The if then else Structure

The simplest form on an if statement is this:
if control-command
then
commands
fi

• First the control-command is executed. If it is successfull (exit status of 0), then the commands between the then and the fi are performed. The words if, then and fi MUST come after a command separator.
• While the normal if statement could be replaced by the conditional operator notation there is the possibility to add an else option.

The general form of the if else construction is this:
if control-command
then
commands
else
commands
fi

• First the control-command is executed. If it is successfull (exit status of 0), then the commands between the then and the else are performed. If it fails, the commands between the else and the fi are executed.
• There is one further extension of the if statement that allows to provide several alternatives (multiple elses) using the keyword elif (short for else if).

The general form of the if then elif then else construction looks like this:
if control-command 1
then
commands
elif control-command 2
then
commands
else
commands
fi

• If the first control-command (control-command1) is successful, then the commands after the first then up to the elif are executed.
• Otherwise, control-command2 after the elif is attempted; if it is successful, then the commands after the following then up to the next elif or else are executed.
• The process continues until the closing fi is reached.