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'"

a="abc"b="def"# Equality Comparisonif [ "$a" == "$b" ]; then    echo "Strings match"else    echo "Strings don't match"fi# Lexicographic (greater than, less than) comparison.if [ "$a" \< "$b" ]; then    echo "$a is lexicographically smaller then $b"elif [ "$a" \> "$b" ]; then    echo "$b is lexicographically smaller than $a"else    echo "Strings are equal"fi

Notes:

1. Spaces between if and [ and ] are important
2. > and < are redirection operators so escape it with \> and \< respectively for strings.