Why do some people put a semicolon after an if condition in shell scripts? [duplicate]

Question

I'm just getting used to shell scripting, and I've come across something which I'm not really sure how to google.

In the tutorials I was reading, it suggests that the correct way to write an if statement is like this:

if [ $a == $b ]; then
  echo "a == b"
fi

However I've seen in our code base places where the semi colon is omitted:

if [ $a == $b ] then
  echo "a == b"
fi

I've also seen double square brackets:

if [[ $a == $b ]]; then
  echo "a == b"
fi

When I've tested all of these in bash, there doesn't seem to be a difference. Is there a difference? Does it have to do with compatibility? What is the correct style to adopt?

Answer 1

if [ $a == $b ]; then
  echo "a == b"
fi

You can use a semicolon, or you can write then on a separate line. Either one is allowed.

if [ $a == $b ]
then
  echo "a == b"
fi

Having neither a ; nor a newline is a syntax error.

$ if [ $a == $b ] then
>   echo "a == b"
> fi
bash: syntax error near unexpected token `fi'

As for [ vs [[, see:

• What's the difference between [ and [[ in Bash?