Check if no command line arguments and STDIN is empty

Does this fit your requirements ?

#!/bin/sh

if test -n "$1"; then
    echo "Read from $1";
elif test ! -t 0; then
    echo "Read from stdin"
else
    echo "No data provided..."
fi

The major tricks are as follow:

• Detecting that you have an argument is done through the test -n $1 which is checking if a first argument exists.

• Then, checking if stdin is not open on the terminal (because it is piped to a file) is done with test ! -t 0 (check if the file descriptor zero (aka stdin) is not open).

• And, finally, everything else fall in the last case (No data provided...).

I searched far and wide to no avail, and finally managed to put this together through much trial and error. It has worked flawlessly for me in numerous use-cases ever since.

#!/bin/bash
### LayinPipe.sh
## Recreate "${@}" as "${Args[@]}"; appending piped input.
## Offers usable positional parameters regardless of where the input came from.
##
## You could choose to create the array with "${@}" instead following
##  any piped arguments by simply swapping the order
##   of the following two 'if' statements.

# First, check for normal positional parameters.
if [[ ${@} ]]; then
    while read line; do
        Args[${#Args[@]}]="${line}"
    done < <(printf '%s\n' "${@}")
fi

# Then, check for piped input.
if [[ ! -t 0 ]]; then
    while read line; do
        Args[${#Args[@]}]="${line}"
    done < <(cat -)
fi

# Behold the glory.
for ((a=0;a<${#Args[@]};a++)); do
    echo "${a}: ${Args[a]}"
done

• Example: (knowing full well that using output of 'ls' as input is discouraged in order to show the flexibility of this solution.)

$ ls
: TestFile.txt 'Filename with spaces'

$ ls -1 | LayinPipe.sh "$(ls -1)"
> 0: Filename with spaces
> 1: TestFile.txt 
> 2: Filename with spaces
> 3: TestFile.txt 

$ LayinPipe.sh "$(ls -1)"
> 0: Filename with spaces
> 1: TestFile.txt 

$ ls -1 | LayinPipe.sh
> 0: Filename with spaces
> 1: TestFile.txt