Unix Bash - Assign if/else to Variable

Question

I have been creating to assign the output of if/else to a variable but keep on getting an error.

For Example:

mathstester=$(If [ 2 = 2 ]
Then echo equal
Else
echo "not equal"

fi)

So whenever I add $mathstester in a script, laid out like this:

echo "Equation: $mathstester"

It should display:

Equation: Equal

Do I need to lay it out differently? Is it even possible?

Answer 1

Putting the if statement in the assignment is rather clumsy and easy to get wrong. The more standard way to do this is to put the assignment inside the if:

if [ 2 = 2 ]; then
    mathstester="equal"
else
    mathstester="not equal"
fi

As for testing variables, you can use something like if [ "$b" = 2 ] (which'll do a string comparison, so for example if b is "02" it will NOT be equal to "2") or if [ "$b" -eq 2 ], which does numeric comparison (integers only). If you're actually using bash (not just a generic POSIX shell), you can also use if [[ "$b" -eq 2 ]] (similar to [ ], but with somewhat cleaner syntax for more complicated expressions), and (( b == 2 )) (these do numeric expressions only, and have very different syntax). See BashFAQ #31: What is the difference between test, [ and [[ ? for more details.

Answer 2

The correct way to use if is:

mathtester=$(if [ 2 = 2 ]; then echo "equal"; else echo "not equal"; fi)

For using this in multiline statements you might consider looking link.