log.sh

#=================================
# ***Logging command output as it is without altering it. (Keep Leading spaces and other such formatting)***

# I want this output:
# $ free -m
#               total        used        free      shared  buff/cache   available
# Mem:          16265       11421        4619          17         223        4713
# Swap:          8196         775        7420

# Append to a date stamp:
# 20200916-09:54:21-0400:               total        used        free      shared  buff/cache   available
# 20200916-09:54:21-0400: Mem:          16265       11421        4619          17         223        4713
# 20200916-09:54:21-0400: Swap:          8196         775        7420

# Log to STDOUT
log() {
  while IFS= read -r line; do
    printf '%s\n' "$(date "+%Y%m%d-%H:%M:%S.%N%z"): $line"
  done
}

# or just output to a file defined in "LOGFILE".
log2file() {
  [[ "$LOGFILE" == "" ]] && {
    LOGFILE=$1
  }
  while IFS= read -r line; do
    printf '%s\n' "$(date "+%Y%m%d-%H:%M:%S.%N%z"): $line" >>"$LOGFILE"
  done
}

# or STDOUT and a file defined in "LOGFILE".
logand2file() {
  [[ "$LOGFILE" == "" ]] && {
    LOGFILE=$1
  }
  while IFS= read -r line; do
    printf '%s\n' "$(date "+%Y%m%d-%H:%M:%S.%N%z"): $line" | tee -a "$LOGFILE"
  done
}

# log Example.
# The '|&' logs ERRORS and STDOUT. Also handles leading spaces.
free -m |& log
# See ERRORS here.
cd ./not_there |& log

# log2file Example
free -m |& log2file /tmp/CASE_NUMBER.log

# logand2file Example
free -m |& logand2file /tmp/CASE_NUMBER.log

# or just output to a file defined in "LOGFILE".
LOGFILE=/tmp/CASE_NUMBER-or-whatever.log
log2file() {
  [[ "$LOGFILE" == "" ]] && {
    LOGFILE=$1
  }
  while IFS= read -r line; do
    printf '%s\n' "$(date "+%Y%m%d-%H:%M:%S.%N%z"): $line" >>"$LOGFILE"
  done
}

# or STDOUT and a file defined in "LOGFILE".
LOGFILE=/tmp/CASE_NUMBER-or-whatever.log
logand2file() {
  [[ "$LOGFILE" == "" ]] && {
    LOGFILE=$1
  }
  while IFS= read -r line; do
    printf '%s\n' "$(date "+%Y%m%d-%H:%M:%S.%N%z"): $line" | tee -a "$LOGFILE"
  done
}

# log2file Example
free -m |& log2file

# logand2file Example
free -m |& logand2file

# Just make a BASH wrapper and call your script like I called "free -m" with "free -m |& log".