Redirect standard input dynamically in a bash script

Question

I was trying to do this to decide whether to redirect stdin to a file or not:

[ ...some condition here... ] && input=$fileName || input=\"&0\"
./         


        

Answer 1

Use eval:

#! /bin/bash

[ $# -gt 0 ] && input="'"$1"'" || input="&1"

eval "./myScript <$input"

This simple stand-in for myScript

#! /usr/bin/perl -lp
$_ = reverse

produces the following output:

$ ./myDemux myScript
pl- lrep/nib/rsu/ !#
esrever = _$

$ ./myDemux
foo
oof
bar
rab
baz
zab

Note that it handles spaces in inputs too:

$ ./myDemux foo\ bar
eman eht ni ecaps a htiw elif

To pipe input down to myScript, use process substitution:

$ ./myDemux <(md5sum /etc/issue)
eussi/cte/  01672098e5a1807213d5ba16e00a7ad0

Note that if you try to pipe the output directly, as in

$ md5sum /etc/issue | ./myDemux

it will hang waiting on input from the terminal, whereas ephemient's answer does not have this shortcoming.

A slight change produces the desired behavior:

#! /bin/bash

[ $# -gt 0 ] && input="'"$1"'" || input=/dev/stdin
eval "./myScript <$input"