Check if bash variable equals 0

Question

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?

cyon · Answer 1 · 2017-09-25T16:15:53.877

263

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

edited Sep 25 '17 at 16:15

answered Oct 26 '12 at 11:36

cyon

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I am getting depth as a result from another program. I tried to echo it and there was nothing in the output. However, when I use [[]] as @Jacek Dominiak suggested, script works correctly (what is quite strange if variable is really unset). I have to admit that I do not really understand whats going on here... – Perlnika Oct 26 '12 at 11:54
7

Your depth variable is unset. That means bash sees an expression which says `[ -eq 0 ]; then` which doesn't make sense to it. `[[ ]]` is the safer version which seems to make bash see it as [[ null -eq 0 ]] which is correct. – cyon Oct 26 '12 at 12:05
3

A safer way to check is to enclose both sides in double quotes, i.e. `if [ "$depth" -eq "0" ]`; this way, an unset variable (`$depth`) evaluates to "" (which of course does not equal zero) – Doktor J Sep 13 '16 at 16:35
1

@DoktorJ Agreed, that's a better way to do it. I'll update the code. – cyon Sep 14 '16 at 09:38
"An unset variable used with the [ command appears empty to bash." that helped me! Thanks – Hasan Rumman Sep 25 '17 at 04:39
2

This gives me: `[: Illegal number:` error. – Daniel Kmak Jun 20 '18 at 21:52
This does not work if the output is a decimal : `[: : integer expression expected` – alper Feb 21 '21 at 15:56

score 82 · Answer 2 · edited Dec 28 '15 at 12:43

82

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

edited Dec 28 '15 at 12:43

Dennis

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answered Oct 26 '12 at 11:57

Raviteja Gubba

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Can you provide some reasoning/evidence why some approaches are good and others bad? – Dennis Mar 09 '13 at 08:58
@Dennis: [This](http://tldp.org/LDP/abs/html/comparison-ops.html) seems to verify the statements in the answer (-> the different brackets for string vs. integer comparison) – mozzbozz Dec 23 '15 at 13:01
`best, $ on age is optional` It's not working without `$` – Jean-Marc Amon Nov 21 '17 at 17:36
1

`if (( age > 21 ))` and `if (( $age > 21 ))` is different sometimes. You can see https://github.com/koalaman/shellcheck/wiki/SC2004 – Jess Chen May 15 '21 at 17:31

score 26 · Answer 3 · answered Oct 26 '12 at 11:37

26

Try:

zero=0;

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

answered Oct 26 '12 at 11:37

Jacek Dominiak

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The double brackets are smarter about preserving unset/null variables as a separate word in the expression. – glenn jackman Oct 26 '12 at 14:44
@pavel, [ is a bash built-in, see https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html#Bourne-Shell-Builtins – Nicholas Sushkin Feb 03 '16 at 16:46

score 25 · Answer 4 · answered Sep 23 '16 at 23:14

25

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

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

answered Sep 23 '16 at 23:14

Gurubaran

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score 7 · Answer 5 · answered Jun 10 '16 at 23:10

7

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

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

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

answered Jun 10 '16 at 23:10

Stephen Niedzielski

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score 3 · Answer 6 · edited Jun 30 '16 at 10:28

3

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.

edited Jun 30 '16 at 10:28

BuZZ-dEE

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answered Jul 03 '14 at 09:53

Prabhat Kumar Singh

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Check if bash variable equals 0

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