LINUX CLASSES - PROGRAMMING

Where's the Logic in That?

Conditional Operations

Let's look at the if/then/else construct in a Bash shell script and see how to control the flow of a script with conditional logic. The general form of if/then/else is shown here, with the actual syntax shown in boldface and the parts you must supply in normal type:

if [ condition is true ]
then
execute these commands
else
execute those commands
fi

The else clause is optional, but you must end the construct with the fi command. You can also have nested if clauses by using the elif command like this:

if [ condition1 is true ]
then
execute these commands
elif [ condition2 is true ]
then
execute these commands
else
execute those commands
fi

So what kind of conditions can we test for? If you're dealing with numbers, here are the conditional expressions you can use. In other words, any of these expressions can go inside the brackets on the if or elif statement:

num1 -eq num2 True if num1 equals num2.

num1 -ne num2 True if num1 is not equal to num2.

num1 -lt num2 True if num1 is less than num2.

num1 -gt num2 True if num1 is greater than num2.

num1 -le num2 True if num1 is less than or equal to num2.

num1 -ge num2 True if num1 is greater than or equal to num2.

If you're comparing character strings, these are the valid conditional expressions:

str1 = str2 True if str1 and str2 are identical.

str1 != str2 True if str1 and str2 are not identical.

-n str1 True if str1 is not null (length is greater than zero).

-z str1 True if str1 is null (length is zero).

You can also test certain file conditions, such as whether or not files exist, the type of file, and so on. Here are the conditional expressions for files:

-f somefile True if somefile exists and is an ordinary file.

-d somefile True if somefile exists and is a directory.

-s somefile True if somefile contains data (the size is not zero).

-r somefile True if somefile is readable.

-w somefile True if somefile is writable.

-x somefile True if somefile is executable.

And finally, here are the logical operators, for performing tests that involve and, or, and not conditions.

cond1 -a cond2 True if both cond1 and cond2 are true.

cond1 -o cond2 True if either cond1 or cond2 is true.

! cond1 True if cond1 is false.

Some if/then/else Examples

Here are some examples using the conditional expressions just listed. Note that the spaces on either side of the square brackets are not optional!

if [ $carprice -gt 20000 ]
then
echo 'Too rich for my blood.'
else
echo 'Can you get that model in blue?'
fi

if [ $maker = 'Buick' ]
then
echo 'Have you driven a Ford lately?'
fi

if [ -r $1 -a -s $1 ]
then
echo "The $1 file is readable and contains data."
fi

The case Statement

Bash provides a case statement that lets you compare a string with several possible values and execute a block of code when it finds a match. Here's an example of the case command, with the syntax shown in boldface and the parts you would supply in normal type:

case $1 in
-a)
commands;;
-f)
commands;;
*)
commands;;
esac

In this example, if the value of $1 was -a, the first block of commands would execute. If the value of $1 was -f, the second block of commands would execute. Otherwise, the third block of commands, following the asterisk clause, would execute. (Think of the asterisk as meaning "match anything.")

You can put as many commands as you need in place of commands in the sample, but be sure to end the block with a double semicolon. Only the first matching block of commands will execute in a case statement, and you must signal the end of the construct with the esac command.

Previous Lesson: Shell Script Variables
Next Lesson: Shell Script Looping

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Copyright © by Bob Rankin
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