Shell scripting is a powerful way to automate tasks and manipulate data in Linux and Unix systems. One common task is to check if a string is empty or null, which means it has no value or content. In this article, you will learn how to use different methods to perform this check in shell script, and what are the advantages and disadvantages of each method.
One of the simplest ways to check if a string is empty or null is to use the -z and -n operators. The -z operator returns true if the string is null or empty, and false otherwise. The -n operator returns true if the string is not null or empty, and false otherwise. For example, you can use these operators in an if statement like this:
str=""
if [ -z "$str" ]; then
echo "The string is empty or null"
else
echo "The string is not empty or null"
fi
The advantage of using these operators is that they are portable and easy to use. The disadvantage is that they may not work properly if the string contains spaces, tabs, or other special characters, unless you quote the string with double quotes.
Another way to check if a string is empty or null is to use the [[ ]] construct, which is a bash extension that provides more features and flexibility than the [ ] construct. The [[ ]] construct also supports the -z and -n operators, but it does not require quoting the string, and it can handle spaces and special characters better. For example, you can use the [[ ]] construct like this:
str=" "
if [[ -z $str ]]; then
echo "The string is empty or null"
else
echo "The string is not empty or null"
fi
The advantage of using the [[ ]] construct is that it is more robust and reliable than the [ ] construct. The disadvantage is that it is not compatible with all shells, and it may not work in some older or non-standard systems.
[[]] is not specific to bash. It originated in the Korn Shell (ksh), and was subsequently implemented in bash and the Z Shell (zsh).
A shorthand option for this would look like: str=" " [[ -z $str ]] && echo "The string is empty or null" || "The string is not empty or null"
A binary and a shell builtin function (using bash) $ which [ /usr/bin/[ $ type [ [ is a shell builtin $ man [ $ man test
This is probably my favorite way; it's clear, simple, and pretty foolproof. These days it's found in just about every implementation of bash.
Another way to check if a string is empty or null is to use the = and != operators, which compare the string with an empty string. The = operator returns true if the string is equal to the empty string, and false otherwise. The != operator returns true if the string is not equal to the empty string, and false otherwise. For example, you can use these operators in an if statement like this:
str="hello"
if [ "$str" = "" ]; then
echo "The string is empty or null"
else
echo "The string is not empty or null"
fi
The advantage of using these operators is that they are also portable and easy to use, and they can work with both the [ ] and the [[ ]] constructs. The disadvantage is that they may not be as clear and intuitive as the -z and -n operators, and they may also require quoting the string.