UNIX for Dummies Questions & Answers

This is probably a stupid question to ask. But could somebody help me clearly distinguish the difference between these two operators in unix '==' and '='?

In unix what?

excuse me if i did not clearly stated the prob. Consider this piece of code:

if( $3 == "R001" )
print "xxx";
else
print "yyy";
fi

my question is why can't I use the following instead of above:

if [[ $3 = "R001" ]]
print "xxx"
else
print "yyy"
fi

Also, is there is a difference in () and [[ ]] in above two examples?

Thanks

UNIX is an operating system, not a commandline. You seem to be talking about the standard UNIX shell, sh or one of it's clones like bash, but could have as easily been using something like korn... or even been programming in C.... = and == are pretty common.

I've never seen code like your first example, probably because it doesn't work. ( ) brackets aren't statement grouping, they define arrays!

Code:

arr=(a = c)
echo ${arr[0]}
echo ${arr[1]}
echo ${arr[2]}

will print a, then =, then c. The = isn't even treated as an operator inside the ().

Inside the [[ ]] brackets, == is a pattern matching operator for strings, and = is a straight equality comparison. Outside of there, = is an assignment operator like variable="something" and I don't think == does anything.

Also, your if statements aren't quite right. Try this:

Code:

if [[ $this = "that" ]]
then
      echo "Something"
else
      echo "Something else"
fi

It appears that the difference is that one command is in a sh based shell and the other a csh based shell.

Thanks both of you, for your replies. The examples that I cited were from ksh actually. I did come across some places and found out that == is equality operator used in arirthmatic operations (e.g. to compare two strings) whereas = is simply an assignment operator.

I guess I am still a little bit confused as far as the different brackets are concerned i.e. is my understand correct per following:

1. (commands...) - open a subshell and execute commands in that subshell ??
2. ((commands...)) - i have no idea about this one.

As far as [ $var1 -eq 0 ] and [[ $var1 -eq 0 ]] type of brackets are concerned, i guess they are equivalent and are used to test whether var1 is set to 0.

Thanks once again
Vikas

[[ ]] must be used when there are multiple tests:

Code:

if [[ "$a" = "one"  && "$b" = "two ]]; then
    do something
fi