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I receive an error: "/bin/ums: 19: [: is_duplicate: unexpected operator" when running the following script:

#! /bin/sh                                                                      
                                                                                
group=""                                                                        
                                                                                
#utility functions                                                              
                                                                                
is_duplicate () {                                                               
                                                                                
  return grep $1 /etc/passwd /etc/group                                         
                                                                                
}                                                                               
                                                                                
#script                                                                         
                                                                                
echo "Welcome to the user management system."                                   
echo "Please enter the name of the group you wish to create:"                   
read group                                                                      
                                                                                
while [ is_duplicate "$group" -eq 0 ]                                           
do                                                                              
  echo "That group name already exists."                                        
  echo "Please enter a new name:"                                               
  read group                                                                    
done

Why?

Note: The script is a work in progress. In this segment I am trying to grep for the name of a group being input in the console. If the group exists in the group file (i.e. grep returns exit code 0) then continue asking for a name.

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1 Answer 1

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To test the success as indicated by the zero exit code no parentheses are needed.

while is_duplicate "$group" ; do
...
done

Similarly, return expects a number, not the string grep. You can call grep without return as the last thing in the function, its exit code will be returned.

But you should be more specific in what you're grepping from the files.

grep $1 /etc/passwd /etc/group

will return true even for user names or shell paths.

At least, quote the $1 (and maybe validate it a bit, you can try what e.g. -v : does).

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  • grep -wF "$1" would be a better match. Or even awk -v ug="$1" '$1==ug {exit 0} END {exit 1}' Commented May 26, 2022 at 15:35
  • Thanks for the insight - removing parenthesis seems to have worked. Why does including them cause the error? Commented May 26, 2022 at 15:42
  • The syntax of [ is explained in man bash. It should start with -z or similar, or with any word followed by -eq and similar. It can't start with two unknown words is_duplicate and the group name (unless the group name is -eq etc., but then only one value is expected, not two, -eq 0.) Commented May 26, 2022 at 15:51

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