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

If you're careful, you can use 'eval' and your first idea.

[ ...some condition here... ] && input=$fileName || input="&1"
eval ./myScript < $input

However, you say that 'myScript' is actually a complex command invocation; if it involves arguments which might contain spaces, then you must be very careful before deciding to use 'eval'.

Frankly, worrying about the cost of a 'cat' command is probably not worth the trouble; it is unlikely to be the bottleneck.

Even better is to design myScript so that it works like a regular Unix filter - it reads from standard input unless it is given one or more files to work (like, say, cat or grep as examples). That design is based on long and sound experience - and is therefore worth emulating to avoid having to deal with problems such as this.