How can bash read from piped input or else from the command line argument

Question

I would like to read some data either from pipe or from the command line arguments (say $1), whichever is provided (priority has pipe).

This fragment tells me if the pipe was open or not but I don't know what to put inside in order not to block the script (test.sh) (using read or cat)

if [ -t 0 ]
then
    echo nopipe
    DATA=$1
else
    echo pipe
    # what here?
    # read from pipe into $DATA
fi

echo $DATA

Executing the test.sh script above I should get the following output:

$ echo 1234 | test.sh
1234
$ test.sh 123
123
$ echo 1234 | test.sh 123
1234

Answer 1

You can read all of stdin into a variable with:

data=$(cat)

Note that what you're describing is non-canonical behavior. Good Unix citizens will:

1. Read from a filename if supplied as an argument (regardless of whether stdin is a tty)
2. Read from stdin if no file is supplied

This is what you see in sed, grep, cat, awk, wc and nl to name only a few.

---

Anyways, here's your example with the requested feature demonstrated:

$ cat script 
#!/bin/bash

if [ -t 0 ]
then
    echo nopipe
    data=$1
else
    echo pipe
    data=$(cat)
fi

echo "$data"

$ ./script 1234
nopipe
1234

$ echo 1234 | ./script
pipe
1234