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"
./myScript < $input

But that doesn't work because when the variable $input is "&0", bash interprets it as a filename.

However, I could just do:

if [ ...condition... ];then
    ./myScript <$fileName
else
    ./myScript

The problem is that ./myScript is actually a long command line that I don't want to duplicate, nor do I want to create a function for it because it's not that long either (it's not worth it).

Then it occurred to me to do this:

[ ...condition... ] && input=$fileName || input=  #empty
cat $input | ./myScript

But that requires to run one more command and a pipe (i.e. a subshell).
Is there another way that's simpler and more efficient?

Answer 1

First of all stdin is file descriptor 0 (zero) rather than 1 (which is stdout).

You can duplicate file descriptors or use filenames conditionally like this:

[[ some_condition ]] && exec 3<"$filename" || exec 3<&0

some_long_command_line <&3

Note that the command shown will execute the second exec if either the condition is false or the first exec fails. If you don't want a potential failure to do that then you should use an if / else:

if [[ some_condition ]]
then
    exec 3<"$filename"
else
    exec 3<&0
fi

but then subsequent redirections from file descriptor 3 will fail if the first redirection failed (after the condition was true).