Clash Royale CLAN TAG#URR8PPP

Why [[ -n $var ]] instead of [[ $var ]]?

In this answer I had some code which read:

if [[ $ZSH_VERSION ]]; then


This was edited to be:

if [[ -n $ZSH_VERSION ]]; then


Update: I just saw the edit comment:

[[ x ]] didn't work until recently in zsh

I looked through the zsh release notes and couldn't find reference to this.

zsh

Which zsh version first allowed [[ x ]]?

zsh

[[ x ]]

2 Answers
2

From the zsh 5.5.1 docs for CONDITIONAL EXPRESSIONS

zsh

CONDITIONAL EXPRESSIONS

For compatibility, if there is a single argument that is not syntactically significant, typically a variable, the condition is treated as a test for whether the expression expands as a string of non-zero length. In other words, [[ $var ]] is the same as [[ -n $var ]]. It is recommended that the second, explicit, form be used where possible.

With the source tree around,

% grep -rl 'if there is a single argument' .
./Doc/Zsh/cond.yo
% git blame ./Doc/Zsh/cond.yo | grep 'if there is a single argument'
d082827c83 (Jun T 2014-05-18 22:03:35 +0900 198) For compat...


Inspection of git log shows that the code change went in a bit earlier than the documentation:

git log

commit 9d47e8398d299e53ffe4e7ddf3731d2fedae9948
...
Date: Tue May 13 08:16:50 2014 -0700

32609: [[ $var ]] behaves as [[ -n $var ]] for bash/ksh compatibility


The mapping of the ChangeLog file to git tag is not clear to me, but it appears zsh 5.0.6 (Thu Aug 28 19:07:04 2014 +0100) is the first version with this change.

ChangeLog

git tag

zsh

It is more explicit showing what the code is doing.

Personally, in this case I'd slightly prefer [[ -n $ZSH_VERSION ]] because it shows that it is testing for non-emptyness of the value of the var.

[[ -n $ZSH_VERSION ]]

But I often have vars meant for boolean meaning in my scripts and I name them accordingly, like is_logged_in or running_in_background. If such vars are unset in my scripts they are false. And in such cases I prefer to use [[ running_in_background ]], just because this reads better without the -n part, like a normal sentence:

is_logged_in

running_in_background

false

[[ running_in_background ]]

-n

if [[ running_in_background ]] ; then
echo "something" | logger -t myprog
fi


But IMO it is just matter of taste, and as such should not have been edited: it's your code.

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