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

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

a="abc"
b="def"

# Equality Comparison
if [ "$a" == "$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.

To compare strings with wildcards, use:

if [[ "$stringA" == *"$stringB"* ]]; then
  # Do something here
else
  # Do something here
fi

I have to disagree one of the comments in one point:

[ "$x" == "valid" ] && echo "valid" || echo "invalid"

No, that is not a crazy oneliner

It's just it looks like one to, hmm, the uninitiated...

It uses common patterns as a language, in a way;

And after you learned the language.

Actually, it's nice to read

It is a simple logical expression, with one special part: lazy evaluation of the logic operators.

[ "$x" == "valid" ] && echo "valid" || echo "invalid"

Each part is a logical expression; the first may be true or false, the other two are always true.

(
[ "$x" == "valid" ] 
&&
echo "valid"
)
||
echo "invalid"

Now, when it is evaluated, the first is checked. If it is false, than the second operand of the logic and && after it is not relevant. The first is not true, so it can not be the first and the second be true, anyway.
Now, in this case is the the first side of the logic or || false, but it could be true if the other side - the third part - is true.

So the third part will be evaluated - mainly writing the message as a side effect. (It has the result 0 for true, which we do not use here)

The other cases are similar, but simpler - and - I promise! are - can be - easy to read!
(I don't have one, but I think being a UNIX veteran with grey beard helps a lot with this.)

The following script reads from a file named "testonthis" line by line and then compares each line with a simple string, a string with special characters and a regular expression. If it doesn't match, then the script will print the line, otherwise not.

Space in Bash is so much important. So the following will work:

[ "$LINE" != "table_name" ] 

But the following won't:

["$LINE" != "table_name"] 

So please use as is:

cat testonthis | while read LINE
do
if [ "$LINE" != "table_name" ] && [ "$LINE" != "--------------------------------" ] && [[ "$LINE" =~ [^[:space:]] ]] && [[ "$LINE" != SQL* ]]; then
echo $LINE
fi
done

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.

I would probably use regexp matches if the input has only a few valid entries. E.g. only the "start" and "stop" are valid actions.

if [[ "${ACTION,,}" =~ ^(start|stop)$ ]]; then
  echo "valid action"
fi

Note that I lowercase the variable $ACTION by using the double comma's. Also note that this won't work on too aged bash versions out there.

I was struggling with the same situation for a while, here is how I could resolve:

   if [ "$var1" == "$var2" ]; then
      #dowhateveryouwant
    fi
  

Be careful with the spaces left before and after the comparison sign, otherwise it won't work or it'll give you an unexpected result.

I've spent so much time on using a single equal(=) sign but didn't work. I Hope it can help.

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

Bash regex operator =~ (official reference)
StackOverflow further examples (here)

In addition to previous answers, you can also use case statement to compare strings once you have a predefined set of values to choose from. For example, given that the user will provide an argument among these predefined values a or b or c or default once running the bash script, the following script demonstrates that.

#!/bin/bash
case "$1" in
    a)
            echo "create the file";;
    b)
            echo "edit the file";;
    c)
            echo "remove the file";;
    *)
            echo "read the file";;
esac

Where "$1" is the argument provided by the user.

I did it in this way that is compatible with Bash and Dash (sh):

testOutput="my test"
pattern="my"

case $testOutput in (*"$pattern"*)
    echo "if there is a match"
    exit 1
    ;;
(*)
   ! echo there is no coincidence!
;;esac

Do you get "unexpected operator" when running a script as a cron job, but not from a console? If yes, your command interpreter is defaulting to dash. You can see it here.

$ which sh
/bin/sh
$ ls -l /bin/sh
/bin/sh -> dash*

Make sure you include the command interpreter

#!/bin/bash

at the top of your shell scripts if you want to use "=="

Just to weigh in on "=" vs "==", although they may both work if you have any coding background "=" should make you uncomfortable in a control statement as it tends to be an assignment operator rather than a test. Inconsistencies like this can make it difficult to shift between languages, and becomes even more painful to keep straight when you have Powershell using -eq for both numeric and string comparison. Bash always seemed backward with numeric evaluations using an operator consisting of a string (-eq) and string comparisons using a numeric operator "==" or "=" just you mess you up. If you ask me I'd say the problem is the distro swapping out bash. That is just my opinion, but this is my fix.

ln /bin/bash /bin/sh -sf

It's arguably not as fast, but has never broken anything.