Using test in Linux scripts

In the last article we talked about the basics of Linux shell scripts. In this one, we’ll mention test, a command-line utility that evaluates and compares conditional expressions. Test is a shell builtin most commonly used with the if statement. Depending on the expression, conditional statement could be unary or binary. For example, -e filepath is an unary command while $VAR1 < $VAR2 is a binary command.  Test has two variations, which are equivalent, apart from the syntax:

test expression

and

[ expression ]

Keep in mind that the second variation (with the brackets) needs to have blank spaces before and after the expression.

As mentioned in the previous article, test command has a termination or exit status, depending on the computed expression. The value of 0 indicates the success of the expression, and the value of 1 indicates the failure of the expression.

String expressions

Test provides the following expressions in order to evaluate strings:

echo "Hello, what is your name?"
read NAME

if [ -z "$NAME" ]; then
    echo "The name is empty!"
    exit 1
fi

if [ -n "$NAME" ]; then
    echo "Hello! ${NAME}"
    if [ "$NAME" = "Tom" ]; then
        echo "Oh it is you Tom!"
    else
        echo "I didn't expect you!"
    fi
fi

Above we have a simple example that uses some of the string expressions in the table. We’re evaluating the variable NAME. We first check whether the string exists. If not, we echo the error message and terminate the script. By echoing $? we can see the exit status which is 1.

Otherwise, if we have the name, then we’ll issue a greeting, and we’ll have another check in which we compare our name to Tom using an if/else condition.

Integer expressions

Expressions that are used with integers are as follows:

echo "Please enter your number"
read NUM

if [ -z "$NUM" ]; then
    echo "The number is empty!."
    exit 1
fi

if [ $NUM -gt 0 ]; then
    echo "The num ${NUM} is greater than zero"
else
    echo "The num ${NUM} is less than zero or zero"
fi

if [ $((NUM % 2)) -eq 0 ]; then
    echo "The num ${NUM} is even."
else 
    echo "The num ${NUM} is odd."
fi

As with the string expression we test a couple of simple scenarios. Whether the number exists, whether the number is greater than zero and whether the number is even or odd (using the modulo operator like in most other programming languages).

File expressions

Expressions that are used with files are the following:

FILE=/home/mehmed/Projects/shell/hello
if [ -e "$FILE" ]; then
    echo "File $FILE exists."

    if [ -d $FILE ]; then
        echo "File $FILE is a directory".
    fi
    if [ -f $FILE ]; then
        echo "File $FILE is a regular file".
    fi
    if [ -s $FILE ]; then
        echo "$FILE has content".
    fi       
    if [ -w $FILE ]; then
        echo "$FILE is writeable".
    fi
    if [ -r $FILE ]; then
        echo "$FILE is readable".
    fi
    if [ -x $FILE ]; then
        echo "$FILE is executable".
    fi
else
    echo "File $FILE does not exist."
    exit 1
fi

Above we’re checking first for the existence of the file and then, as the script goes on whether the file is a directory, a regular file, whether it has content, or whether it is writeable, readable, and executable. If the file doesn’t exist, we echo the message, and exit with the status code of 1.

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