Conditional structures

If-then-else and elif

Conditional expression verify if a statement is true or false. If the statement is true, the code inside the if block is executed. If the statement is false, the code inside the else block is executed. In the first example only the if block is executed when the condition is true.

Simple conditional expression

if [ $a -eq $b ]; then
    echo "a is equal to b"
fi

Beside executing the code inside the block if the statement is true, the code inside the else block is executed when the condition is false.

Conditional expression with else clause

if [ $a -eq $b ]; then
    echo "a is equal to b"
else
    echo "a is not equal to b"
fi

If the condition is false, the code inside the else block is executed. The else block can also be used as a second if-then-else structure by using the elif keyword.

Conditional expression with else if clause

if [ $a -eq $b ]; then
    echo "a is equal to b"
elif [ $a -eq $c ]; then
    echo "a is not equal to b"
else
    echo "a is not equal to b or c"
fi

Test Operators

For conditional structures to function, the condition must be expressed in a form that can be evaluated by the shell. The command test is used to evaluate the condition and the short form is the [ sign.

File system tests

The test command has numerous options to test the existence of files and directories as in the following example. After the test command, the expression is evaluated as a boolean expression. So with the option -e, the expression is evaluated as a file existence test, but no other test is performed.

Test command with file existence test

if [ -e filename ]; then
    echo "file with name `filename` exists"
fi

As everything on a Unix file system is a file, the test command can be used to test the existence of files and directories. To be more explicit, the test command can be used to test if a regular file for example. With the option -f, the expression is evaluated as a file existence test and the test is performed on the file type.

Test command with file type test

if [ -f filename ]; then
    echo "file exists and is a regular FILE"
fi

The same can be done with the test command to test if a directory exists. The example below tests if the directory dirname exists and if the directory doesn’t exist, the directory is created.

Test command with directory existence test

if [ ! -d dirname ]; then
    mkdir dirname
fi

The previous examples only showed a couple of cases, but the table below shows all the available options and their meaning. Understanding the direct options is important to understand the meaning of the test command and write the correct conditional expression.

Operator	Description
FILE1 -ef FILE2	FILE1 and FILE2 have the same device and inode numbers
FILE1 -nt FILE2	FILE1 is newer (modification date) than FILE2
FILE1 -ot FILE2	FILE1 is older than FILE2
-b FILE	Test if FILE exists
-c FILE	Test if FILE exists and is block special
-d FILE	Test if FILE exists and is character special
-e FILE	Test if FILE exists
-f FILE	Test if FILE exists and is a regular file
-g FILE	Test if FILE exists and is set-group-ID
-G FILE	Test if FILE exists and is owned by the effective group ID
-h FILE	Test if FILE exists and is a symbolic link
-k FILE	Test if FILE exists and has its sticky bit set
-L FILE	Test if FILE exists and is a symbolic link
-N FILE	Test if FILE exists and has been modified since it was last read
-O FILE	Test if FILE exists and is a directory
-p FILE	Test if FILE exists and is a named pipe
-r FILE	Test if FILE exists and a read permission is granted
-s FILE	Test if FILE exists and has a size greater than zero
-S FILE	Test if FILE exists and is a socket
-t FD	Test if file descriptor FD is opened on a terminal
-u FILE	Test if FILE exists and its set-user-ID bit is set
-w FILE	Test if FILE exists and write permission is granted
-x FILE	Test if FILE exists and execute (or search) permission is granted

String comparison

if [ -n variable ]; then
    echo "variable is not empty"
fi
if [ variable ]; then
    echo "variable is not empty"
fi

if [ -z variable ]; then
    echo "variable is empty"
fi

if [ variable1 = variables ]; then
    echo "variable1 is equal to variable2"
fi

if [ variable1 != variables ]; then
    echo "variable is not equal to variable2"
fi

Integer comparison

if [ variable1 -eq variables ]; then
    echo "variable1 is equal to variable2"
fi

if [ variable1 -ne variables ]; then
    echo "variable1 is not equal to variable2"
fi

if [ variable1 -ge variables ]; then
    echo "variable1 is greater than or equal to variable2"
fi
if [ variable1 -gt variables ]; then
    echo "variable1 is greater than variable2"
fi
if [ variable1 -le variables ]; then
    echo "variable1 is less or equal to variable2"
fi
if [ variable1 -lt variables ]; then
    echo "variable1 is less than variable2"
fi

Combined comparison

Conditional expression with the and test operators

if [ -d dirname -a -g dirname ]; then
    echo "directory exists"
fi

Conditional expression with the or test operators

if [ -d dirname -o -g dirname ]; then
    echo "directory exists"
fi

Case

The case statement is used to test a value and execute a statement depending on the value. It is an alternative to the elif statement and easier to write and read.

read -p "Enter a number: " a
case $a in
    1)
        echo "a is 1"
        ;;
    2)
        echo "a is 2"
        ;;
    3)
        echo "a is 3"
        ;;
    *)
        echo "a is not 1, 2 or 3"
        ;;
esac

Select

With control structures, it is possible to select one of several possible actions depending on the value of a variable. The select statement is used to do this as it is a way to create a menu in a controlled way. The select statement is similar to the case statement, but it is more flexible. The select statement is used to test a value and execute a statement depending on the value. In th example below a case statement is executed when the selected value is correct.

PS3="Enter your choice: "
select opt in jack john jane quit
do
    case $opt in
        jack)
            echo "The chosen one is Jack"
            ;;
        john)
            echo "The chosen one is John"
            ;;
        jane)
            echo "The chosen one is Jane"
            ;;
        quit)
            echo "Goodbye"
            break
            ;;
        *)
            echo "Invalid option $REPLY"
            ;;
    esac
done