How to compare strings in Bash

Using variables in if statements

if [ "$x" = "valid" ]; then
  echo "x has the value 'valid'"
fi

If you want to do something when they don't match, replace = with !=. You can read more about string operations and arithmetic operations in their respective documentation.

Why do we use quotes around $x?

You want the quotes around $x, because if it is empty, your Bash script encounters a syntax error as seen below:

if [ = "valid" ]; then

---

Non-standard use of == operator

Note that Bash allows == to be used for equality with [, but this is not standard.

Use either the first case wherein the quotes around $x are optional:

if [[ "$x" == "valid" ]]; then

or use the second case:

if [ "$x" = "valid" ]; then

Or, if you don't need an else clause:

[ "$x" == "valid" ] && echo "x has the value 'valid'"