How does "cat << EOF" work in bash?

How does "cat << EOF" work in bash?

Have you ever come across the cat << EOF syntax in a bash script and wondered what it does? If so, you're not alone! This syntax, known as a "here document," is a powerful way to input multi-line strings or commands into a program. In this blog post, we'll dive into the details of how it works and provide easy solutions to common issues you may encounter. So, let's get started! 🚀

What does "cat << EOF" do?

The cat << EOF syntax is used to create a here document in bash. A here document is a way to redirect input into a command or program. It allows you to specify a block of text or commands directly in your script without the need for external files.

In the context of your example, cat << EOF is being used to construct a multi-line string and pass it as input to the psql program. The EOF is a delimiter that marks the end of the here document. Everything between cat << EOF and EOF is treated as input and is passed to the psql program.

How does it work?

When the cat << EOF command is encountered in a bash script, it starts reading and storing any subsequent lines until it encounters the EOF delimiter. It then outputs the concatenated lines as if they were read from a file. In the case of your example, the output of cat << EOF is piped to the psql program.

Think of it as a way to write multi-line strings or commands inline, right within your script!

Common Issues and Solutions

Issue 1: Syntax errors within the here document

One common issue you may encounter is syntax errors within the here document. For example, if you have quotes or special characters within the here document, they may interfere with the interpretation of the text.

Solution:

To avoid these issues, you can use single quotes around the EOF delimiter. This prevents variable substitution and special character interpretation within the here document.

<pre><code>cat &lt;&lt; 'EOF' | psql ---params BEGIN; `pg_dump ----something` update table .... statement ...; END; EOF </code></pre>

Issue 2: Indentation of the here document

Another issue that can occur is indentation problems within the here document. If the here document is indented, the leading whitespace will be included in the output. This can cause unexpected behavior, especially when passing commands to a program.

Solution:

To address this issue, use the - option with the << operator. This removes leading tabs from the here document, allowing for clean and consistent indentation.

<pre><code>cat &lt;&lt;- EOF | psql ---params BEGIN; `pg_dump ----something` update table .... statement ...; END; EOF </code></pre>

Man page for "cat << EOF"

Although cat and EOF have their own individual man pages, the combination of cat << EOF does not have a specific man page. However, you can refer to the man pages for cat and bash for more information on their individual functionalities.

Your Turn!

Now that you understand how the cat << EOF syntax works, it's your turn to put it into action! Try using a here document in your next bash script and see how it simplifies the input of multi-line strings or commands.

If you have any questions or want to share your experience with here documents, leave a comment below. Happy scripting! 😄

*[EOF]: End of File