "$test" != "AB" ] || [ "$test" != "BC" ]

[ "AB" != "AB" ] || [ "AB" != "BC" ]

false || true

true

The then part (No match) is executed.

Try not to use negative checks (!=) try positive once (==) whenever possible. Have a look at these 2 examples:

Code:

#!/bin/bash

test="$1"
if [  ! -z $test ]
then
    if [ "$test" == "AB" ]
    then
         echo "first: Match"
    else
         echo "first: No Match"
    fi
fi


    if [ "$test" == "AB" ] ||  [ "$test" == "BC" ]
    then
         echo "second: Match"
    else
         echo "second: No Match"
    fi

Hope this helps.

MTK358 · 06-13-2011, 08:18 PM

Quote:

Originally Posted by nano2

I am getting False result no match I have now tried to pass it in as command line

like so

Code:

test="$1"
if [  ! -z $test ]
then 
    if [ "$test" != "AB"] ||  [ "$test" != "BC"] 
    then
         echo "No Match"
    else
         echo "Exact Match"
    fi
fi

so when i pass in "AB" i still get the wrong match -can any one spot whats going on ?

Why can i not get the exact match

So you want it to say "Exact Match" when the string is "AB" or BC"?

Basically, you're saying that it doesn't match if it's not "AB" or not "BC". If it's "AB", then it's not "BC", and you wrote that that means "No Match". Get it now?

XavierP · 06-14-2011, 03:06 AM

Moved to Programming

sundialsvcs · 06-14-2011, 06:37 AM

perl -e 'print ($test =~ /^(AB|BC)$/ ? "match" : "no match");'

so to speak ... it won't work quite as-writ but "you get the idea."

A "shell command" can be written in any programming language, thanks to "shebang" (#!), and you've probably got half-a-dozen of 'em out there on your computer right now.

rustek · 06-22-2011, 02:57 AM

Replace || with && and it will do what you want.

Russ