Checking for null string in bash

Question

Is there any difference between the following tests?

[[ "$STRING" = "" ]] && exit 1;


[[ "x$STRING" = "x" ]] && exit 1;


[[ -z $STRING ]] && exit 1;

Sodved · Accepted Answer · 2011-07-12 14:14:27Z

13

Nope, they are all the same. But couple of defensive habits to get into.

You should quote the $STRING in the -z one as well
If you are running with the -u option (I always do) then you should reference the possibly optional variable as ${STRING-} just in case its not set at all

answered Jul 12, 2011 at 14:14

Sodved

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1 Comment

toxalot Over a year ago

Why would you quote $STRING when using the [[ keyword?

Guilherme David da Costa · Accepted Answer · 2011-07-12 14:22:23Z

1

Apparently, they all do the same thing, that is check if the given string its "empty", except that the first one checks if $string its empty, the second checks whether x plus $string its equals to x and finally, -z that checks the length. Personally I'd ratter go with -z that's more realiable.

answered Jul 12, 2011 at 14:22

Guilherme David da Costa

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2 Comments

Guilherme David da Costa Over a year ago

You don't need to quote $string in the -z one, and on the first example too, specially if you know that $string will contain a string.

Lara Bailey Over a year ago

Quoting strings is a good habit and makes a real difference if you do more complex tests like: [[ -z "$STRING" && -z "$OTHER" ]].

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