String comparison in bash. [[: not found

How you are running your script? If you did with

$ sh myscript

you should try:

$ bash myscript

or, if the script is executable:

$ ./myscript

sh and bash are two different shells. While in the first case you are passing your script as an argument to the sh interpreter, in the second case you decide on the very first line which interpreter will be used.

[[ is a bash-builtin. Your /bin/bash doesn't seem to be an actual bash.

From a comment:

Add #!/bin/bash at the top of file

Is the first line in your script:

#!/bin/bash

or

#!/bin/sh

the sh shell produces this error messages, not bash

As @Ansgar mentioned, [[ is a bashism, ie built into Bash and not available for other shells. If you want your script to be portable, use [. Comparisons will also need a different syntax: change == to =.

if [ $MYVAR = "myvalue" ]; then
    echo "true"
else
    echo "false"
fi