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'"

You can also use use case/esac:

case "$string" in
 "$pattern" ) echo "found";;
esac

Bash 4+ examples. Note: not using quotes will cause issues when words contain spaces, etc. Always quote in Bash, IMO.

Here are some examples in Bash 4+:

Example 1, check for 'yes' in string (case insensitive):

    if [[ "${str,,}" == *"yes"* ]] ;then

Example 2, check for 'yes' in string (case insensitive):

    if [[ "$(echo "$str" | tr '[:upper:]' '[:lower:]')" == *"yes"* ]] ;then

Example 3, check for 'yes' in string (case sensitive):

     if [[ "${str}" == *"yes"* ]] ;then

Example 4, check for 'yes' in string (case sensitive):

     if [[ "${str}" =~ "yes" ]] ;then

Example 5, exact match (case sensitive):

     if [[ "${str}" == "yes" ]] ;then

Example 6, exact match (case insensitive):

     if [[ "${str,,}" == "yes" ]] ;then

Example 7, exact match:

     if [ "$a" = "$b" ] ;then

Enjoy.

Are you having comparison problems? (like below?)

var="true"
if [[ $var == "true" ]]; then
  # It should be working, but it is not...
else
  # It is falling here...
fi

Try like the =~ operator (regular expression operator) and it might work:

var="true"
if [[ $var =~ "true" ]];then
  # Now it works here!!
else
  # No more inequality
fi