Bash -eq and ==, what's the diff?

Question

Why this works:

Output=$( tail --lines=1 $fileDiProva )
##[INFO]Output = \"OK\"

if [[ $Output == $OK ]]; then
    echo \"OK\"
else
    echo \"No Match\"
fi


        

Answer 1

-eq, -lt, -gt is only used for arithmetic value comparison(integers).

== is used for string comparison.

Answer 2

-eq is an arithmetic test.

You are comparing strings.

From help test:

Other operators:

  arg1 OP arg2   Arithmetic tests.  OP is one of -eq, -ne,
                 -lt, -le, -gt, or -ge.

When you use [[ and use -eq as the operator, the shell attempts to evaluate the LHS and RHS. The following example would explain it:

$ foo=something
+ foo=something
$ bar=other
+ bar=other
$ [[ $foo -eq $bar ]] && echo y
+ [[ something -eq other ]]
+ echo y
y
$ something=42
+ something=42
$ [[ $foo -eq $bar ]] && echo y
+ [[ something -eq other ]]
$ other=42
+ other=42
$ [[ $foo -eq $bar ]] && echo y
+ [[ something -eq other ]]
+ echo y
y

Answer 3

Have a look at this explanation of if.

The first one == is in the section of string compare operators and it can only compare two strings.

The second one -eq is in the last section ARG1 OP ARG2(last one) and its documentation says "ARG1" and "ARG2" are integers.