demo.sh

#!/bin/bash
#
# Usage:
#   ./demo.sh <function name>

set -o nounset
set -o pipefail
set -o errexit

#
# Improving the Style of Shell from Nix Package Manager
#

# from https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/setup.sh#L318

# Return success if the specified file is an ELF object.
isELF() {
    local fn="$1"
    local fd
    local magic
    exec {fd}< "$fn"
    read -r -n 4 -u "$fd" magic
    exec {fd}<&-
    if [[ "$magic" =~ ELF ]]; then return 0; else return 1; fi
}

# My simpler rewrite.
isElfSimple() {
  local path=$1  # double quotes never necessary on RHS
  local magic

  # read 4 bytes from $path, without escaping, into $magic var
  read -r -n 4 magic < "$path"

  # Return the exit code of [[
  [[ "$magic" =~ ELF ]]
}

isElfSimpleWithStdin() {
  seq 3 > /tmp/3.txt
  while read line; do
    echo $line
    isElfSimple /bin/true && echo YES
    isElfSimple $0 || echo NO
    echo
  done < /tmp/3.txt
}

compareIsElf() {
  local path=$1
  isELF "$path" && echo "YES isELF $path" || echo "NO isELF $path"
  isElfSimple "$path" && echo "YES isElfSimple $path" || echo "NO isElfSimple $path"
}

testIsElf() {
  for path in /bin/sh /bin/true $0; do
    compareIsElf "$path"
    echo
  done
}

#
# Improving the Style of Shell from Apache Yetus
#

doWork() {
  echo 'FOO'
  echo 'BAR'
  echo
}

doWorkAndLog() {
  # https://github.com/apache/yetus/blob/10d4d13cc95a814eac97a976a8de525531ac986a/precommit/core.d/builtin-bugsystem.sh#L50
  if [[ -n "${CONSOLE_REPORT_FILE}" ]]; then
    echo "--- Logging to ${CONSOLE_REPORT_FILE}"
    exec 6>&1 1>"${CONSOLE_REPORT_FILE}"
  fi

  doWork

  if [[ -n "${CONSOLE_REPORT_FILE}" ]]; then
    exec 1>&6 6>&-
    echo "--- Contents of ${CONSOLE_REPORT_FILE}:"
    cat "${CONSOLE_REPORT_FILE}"
  fi
}

doWorkAndLogSimple() {
  if [[ -n "${CONSOLE_REPORT_FILE}" ]]; then
    echo "--- Logging to ${CONSOLE_REPORT_FILE}"
    doWork > ${CONSOLE_REPORT_FILE}
    echo "--- Contents of ${CONSOLE_REPORT_FILE}:"
    cat "${CONSOLE_REPORT_FILE}"
  else
    doWork
  fi
}

testDoWorkAndLog() {
  set +o nounset

  doWorkAndLog
  CONSOLE_REPORT_FILE=/tmp/$0-$$.log doWorkAndLog

  echo --- SIMPLE VERSION ---

  doWorkAndLogSimple
  CONSOLE_REPORT_FILE=/tmp/$0-$$.log doWorkAndLogSimple
}

"$@"