Error in if else statement in linux shell script

Question

Being newbie in linux shell scripting, I had tried to implement the following code in test.sh :

#!/bin/sh
# This is some secure program that uses security.
clear 
VALID_PASSWORD="secret" #this is our password.

echo "Please enter the password:"
read PASSWORD

if [ "$PASSWORD" == "$VALID_PASSWORD" ]; then
    echo "You have access!";
else
    echo "ACCESS DENIED!";
fi

But while executing the script, it shows me following error:

Please enter the password:
1234
./tst1.sh: 9: [: 1234: unexpected operator
ACCESS DENIED!

I am unable to debug this error. Appreciate the much needed help.

Answer 1

If you are using Shell, you need to use the POSIX comparison =.

That is, use:

if [ "$PASSWORD" = "$VALID_PASSWORD" ]; then
#                ^

Instead of

if [ "$PASSWORD" == "$VALID_PASSWORD" ]; then
#                ^^

You can have a good read to What is the difference between test, [ and [[ ? in order to see all the variances. Basically, if you are in Shell, use the POSIX tiny set of possibilities; if you are in Bash, you can use other things such as [[:

            |   new test [[   |   old test [
----------------------------------------------
            |      >          |      \>
string      |      <          |      \<
comparison  |   = (or ==)     |      =
            |      !=         |      !=

Answer 2

The comparison operator == you use to compare the two strings is a Bash extension, it doesn't exist in basic sh, which instead uses the test command which uses single = to compare strings.

Answer 3

Correct Syntax for If - Else in Linux is below

if [ "$1" = "cool" ]
then
    echo "Cool Beans"
else
    echo "Not Cool Beans"
fi