Bash [[ tests, quoting variables

Question

I want to decide whether to always omit quotes for variables appearing withing a Bash [[ test. I interpret the man page to say that it is permissible to do so without any loss of correctness.

---

I devised this simplistic "test" to verify my thinking and the "expected behaviour" but it may prove absolutely nothing, take a look at it:

x='1 == 2 &&'
if [[ $x == '1 == 2 &&' ]]; then
    echo yes
else
    echo no
fi

Note I am not writing this as such:

x='1 == 2 &&'
if [[ "$x" == '1 == 2 &&' ]]; then
    echo yes
else
    echo no
fi

which so far has always been my style, for consistency if nothing else.

---

Is is safe to switch my coding convention to always omit quotes for variables appearing within [[ tests?

I am trying to learn Bash and I am trying to do so picking up good habits, good style and correctness..

Answer 1

The key thing to remember is that quotes and escaping within pattern matching contexts always cause their contents to become literal. Quoting on the left hand side of an == within [[ is never necessary, only the right side is interpreted as a pattern. Quoting on the right hand side is necessary if you want a literal match and to avoid interpretation of pattern metacharacters within the variable.

In other words, [ "$x" = "$x" ] and [[ $x == "$x" ]] are mostly equivalent, and of course in Bash the latter should be preferred.

One quick tip: think of the operators of the [[ ]] compound comand as being the same grammar-wise as other control operators such as elif, do, ;;, and ;;& (though technically in the manual they're in their own category). They're really delimiters of sections of a compound command, which is how they achieve seemingly magical properties like the ability to short-circuit expansions. This should help to clarify a lot of the behavior of [[, and why it's distinct from e.g. the arithmetic operators, which are not like that.

More examples: http://mywiki.wooledge.org/BashFAQ/031#Theory

Answer 2

No. You should not get in the habit of always omitting quotes, even if they appear within [[ tests. Bash is famous for burning people for leaving off quotes :-)

In bash the [[ ]] should always evaluate as an expression, so the script will continue to function. The risk is that a logic error may pop up unnoticed. In all cases that I can think of off the top of my head it would be fine. However, quotes allow you to be specific about what you want, and are also self-documenting in addition to being safer.

Consider this expression:

if [[ "$INT" =~ ^-?[0-9]+$ ]]; then

It would still work without the quotes because it is in between [[ ]], but the quotes are clarifying and do not cause any issues.

At any rate, this is my opinion as a guy who has received a royal hosing at the hand of Bash because I failed to put " " around something that needed them :'(

My bash hacking friend once said, "use quotes liberally in Bash." That advice has served me well.