The "If" control flow is used to execute a command list only if a given command returns zero.
The general form is:
if command1 then command2 else command3 fi
If command1 returns zero (success status of a command or test) then command2 is executed. Otherwise command3 is executed.
Nested if statements can also be used.
if command1 then command2 else if command3 then command4 fi fi
This can also be written using the abbreviated form:
if command1 then command2 elif command3 then command4 fi
Both these statements execute command2 if command1 returns zero. If command1 returns non-zero and command3 does return zero, then command4 is executed.
Our page on Test Operators has some examples on using the "test" command to test for conditions, however, we can explore one complicated case here.
In this example we read in the answers to two questions, then check the answers using an if loop and a nested if loop.
echo "Test 1: What is a female dear called?"
read answer1
echo "Test 2: What is a long long way to go?"
read answer2
if test "$answer1" = "doe"
then
echo "You passed test 1"
if test "$answer2" = "far"
then
echo "and you also passed test 2"
else
echo "but you failed test 2"
fi
elif test "$answer2" = "far"
then
echo "You failed test 1 but passed test 2"
else
echo "You failed both test 1 and test 2"
fi
If the above is written to a file called ifexam and run by a clever user we get the following:
sh$ sh ifexam Test 1: What is a female dear called? doe Test 2: What is a long long way to go? far You passed test 1 and you also passed test 2
Perhaps you can trace through the if loops and see what would happen if one or both of the answers were incorrect!