GitHub Actions Multi-line Output Variable

When building GitHub Actions it’s common to want to output multi-line text from a composite action or step. GitHub Actions provides the $GITHUB_OUTPUT environment file mechanism to pass outputs between steps. The syntax is simple, if you are familiar with a Linux shell: echo "test=hello" >> "$GITHUB_OUTPUT". This will set a variable from that job-step called test with the value hello and can be called from another step using the syntax ${{ steps.<jobid>.outputs.test }}. This deals with single line outputs well, but what about multis-line outputs? That can be more complex.

Bash has the traditional EOF delimiter for dealing with this, but it can be complex, especially when it’s not your machine (i.e. a GitHub hosted runner). Usually the syntax is straightforward: you write the output name followed by <<DELIMITER, then the value, and finally then delimetirer again. For example:

    {
      echo "message<<<EOF"
      echo "Line 1"
      echo "Line 2"
      echo "Line 3"
      echo "EOF"
    } >> "$GITHUB_OUTPUT"

In this case the variable will have the name message.

However, a common gotcha arises when the multi-line content comes from a file or command. GitHub’s parser is very strict, but the community have found this to be not so well documented. The closing delimiter must be on its own line, with no extra spaces, and must follow a newline that isn’t swallowed by the file content. This means that if your file doesn’t end with a newline, or the shell appends the closing delimiter immediately after the last line of content, GitHub will throw the error:

Error: Unable to process file command 'output' successfully.
Error: Invalid value. Matching delimiter not found 'EOF'

WARNING This code will fail:

{
  echo "commits<<EOF"
  cat some-file.txt
  echo "EOF"
} >> "$GITHUB_OUTPUT"

The solution is simple and reliable: ensure a newline between the file content and closing delimeter. Using printf '\n' guarantees this:

{
  echo "commits<<EOF"
  cat commits.txt
  printf '\n'
  echo "EOF"
} >> "$GITHUB_OUTPUT"