Posix conformant support for synchronous data I/O in a named pipe.

README.md

Synchronous Data

All data written and read by these functions is formatted as a multi-line string inside the named pipe. Parsing of this data requires understanding of working with multi-line strings inside shell scripts.

In most cases, piping data to sed without the -i option should suffice.

Usage of pipe_w

$ pipe_w <pipe_name> <double_quoted_data_to_write> [write-flag]

CAUTION:

Sanitization (escaping) of data to be written is currently the responsibility of the end user.

Value of write-flag	Meaning
--append	Appends write of incoming data to existing data (default).
--overwrite	Erases all existing data before writing incoming data.

---

Usage of pipe_r

$ pipe_r <pipe_name> <item_number> [read-flag]

NOTE:

pipe_r can read all data in the named pipe when 0 is passed as the item_number.

Value of read-flag	Meaning
--no-delete	All data in pipe persists after read (default).
--item-only	Only requested item persists after read. (item_number cannot be 0)
--delete-item	Deletes the requested item after read; changes index of remaining items, deletes named pipe once empty. (item_number cannot be 0)
--delete-all	All data and named pipe are deleted after read.

pipe_read.sh

#!/bin/sh

# Tested on Alpine 3.10.5+

function pipe_r {
  local pipe=${1:-"test"}
  local item=${2:-0}
  local flag=${3:-"--no-delete"}
  local sync=
  local data=

  # NOTE: Require the pipe to exist so we can read from it.
  
  if [ -p $pipe ] && [[ ! -z $pipe ]]
  then
    # Extract the contents of the pipe.
    sync="$(cat < ${pipe})"

    if [ $item -eq 0 ]
    then
      # Place all `sync` data into requested `data`.
      data="${sync}"

    elif [ $item -gt 0 ]
    then
      # Place requested item from `sync` into `data`.
      data="$(echo "${sync}" | sed "${item}q;d")"

    fi

    if [ "${flag}" == "--no-delete" ]
    then
      # Pass all previous `sync` data back into pipe.
      ( echo "${sync}" > $pipe & ) > /dev/null 2>&1

    elif [ "${flag}" == "--item-only" ] && [ $item -gt 0 ]
    then
      # Pass only requested `data` (item) back into pipe.
      ( echo "${data}" > $pipe & ) > /dev/null 2>&1

    elif [ "${flag}" == "--delete-item" ] && [ $item -gt 0 ]
    then
      # Parse the remaining items, if any.
      local remaining="$(echo "${sync}" | sed "${item}d")"

      if [[ -z $remaining ]]
      then
        # Delete the now empty pipe, silently.
        ( rm -f $pipe ) > /dev/null 2>&1

      else
        # Pass mutated data back into pipe with `item` removed.
        ( echo "${remaining}" > $pipe & ) > /dev/null 2>&1

      fi

    elif [ "${flag}" == "--delete-all" ]
    then
      # Delete the pipe, silently.
      ( rm -f $pipe ) > /dev/null 2>&1

    fi

    # Send the requested `data` to stdout.
    echo "${data}"
  fi
}

pipe_write.sh

#!/bin/sh

# Tested on Alpine 3.10.5+

function pipe_w {
  local pipe=${1:-"test"}
  local data=${2:-"the quick brown fox jumps over the lazy dog"}
  local flag=${3:-"--append"}
  local first_run=0
  local sync=

  if [ ! -p $pipe ]
  then
    # Create the pipe if it doesn't exist.
    mkfifo $pipe
    
    # Set `first_run` for proper data handling below.
    first_run=1
  fi

  if [ $first_run -eq 0 ] && [ "${flag}" == "--overwrite" ]
  then
    # Empty the pipe completely, and silently.
    ( ( echo ' ' >> $pipe & ) && echo "$(cat < ${pipe})" ) > /dev/null 2>&1
    
    # Set `first_run` for proper data handling below.
    first_run=1
  fi

  if [ $first_run -eq 0 ] && [[ -z $sync ]]
  then
    # If we are appending data, front-load contents of pipe.
    sync="$(cat < ${pipe})"
  fi

  for item in $data
  do
    if [ $first_run -eq 1 ]
    then
      # Pipe is empty, place first item into `sync`.
      sync="${item}"

      # Unset `first_run` to indicate start of data collection.
      first_run=0
    else
      # Data has collected at least one thing, append item to `sync`.
      sync="$(printf "%s\n" "${sync}" "${item}")"
    fi
  done

  # Silently place contents of `sync` into pipe. 
  ( echo "${sync}" > $pipe & )
}