SC2309

-eq treats this as a variable. Use = to compare as string (or expand explicitly with $var)

Problematic code:

read -p "Continue? [y/n] " var
[ "$var" -eq "n" ​] && exit 1

Correct code:

#read -p "Continue? [y/n] " var
[ "$var" = "n" ​] && exit 1

Rationale:

ShellCheck found a string used as an argument to a numerical operator like -eq, -ne, -lt, -ge. Such strings will be treated as arithmetic expressions, meaning n will refer to a variable $n, and 24/12 will be evaluated into 2.

In the problematic example, the intention was instead to compare "n" as a string, so it should use the equivalent string operator instead, in this case =.

Exceptions:

It is perfectly valid to use variables as operands. ShellCheck will not flag any value that is an unquoted variable name assigned in the script:

a=42; [[ "a" -eq 0 ]]  # Flagged due to quotes
      [[ b -eq 0 ]]    # Flagged due to not being assigned
c=42; [[ c -eq 0 ]]    # Not flagged

However, ShellCheck does not know whether you intended foo/bar to be division or a file path.

If you intended to divide $foo and $bar, you can either make it explicit with [[ $((foo/bar)) -ge 0 ]], or simply ignore the warning.

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