The test command in Linux

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The test command in Linux evaluates the conditional expression and checks the file types. It is commonly used in shell scripts to perform checks like comparing numbers or strings, and verifying file attributes such as existence, type, and permissions.

Table of Contents

Here is a comprehensive guide to the options available with the test command in Linux −

• Syntax of test Command
• Options of test Command
• Expressions of test Command
• Using test Command in Linux

Syntax of test Command

The syntax of the test command in Linux is as follows −

test [options] [expression]

In the above syntax, the [options] argument is optional used to specify options to view the command's help and version. The [expression] argument is used to specify the expression that needs to be evaluated.

Options of test Command

The options of the test command are listed below −

Expressions of test Command

The following expressions are used with the test command −

Using test Command in Linux

This section discusses how to use the test command in Linux with examples −

Testing Files using the test Command

To check whether the file exists or not, use the -e option followed by the filename −

test -e file.txt

The test command does not display output, but it returns a 0 exit code if the condition is true, and 1 if false. For instance, in the above example, if the file exists, the exit code will be 0, else 1. To check the exit code, run the following command −

echo $?

Since the file exists in the current working directory, the exit code is 0, as shown in the above output image.

To check if the specified argument is a directory or not, use the -d option −

test -d /etc

To check if a file contains content or not, use the -s option −

test -s file.txt

To check if the file is writable or not, use the -w option −

test -w file.txt

Comparing Strings using the test Command

To check if two strings are equal or contain the same content, use the test command in the following way −

test "$str1" = "$str2"

In this case, the exit code will be 1, because the two strings do not match, as shown in the following output image −

Checking if a string is empty or not −

test -z "$str1"

Comparing Integers using the test Command

To compare if a number is less than the second number, use the test command in the following way −

test "$num1" -lt "$num2"

Since 3 is less than 5, the exit code is 0 as shown in the above image.

To check if a number is equal to another number, use the following command −

test "$num1" -eq "$num2"

Using Logical Operators with the test Command

Integers, strings, and files can also be compared using the logical operators with the test command. For example, to check if the file exists and is readable, use the following command −

test -f file.txt && test -r file.txt

The square brackets are aliases of the test command, so the above command can also be executed in the following way −

[ -f file.txt ] && [ -r file.txt ]

The exit code of the above command will be 0 if the file.txt exists in the current directory and is readable, otherwise 1.

Similarly, the exit status of the following command will be 0 if any of the specified conditions are true −

[ "$num1" -gt 0 ] || [ "$num2" -lt 5 ]

Displaying Usage Help

If the --help option is used with the test command, then --help is treated as a string. To avoid it, use the /usr/bin/[ followed by --help option to display the usage help −

/usr/bin/[ --help

Conclusion

The test command in Linux is used to evaluate expressions and check file types, attributes, strings, and integers. It is a fundamental tool in shell scripting for making decisions based on conditions.

The test command returns an exit code instead of producing output, where 0 indicates success (true) and 1 indicates failure (false). By using various options and logical operators, different file properties, string comparisons, and numeric conditions can be tested efficiently.