Check if bash variable equals 0

I have a bash variable depth and I would like to test if it equals 0. In case yes, I want to stop executing of script. So far I have:

zero=0;

if [ $depth -eq $zero ]; then
	echo "false";
	exit;
fi

Unfortunately, this leads to:

 [: -eq: unary operator expected

(might be a bit inaccurate due to translation)

Please, how can I modify my script to get it working?

Looks like your depth variable is unset. This means that the expression [ $depth -eq $zero ] becomes [ -eq 0 ] after bash substitutes the values of the variables into the expression. The problem here is that the -eq operator is incorrectly used as an operator with only one argument (the zero), but it requires two arguments. That is why you get the unary operator error message.

EDIT: As Doktor J mentioned in his comment to this answer, a safe way to avoid problems with unset variables in checks is to enclose the variables in "". See his comment for the explanation.

if [ "$depth" -eq "0" ]; then
   echo "false";
   exit;
fi

An unset variable used with the [ command appears empty to bash. You can verify this using the below tests which all evaluate to true because xyz is either empty or unset:

• if [ -z ] ; then echo "true"; else echo "false"; fi
• xyz=""; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fi
• unset xyz; if [ -z "$xyz" ] ; then echo "true"; else echo "false"; fi

Double parenthesis (( ... )) is used for arithmetic operations.

Double square brackets [[ ... ]] can be used to compare and examine numbers (only integers are supported), with the following operators:

· NUM1 -eq NUM2 returns true if NUM1 and NUM2 are numerically equal.

· NUM1 -ne NUM2 returns true if NUM1 and NUM2 are not numerically equal.

· NUM1 -gt NUM2 returns true if NUM1 is greater than NUM2.

· NUM1 -ge NUM2 returns true if NUM1 is greater than or equal to NUM2.

· NUM1 -lt NUM2 returns true if NUM1 is less than NUM2.

· NUM1 -le NUM2 returns true if NUM1 is less than or equal to NUM2.

For example

if [[ $age > 21 ]] # bad, > is a string comparison operator

if [ $age > 21 ] # bad, > is a redirection operator

if [[ $age -gt 21 ]] # okay, but fails if $age is not numeric

if (( $age > 21 )) # best, $ on age is optional

Try:

zero=0;

if [[ $depth -eq $zero ]]; then
  echo "false";
  exit;
fi

you can also use this format and use comparison operators like '==' '<='

  if (( $total == 0 )); then
      echo "No results for ${1}"
      return
  fi

Specifically: ((depth)). By example, the following prints 1.

declare -i x=0
((x)) && echo $x

x=1
((x)) && echo $x

You can try this:

: ${depth?"Error Message"} ## when your depth variable is not even declared or is unset.

NOTE: Here it's just ? after depth.

or

: ${depth:?"Error Message"} ## when your depth variable is declared but is null like: "depth=". 

NOTE: Here it's :? after depth.

Here if the variable depth is found null it will print the error message and then exit.