How to check if stdin is from the terminal or a pipe in a shell script?

Question

I am writing a POSIX shell script that may or may not receive input from stdin as in foo.sh < test.txt, non-interactively.

How do I check whether there is anything on stdin, to avoid halting on while read -r line...?

Answer 1

To answer the question literally (but not what you actually want): read -t 0

Timeout, zero seconds.

1. this is a race-condition, depending on when the left-hand-side is ready to provide data. You can specify -t 5 but on a thrashing system even that is problematic.
2. read -t is not standardised in SUSv3. It is in BSD sh, bash, zsh. It's not in ksh or dash.

So you can't just use #!/bin/sh and expect to have this.

The basic problem is that even if there's nothing on stdin now, that doesn't mean there won't be soon. Invoking a program normally leaves stdin connected to the terminal/whatever, so there's no way to tell what's needed.

So, to answer your question literally, you can do it, but in practice your options are:

1. test if stdin is a tty: [ -t 0 ]
2. use argv to control behaviour

Answer 2

If I get the question right, you may try the following:

#!/bin/sh if [ -t 0 ]; then     echo running interactivelly else     while read -r line ; do         echo $line     done fi