Passing Input to a Bash Function via Arguments or stdin

I have a collection of Bash functions that I often use. For some of them, I needed to ability to read input from all of the following:

• positional arguments
• stdin (standard input)
• here-strings

This was not as straight-forward as I would have hoped. After much trial and error, though, here’s what I came to. We’ll use the length function as an example:

Bash Function #

length() {
  local input=""

  if [[ -p /dev/stdin ]]; then
    input="$(cat -)"
  else
    input="${@}"
  fi

  if [[ -z "${input}" ]]; then
    return 1
  fi

  echo "${#input}"
}

Usage #

It can be used in the following ways:

$ length "hey"
3

$ echo "hey" | length
3

$ length <<< echo "hey"
3

How it Works #

First, check if stdin was given:

[[ -p /dev/stdin ]]

If it was, then read from it:

cat -

If not, then read in all positional arguments:

${@}

All of this is assigned to the local variable input. Next, check if input is empty:

[[ -z "${input}" ]]

If it is, then return a non-zero exit code:

return 1

Finally, if input has a value, then get its length:

echo "${#input}"