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

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).