README.md

Adding filter capabilities to any Bash function

If you have a bash function that can transform lines taken as arguments, the following trick can be used to give that function line-filter or file-filter capabilities, depending on how the function treats newlines.

mylinefunc() {
  __line_filter "$@" && return $?

  # transform a single line of input
  echo "${1-#}" "$*"
}

__line_filter() {
  [[ -n "$1" ]] && return 1
  while IFS= read -ra args; do
    "${FUNCNAME[1]}" "${args[@]}"
  done
}

Explanation:

• All arguments are passed to __line_filter first. __line_filter checks to see if arguments exist (if the length of $1 is not zero).

• If the argument list is not empty, the __line_filter function returns so that the original function can process the arguments as usual.

• If the argument list is empty, the __line_filter function takes input from stdin, and calls the original function, passing each line from stdin as a line of arguments.

  FUNCNAME is somewhat of a call stack, so FUNCNAME[1] would evaluate to the name of the previous function in the stack

If the function can handle line newlines, use __file_filter instead of __line_filter.

__file_filter() {
  [[ -n "$1" ]] && return 1
  "${FUNCNAME[1]}" "$(</dev/stdin)"
}

• Adding these filter mechanisms to an existing command is like using xargs on that command

Related:

• 20220112092321 CLI IO Schemes/Methods
• 20220112093437 Unix Filter IO Scheme

References:

• What is a UNIX/Linux Filter? - rwxrob: https://github.com/rwxrob/zet/blob/e2ab6dbfdc73d54d3aef1634fa659d29d609b1c1/20210809223847/README.md
• man bash
• help read

Tags:

#literature-note #unix #io #bash #scripting #bash-tricks