Is the semicolon now optional in bash conditionals with [[ .. ]] in version 5.2?

Question

The following bash script:

#!/bin/bash

TEST_VAR=""

if [[ -z $TEST_VAR ]] then
    echo "empty"
else
    echo "$TEST_VAR"
fi

despite missing a semicolon before "then", does not throw an error in bash 5.2 and completes successfully, even returning 0.

Running the above script on a system running bash 5.1 or 5.0 fails as expected.

Why is this? Did bash change their parser in version 5.2? After searching I can't find any reference to this change. Did bash intentionally change behavior here, or is this a bug?

Answer 1

Thanks to oguz ismail, who provided proofs to back up my intuition that I wasn't able to find - both the relevant mailing list discussion and the git commit that implemented this.

---

In a nutshell, following a mailing list discussion happened before the release of 5.2, it had been found that the behavior of [[]] requiring a semicolon at the end was inconsistent with that of other compound commands, which didn't require a semicolon at the end, and [[]]'s behavior was changed to conform to those before 5.2 rolled out.

---

]] is a reserved word:

$ LANG=C type ]]
]] is a shell keyword

From Reserved Words (Bash Reference Manual) - emphasys mine:

Reserved words are words that have special meaning to the shell. They are used to begin and end the shell’s compound commands.

And [[ expression ]] is, in fact, a compound command.

Historically, other compound commands such as {} or () never required a semicolon after them:

$ if { true; } then echo foo; fi # no semicolon after the first `}`, no syntax error
foo
$ if ( true ) then echo foo; fi # no semicolon after the first `)`, no syntax error
foo
$ if { true; } then { echo foo; } fi # no semicolon after the second `}`, no syntax error either
foo

So [[]]'s behavior was changed to conform to those before 5.2 rolled out.