Automatically trace output file of a background job

trc.bash

# The following Bash function will automatically pick up the stdout (and/or stderr,
# depending on command line options) of another Bash background job and tail it
# until the given job terminates.
#
# example usage:
# $ ls -R / &> ls.out & trc
# will run recursive "ls" in the background and redirect its output to a file
# then start "tail --pid <background job pid> -F <backgroun jobs' stdout file>"
#
# The default is to tail stdout (file descriptor "1") of the given job
# Default job spec is "%" ("current" job)
#
# Dependency:
# gnu tail available as "gtail", change to "tail" if required
# "lsof" - to fetch the file descriptors

if [ -n "$(which gtail)" ]
then
  TAIL=gtail
else
  TAIL=tail
fi

trc() {
  trc_usage() {
    echo "trc: [-e -o] [jobspec]
     -e: tail stderr
     -o: tail stdout (default if no flags)" >&2
  }

  local OPTIND f # argument processing vars
  local ERR OUT JOBSPEC # options
  ERR=0
  OUT=0
  while getopts "eoj:" f; do
    case "$f" in
      e)
        ERR=1
        ;;
      o)
        OUT=1
        ;;
      *)
        trc_usage
        return
        ;;
    esac
  done
  shift $((OPTIND-1))

  case "$#" in
    0)
      JOBSPEC=%
      ;;
    1)
      JOBSPEC="$1"
      ;;
    *)
      echo trc: Too many arguments >&2
      trc_usage
      return
      ;;
  esac

  if [[ $ERR == 0 ]]
  then
    OUT=1
  fi
  if [[ $OUT == 1 ]]
  then
    DESCS=1
  fi
  if [[ $ERR == 1 ]]
  then
    DESCS="$DESCS,2"
  fi

  FILENAMES="$(jobs -x lsof +w -d "$DESCS" -a -p "$JOBSPEC" -Fn0 2>/dev/null | \
    sed -n -e '/^n\//s/^n//p' | sort -u)"
  if [[ -z "$FILENAMES" ]]
  then
    echo trc: No file names found >&2
    trc_usage
    return
  fi

  jobs -x $TAIL --pid "$JOBSPEC" -F $FILENAMES
}