bash heredoc. how to trim spacing

When working with Bash, heredocs (short for "here documents") are a powerful feature that allows you to define multiline strings conveniently. However, managing whitespace, especially leading spaces or tabs, can sometimes be challenging. This guide will walk you through how to trim spacing in Bash heredocs, ensuring your scripts remain clean and readable.

Understanding Heredocs in Bash

A heredoc allows you to create a block of text that can be passed directly to a command. The basic syntax involves the << operator followed by a delimiter which signifies the start and end of the heredoc. For example:

cat << EOF
This is a heredoc.
It can span multiple lines.
EOF

In this example, everything between EOF markers will be included in the output of the cat command.

The Challenge of Whitespace

By default, leading spaces or tabs in heredocs will be preserved, which can lead to unwanted formatting issues in your scripts. To effectively handle this, there are specific techniques:

Using the <<- Syntax

Bash provides an option with the <<- syntax, which allows you to strip leading tabs. This can be particularly useful when you want to indent your heredoc for readability without affecting the actual text.

cat <<- EOF
    This text will be printed without leading tabs.
EOF

Note that only tabs are removed, and spaces will remain. If your heredoc uses spaces for indentation, you will need another method.

Trimming Spaces with sed or Parameter Expansion

To remove leading spaces (not just tabs), you can use tools like sed, or utilize Bash's parameter expansion capabilities. Here’s how to do it with each method:

Using sed:

You can pipe the heredoc output to sed to remove leading spaces:

result=$(cat << EOF | sed 's/^[ \t]*//'
    This is a line with leading spaces.
    So is this one.
EOF
)

echo "$result"

This sed command substitutes (s) leading spaces or tabs (^[ \t]*) with nothing.

Using Parameter Expansion:

Alternatively, Bash’s parameter expansion can be a cleaner approach:

read -r -d '' result << EOF
    This line has leading spaces.
    This one does too.
EOF

result="${result#"${result%%[![:space:]]*}"}"

echo "$result"

In this example, the read command collects the heredoc into a variable, and the parameter expansion trims the leading whitespace.

Summary

When dealing with heredocs in Bash, especially when formatting is important, understanding how to manage leading whitespace effectively is crucial. You can use the <<- syntax for tab removal, or lean on tools such as sed or Bash’s built-in parameter expansion for more comprehensive trimming. Each method serves a unique purpose, allowing for flexibility depending on your specific requirements.

For a deeper dive into Bash heredocs and further examples, check out PhoenixNAP and Tecmint. With these techniques, you'll ensure that your Bash scripts are not only functional but also clean and maintainable.