Bash – Difference between [[ $variable ]] and [[ -n $variable ]]

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In bash, are [[ $variable ]] and [[ -n $variable ]] completely equivalent? It appears to be the case judging by the output below, but I see both forms of usage prevalent in shell scripts.

$ z="abra"
$ [[ $z ]]
$ echo $?
0
$ [[ -n $z ]]
$ echo $?
0
$ z=""
$ [[ $z ]]
$ echo $?
1
$ [[ -n $z ]]
$ echo $?
1
$ unset z
$ [[ $z ]]
$ echo $?
1
$ [[ -n $z ]]
$ echo $?
1

Best Answer

[ "$var" ] is equivalent to [ -n "$var" ] in bash and most shells nowadays. In other older shells, they're meant to be equivalent, but suffer from different bugs for some special values of "$var" like =, ( or !.

I find [ -n "$var" ] more legible and is the pendant of [ -z "$var" ].

[[ -n $var ]] is the same as [[ $var ]] in all the shells where that non-standard ksh syntax is implemented.

test "x$var" != x would be the most reliable if you want to be portable to very old shells.

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