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Bash unary operator expected

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mamiemando Posted messages 33540 Registration date   Status Modérateur Last intervention   -
Hello,

I am a beginner in bash and I am trying to create a very simple script that only stops when the user says yes. 

#!/bin/bash until [ "$var" = 'oui' ] do read -p 'say yes: ' var done


It gives me the error: 
line 3: [: =: unary operator expected
.

I know it's probably simple, but I searched and couldn't find it. If anyone could explain it to me, thank you.

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Réponse 1 / 1
mamiemando Posted messages 33540 Registration date   Status Modérateur Last intervention   7 927
 
Hello

Problem Explanation

In your example, 
var
is not initialized, 
$var
is substituted with "nothing", resulting in the test: 

until [ = 'yes' ] do ... done


Hence the error: 
=
is a binary operator (a left operand, a right operand) and consequently bash tells you that it might take a unary operator (like 
!
)... but not binary.

How to solve your problem

You need to:
- either test in advance if 
var
is a non-empty string (using 
-z
)
- or (which I recommend) put 
$var
in quotes every time you evaluate it in a test.

Personally, I advocate the second solution. Generally, a string in bash can contain spaces, and you'll run into the same kind of problem if you don't take this precaution. 

 #!/bin/bash until [ "$var" = 'yes' ] do read -p 'say yes: ' var done exit 0


Test: 

(mando@aldur) (~) $ sh toto.sh 
say yes: 
say yes: no
say yes: yes yes
say yes: yes


Good luck
7
bash
 
Thank you very much for taking the time to reply to me, mamiemando. My script works and I understand now, thank you!!
1
mamiemando Posted messages 33540 Registration date   Status Modérateur Last intervention   7 927
 
You're welcome :-) Have a great day!
1