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" ]]