How to make a bash function which can read from standard input?

Question

I have some scripts that work with parameters, they work just fine but i would like them to be able to read from stdin, from a pipe for example, an example, suppose this is called read:

#!/bin/bash function read() {  echo $* }  read $* 

Now this works with read "foo" "bar", but I would like to use it as:

echo "foo" | read 

How do I accomplish this?

Answer 1

It's a little tricky to write a function which can read standard input, but works properly when no standard input is given. If you simply try to read from standard input, it will block until it receives any, much like if you simply type cat at the prompt.

In bash 4, you can work around this by using the -t option to read with an argument of 0. It succeeds if there is any input available, but does not consume any of it; otherwise, it fails.

Here's a simple function that works like cat if it has anything from standard input, and echo otherwise.

catecho () {     if read -t 0; then         cat     else         echo "$*"     fi }  $ catecho command line arguments command line arguments $ echo "foo bar" | catecho foo bar 

This makes standard input take precedence over command-line arguments, i.e., echo foo | catecho bar would output foo. To make arguments take precedence over standard input (echo foo | catecho bar outputs bar), you can use the simpler function

catecho () {     if [ $# -eq 0 ]; then         cat     else         echo "$*"     fi } 

(which also has the advantage of working with any POSIX-compatible shell, not just certain versions of bash).

Answer 2

You can use <<< to get this behaviour. read <<< echo "text" should make it.

Test with readly (I prefer not using reserved words):

function readly() {  echo $*  echo "this was a test" }  $ readly <<< echo "hello" hello this was a test 

With pipes, based on this answer to "Bash script, read values from stdin pipe":

$ echo "hello bye" | { read a; echo $a;  echo "this was a test"; } hello bye this was a test