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Importantly, auto_log reports an error and terminates script execution if no tag string is specified, via exit 255 (we'll see much more on error handling shortly).

The test is made using a bash if/else/fi block, where double brackets ('"[[ ]]") enclose the test, always followed by a semicolon (";"). Here the test is for string equality (see https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html for a complete list of bash comparison operators). Note that there must always be a space after the open brackets, and one before the close brackets.

# Sets up auto-logging to a log file in the current directory
# using the specified logFileTag (arg 1) in the log file name.
auto_log() {
  local logFileTag="$1"
  if [[ "$logFileTag" != "" ]]; then
    local logFilePath="./autoLog_${logFileTag}.log"
    echo_se ".. logging to $logFilePath"
    exec 1> >(tee "$logFilePath") 2>&1
  else
    echo_se "** ERROR in auto_log: no logFile argument provided"
    exit 255
  fi
}

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• Starts automatic logging to a log file named using its 1st logFileTag argument, by calling the auto_log function.
• Uses echo -e where the -e argument to echo enables interpretation of backslash escapes.
  • e.g. "\nt" will be interpreted as a newline, tab character and "\tn" will be interpreted as a tab characternewline
• Calls stdStreams with its 2nd and 3rd arguments.
• Calls echo capturing its output in a local variable using backtick execution syntax, then displays the captured text.
  • see Quoting in the shell
• Calls the echo_se function with some text.
• Calls the echo_se function again, capturing its output in a local variable, then displays the captured text.
  
  

# function that illustrates auto-logging and capturing function output
#  arg 1 - (required) tag to identify the logfile
#  arg 2 - (optional) text for standard output
#  arg 3 - (optional) text for standard error
function testAutolog() {
  local logFileTag="$1"
  local outTxt=${2:-"text for standard output"}
  local errTxt=${3:-"text for standard error"}

  auto_log "$logFileTag"

  echo -e "\n1) Call stdStreams with output and error text:"
  stdStreams "$outTxt" "$errTxt"

  echo -e "\n2) Capture echo output in a variable and display it:"
  local output=`echo $outTxt`
  echo -e "   echo output was:\n$output"

  echo -e "\n3) Call echo_se with error text:"
  echo_se "$errTxt"

  echo -e "\n4)Capture echo_se function output in a variable and display it:"
  output=`echo_se "$errTxt"`
  echo -e "echo_se output was: '$output'"
}

exercise 4

Call the testAutoLog testAutolog command with no further command line arguments. What happens, and why?

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What log file is produced, what ? What are its contents, and why?

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