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Accepted
Valentyn Hruzytskyi
Asked: 2018-11-16 14:11:38 +0800 CST

Problem with using if/then statement: `((: == : syntax error: operand expected (error token is "== ")`

  • 772

I try to implement positional parameters in bash script and check them, but have some issue in if/then switch statement.

Script:

#set "default" parameter
query=""                                          

while [ "$1" != "" ]; do
    case $1 in
        -q | --query )          shift
                                query=$1
                                ;;
    esac
    shift
done                            

#query cannot be empty
if (($query == ""))                               
then
 echo 'no query'
 exit
fi

echo "query - $query"

All works well when I set the correct parameters:

$ ./script.sh -q request
query - request

-q parameter cannot be empty and I need to validate it. So, when the command line looks like:

$ ./script.sh -q

or

$ ./script.sh

I get this error:

./main.sh: line 13: ((: == : syntax error: operand expected (error token is "== ")
query -

How to correctly implement if operator in this case?

scripts command-line bash

1 Answers

  1. Best Answer

    (( ... )) is for arithmetic evaluation.

    You should use [ ... ] (just like you did in the while condition) or bash's extended test syntax [[ ... ]] - and remember to quote "$query" and leave whitespace around the [ and ] operators:

    if [ "$query" == "" ]

    Alternatively, use the -z empty string test:

    if [ -z "$query" ]

    For general help with questions like this, try www.shellcheck.net or install the shellcheck package from the universe repository.

    • 2