How to read stdin when no arguments are passed?

Question

Script doesn't work when I want to use standard input when there are no arguments (files) passed. Is there any way how to use stdin instead of a file in this code?

I tried this:

if [ ! -n $1 ] # check if argument exists
   then
   $1=$(</dev/stdin)  # if not use stdin as an argument
   fi

var="$1"
while read line
   do
   ...                # find the longest line
   done <"$var"

Answer 1

For a general case of wanting to read a value from stdin when a parameter is missing, this will work.

$ echo param | script.sh
$ script.sh param

script.sh

#!/bin/bash

set -- "${1:-$(</dev/stdin)}" "${@:2}"

echo $1

Answer 2

Just substitute bash's specially interpreted /dev/stdin as the filename:

VAR=$1
while read blah; do
  ...
done < "${VAR:-/dev/stdin}"

(Note that bash will actually use that special file /dev/stdin if built for an OS that offers it, but since bash 2.04 will work around that file's absence on systems that do not support it.)