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diff --git a/src/fluent-bit/tests/lib/shunit2/examples/mock_file.sh b/src/fluent-bit/tests/lib/shunit2/examples/mock_file.sh
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+#!/bin/sh
+#
+# shUnit2 example for mocking files.
+#
+# This example demonstrates two different mechanisms for mocking files on the
+# system. The first method is preferred for testing specific aspects of a file,
+# and the second method is preferred when multiple tests need access to the
+# same mock data.
+#
+# When mocking files, the key thing of importance is providing the code under
+# test with the correct file to read. The best practice for writing code where
+# files need to be mocked is either:
+# - Pass the filename to be tested into a function and test that function, or
+# - Provide a function that returns the name of the filename to be read.
+#
+# The first case is preferred whenever possible as it allows the unit test to
+# be explicit about what is being tested. The second case is useful when the
+# first case is not achievable.
+#
+# For the second case, there are two common methods to mock the filename
+# returned by the function:
+# - Provide a special value (e.g. a mock variable) that is only available
+#   during testing, or
+# - Override something (e.g. the constant) in the test script.
+#
+# The first case is preferred as it doesn't require the unit test to alter code
+# in any way. Yes, it means that the code itself knows that it is under test,
+# and it behaves slightly differently than under normal conditions, but a
+# visual inspection of the code by the developer should be sufficient to
+# validate proper functionality of such a simple function.
+
+# Treat unset variables as an error.
+set -u
+
+PASSWD='/etc/passwd'
+
+# Read the root UID from the passwd filename provided as the first argument.
+root_uid_from_passed_filename() {
+  filename=$1
+  root_uid "${filename}"
+  unset filename
+}
+
+
+# Read the root UID from the passwd filename derived by call to the
+# passwd_filename() function.
+root_uid_from_derived_filename() {
+  root_uid "$(passwd_filename)"
+}
+
+passwd_filename() {
+  if [ -n "${MOCK_PASSWD:-}" ]; then
+    echo "${MOCK_PASSWD}"  # Mock file for testing.
+    return
+  fi
+  echo "${PASSWD}"
+}
+
+
+# Extract the root UID.
+root_uid() { awk -F: 'u==$1{print $3}' u=root "$1"; }
+
+
+main() {
+  echo "root_uid_from_passed_filename:"
+  root_uid_from_passed_filename "${PASSWD}"
+
+  echo
+
+  echo "root_uid_from_derived_filename:"
+  root_uid_from_derived_filename
+}
+
+
+# Execute main() if this is run in standalone mode (i.e. not in a unit test).
+ARGV0="$(basename "$0")"
+argv0="$(echo "${ARGV0}" |sed 's/_test$//;s/_test\.sh$//')"
+if [ "${ARGV0}" = "${argv0}" ]; then
+  main "$@"
+fi