FreeKB - How to use if, else, then, elif statements in a BASH script

In bash, an if statement has the following structure.

if [ comparison ]; then
  --do something--
elif [ comparison ]
  --do something
else
  --do something--
fi

For example.

if [ $foo -eq 1 ]; then
  echo "Foo equals 1"

elif [ $foo -eq 2 ]
  echo "Foo equals 2"

else
  echo "Foo does not equal 1 or 2"

fi

The following table contains commonly used comparison operators.

Equals (integers)	if [ $foo -eq 1 ]
Does not equal (integers)	if [ $foo -ne 1 ]
Equals (strings)	if [ "$foo" == "Hello" ]
Does not equal (strings)	if [ "$foo" != "Hello" ]
Greater than	if [ $foo -gt 1 ]
Greater than or equal to	if [ $foo -ge 1 ]
Less than	if [ $foo -lt 1 ]
Less than or equal to	if [ $foo -le 1 ]
Variable contains no value (null / empty)	if [ -z "$foo" ]
Variable contains a value (not null / not empty)	if [ ! -z "$foo" ]
File exists	if [ -e "/path/to/file" ]
File does not exist	if [ ! -e "/path/to/file" ]
File is empty	if [ ! -s "/path/to/file" ]
File is not empty	if [ -s "/path/to/file" ]
File contains	if grep -q "Hello World" /path/to/file;
File does not contain	if ! grep -q "Hello World" /path/to/file;
Directory exists	if [ -d "/home/test" ]
Directory does not exist	if [ ! -d "/home/test" ]
Directory empty	if [ -z "$(ls -A /path/to/directory)" ]
Directory not empty	if [ ! -z "$(ls -A /path/to/directory)" ]
Variable contains	if [[ $foo = Hello ]]
Variable does not contain	if [[ $foo != Hello ]]
Variable begins with	if [[ $foo =~ ^Hello ]]
Variable does not begin with	if [[ ! $foo =~ ^Hello ]]
Variable ends with (do not place double quotes around value)	if [[ $foo =~ Hello$ ]]
Variable does not end with (do not place double quotes around value)	if [[ ! $foo =~ Hello$ ]]

Spacing

Many if statements requre a single white space inside of the brackets. I also use a single white space as my practice.

And (&&), Or (||)

Double && characters can be used to link statements together as an and.

if [ $foo -eq 1 ] && [ $bar -eq 1 ]

if [[ $foo == "Hello" && $bar == "World" ]]

Double || characters can be used to link statements together as an or.

if [ $foo -eq 1 ] || [ $bar -eq 1 ]

if [[ $foo == "Hello" || $bar == "World" ]]