eval_git_sha.sh
#!/bin/bash

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GIT_SHA_FILENAME=tools_git_sha

unset GIT_SHA

# Check if the current directory is inside a Git repository.
if git rev-parse --is-inside-work-tree > /dev/null 2>&1; then
  # If inside a Git repository, retrieve commit hash.
  GIT_SHA=$(git rev-parse --short HEAD)
else
  # If not in a Git repository, check if GIT_SHA_FILENAME exists.
  if [ -f "$GIT_SHA_FILENAME" ]; then
    # If the file exists, assume it contains the commit hash.
    GIT_SHA=$(<"$GIT_SHA_FILENAME")
  else
    GIT_SHA="N/A"
  fi
fi

if [ -z "$GIT_SHA" ]; then
  echo "an unexpected error occurred"
  exit 1
fi

# Calling code expects evaluated commit hash in STDOUT. In addition we
# refresh the content of GIT_SHA_FILENAME to ensure it is up to date.
echo $GIT_SHA | tee "$GIT_SHA_FILENAME"
