Redirect stdout and stderr to Function

It's an old thread.. but I have used it to help me write a log function that will output also multiple lines of a command output:

# Defines function to grab a time stamp #
get_Time () { Time=$(date +%Y-%m-%d\ %H:%M:%S) ; }

write_Log()
{
get_Time
if [ -n "${1}" ]; then         # If it's from a "<message>" then set it
    IN="${1}"
    echo "${Time} ${IN}" | tee -a ${log_File}
else
    while read IN               # If it is output from command then loop it
    do
        echo "${Time} ${IN}" | tee -a ${log_File}
    done
fi
}

Based on the previous answers, I put together some generic functions that work with or without a log file, as listed at the end of this post. These are handy for more complex scripts. I generally print terminal window messages to stderr so as to not interfere with legitimate program output that may need to be redirected. The functions can be called as follows:

scriptFolder=$(cd $(dirname "$0") && pwd)
scriptName=$(basename $scriptFolder)
# Start a log file that will be used by the logging functions
logFileStart ${scriptName} "${scriptFolder)/${scriptName}.log"

# The following logs the message string passed to the function.
# - use a space for empty lines because otherwise the logging function
#   will hang waiting for input
logInfo " "
logInfo "Starting to do some work."

# The following will log each 'stdout` and `stderr` line piped to the function.
someOtherProgram 2>&1 | logInfo

Functions...

# Echo to stderr
echoStderr() {
  # - if necessary, quote the string to be printed
  # - redirect stdout from echo to stderr
  echo "$@" 1>&2
  # Or, use an alternate echo such one that colors textT
  # ${echo2} "$@" 1>&2
}

# Print a DEBUG message
# - prints to stderr and optionally appends to log file if ${logFile} is defined globally
#   - see logFileStart() to start a log file
# - call with parameters or pipe stdout and stderr to this function: 2>&1 | logDebug
# - print empty lines with a space " " to avoid hanging the program waiting on stdin input
logDebug() {
  if [ -n "${1}" ]; then
    if [ -n "${logFile}" ]; then
      # Are using a log file
      echoStderr "[DEBUG] $@" 2>&1 | tee --append $logFile
    else
      # Are NOT using a log file
      echoStderr "[DEBUG] $@"
    fi
  else
    while read inputLine; do
      if [ -n "${logFile}" ]; then
        # Are using a log file
        echoStderr "[DEBUG] ${inputLine}" 2>&1 | tee --append $logFile
      else
        # Are NOT using a log file
        echoStderr "[DEBUG] ${inputLine}"
      fi
    done
  fi
}

# Print an ERROR message
# - prints to stderr and optionally appends to log file if ${logFile} is defined globally
#   - see logFileStart() to start a log file
# - call with parameters or pipe stdout and stderr to this function: 2>&1 | logError
# - print empty lines with a space " " to avoid hanging the program waiting on stdin input
logError() {
  if [ -n "${1}" ]; then
    if [ -n "${logFile}" ]; then
      # Are using a log file
      echoStderr "[ERROR] $@" 2>&1 | tee --append $logFile
    else
      # Are NOT using a log file
      echoStderr "[ERROR] $@"
    fi
  else
    while read inputLine; do
      if [ -n "${logFile}" ]; then
        # Are using a log file
        echoStderr "[ERROR] ${inputLine}" 2>&1 | tee --append $logFile
      else
        # Are NOT using a log file
        echoStderr "[ERROR] ${inputLine}"
      fi
    done
  fi
}

# Start a new logfile
# - name of program that is being run is the first argument
# - path to the logfile is the second argument
# - echo a line to the log file to (re)start
# - subsequent writes to the file using log*() functions will append
# - the global variable ${logFile} will be set for use by log*() functions
logFileStart() {
  local newLogFile now programBeingLogged
  programBeingLogged=$1
  # Set the global logfile, in case it was not saved
  if [ -n "${2}" ]; then
    logFile=${2}
  else
    # Set the logFile to stderr if not specified, so it is handled somehow
    logFile=/dev/stderr
  fi
  now=$(date '+%Y-%m-%d %H:%M:%S')
  # Can't use logInfo because it only appends and want to restart the file
  echo "Log file for ${programBeingLogged} started at ${now}" > ${logFile}
}

# Print an INFO message
# - prints to stderr and optionally appends to log file if ${logFile} is defined globally
#   - see logFileStart() to start a log file
# - call with parameters or pipe stdout and stderr to this function: 2>&1 | logInfo
# - print empty lines with a space " " to avoid hanging the program waiting on stdin input
logInfo() {
  if [ -n "${1}" ]; then
    if [ -n "${logFile}" ]; then
      # Are using a log file
      echoStderr "[INFO] $@" 2>&1 | tee --append $logFile
    else
      # Are NOT using a log file
      echoStderr "[INFO] $@"
    fi
  else
    while read inputLine; do
      if [ -n "${logFile}" ]; then
        # Are using a log file
        echoStderr "[INFO] ${inputLine}" 2>&1 | tee --append $logFile
      else
        # Are NOT using a log file
        echoStderr "[INFO] ${inputLine}"
      fi
    done
  fi
}

# Print an WARNING message
# - prints to stderr and optionally appends to log file if ${logFile} is defined globally
#   - see logFileStart() to start a log file
# - call with parameters or pipe stdout and stderr to this function: 2>&1 | logWarning
# - print empty lines with a space " " to avoid hanging the program waiting on stdin input
logWarning() {
  if [ -n "${1}" ]; then
    if [ -n "${logFile}" ]; then
      # Are using a log file
      echoStderr "[WARNING] $@" 2>&1 | tee --append $logFile
    else
      # Are NOT using a log file
      echoStderr "[WARNING] $@"
    fi
  else
    while read inputLine; do
      if [ -n "${logFile}" ]; then
        # Are using a log file
        echoStderr "[WARNING] ${inputLine}" 2>&1 | tee --append $logFile
      else
        # Are NOT using a log file
        echoStderr "[WARNING] ${inputLine}"
      fi
    done
  fi
}

Thanks to people who posted their responses. I came up with my version which will add timestamp once per message.

#!/bin/bash
CURRENT_PID=$$
PROCESS_NAME=$(basename $0)

LOGFILE=/var/log/backup-monitor.log
function log_message {
  if [ -n "$1" ]; then
      MESSAGE="$1"
      echo -e "$(date -Iseconds)\t$PROCESS_NAME\t$CURRENT_PID\t$MESSAGE" | tee -a $LOGFILE
  else
      MESSAGE=$(tee)
      echo -e "$(date -Iseconds)\t$PROCESS_NAME\t$CURRENT_PID\t$MESSAGE" | tee -a $LOGFILE
  fi
}

log_message "Direct arguments are working!!"

echo "stdin also working" | log_message

You can create a function that optionally writes the STDOUT and STDERR from system commands to a log file or accepts arguments that will be written to a log file by doing the following:

_LOG='/some_dir/some_file'

function Log_Msg {

    #If no arguments are given to function....
    if [ -z "$@" ]; then
        
        #...then take STDOUT/STDERR as input and write to log file
        read && echo "$REPLY" | tee -a $_LOG \

    else
        #Take arguments that were given to function and write that to log file
        echo "$@" | tee -a $_LOG

    fi

}

#Logging from system commands example. The "|&" operator pipes STDOUT and STDERR to Log_Msg function
bad command |& Log_Msg

or

#Taking an argument as input and writing to log file
Log_Msg "Write this to log file"

Hope this helps!