Why testing for non-empty string using -n always true?

Question

Hello I have a simple example which I am confused about:

#!bin/bash
#test_str is a script to test whether a string is empty or not

x=''
if [ -n $x ]
then  
    echo "'$x' is not empty"
else 
    echo "'$x' empty"
fi

chmod u+x test_str
./test_str

The output:

'' is not empty

This occurs also if I've not declared a variable (x here).

If I use the flag -z to test for emptiness it works fine:

  if [ -z $x ]
  then  
      echo "'$x' is  empty"
  else 
      echo "'$x' not empty"
  fi

The output:

'' is empty

So how can I use -n correctly? Thank you!

Quote it! `[ -n "$x" ]` without it, construct is evaluated as `[ -n ]` which is _always_ true — Inian, Sep 24 '20 at 11:06
Duplicate maybe https://stackoverflow.com/questions/6852612/test-for-empty-string-with-x it's hard to find a duplicate when searching for `-n` :/ — KamilCuk, Sep 24 '20 at 11:07
@Inian: Can you explain more please. or add it as an answer? — Maestro, Sep 24 '20 at 11:09
@Inian: It works fine thanks. One last question: should I uses `""` for the flag `-z` too or not? and if not why? — Maestro, Sep 24 '20 at 11:12
When in doubt, always look up the manual - https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html . Search for `-z string` and `-n string` — Inian, Sep 24 '20 at 11:14
@Maestro : You will notice, that i.e. [ -ge ] also is true. See _man test_, where it says: _-n STRING : the length of STRING is nonzero_ and in the next line: _STRING equivalent to -n STRING_ Since your `[ -n $x ]` is, after expansion of the empty x, equiavlent to `test -n`, the first variant does not apply (there is no _STRING_ anymore), but the second does: The statement is interpreted as `test -n -n`, i.e. whether the string _-n_ is not empty. — user1934428, Sep 24 '20 at 12:29

score 3 · Accepted Answer · edited Sep 24 '20 at 11:16

3

This is because [ -n $n ] is expanded to [ -n ] so there is nothing to compare and it will return true.

Instead, you should use [ -n "${n}" ] to prevent such errors and to prevent globbing.

edited Sep 24 '20 at 11:16

Jens

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answered Sep 24 '20 at 11:07

Emanuel Bennici

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Side note: if the glob character is in the value of $n, there's no globbing. – Jens Sep 24 '20 at 11:10
and this `[ -z $n ]` how it is interpreted? – Maestro Sep 24 '20 at 11:49
1

`-z` does check the _existenze_ of the variable, not the value. But you should also use `[ -z "$n" ]` instead. – Emanuel Bennici Sep 24 '20 at 12:04

Why testing for non-empty string using -n always true?

1 Answers1