I know for bash and zsh, one can use e.g. [ -t 1 ] to determine if STDIN is an interactive tty session.
However, there doesn't seem to be a way to test whether stdin is being redirected from a file, versus being piped in from a command:
foo < ./file
bar | foo
Is there any way to detect the difference between these two? Separately, is there any way to get the path of the file being redirected from (outside of /proc/self, which is unavailable on macOS)?
You can check if /dev/stdin is a regular file or a pipe:
$ cat tmp.sh
#!/bin/bash
if [ -f /dev/stdin ]; then
echo "file"
elif [ -p /dev/stdin ]; then
echo "pipe"
fi
$ bash tmp.sh < foo.txt
file
$ echo foo | bash tmp.sh
pipe
This relies on /dev/stdin being in your file system, though.
You can also use the stat command, which will return information about standard input given no file name argument. As you mentioned you are using macOS, you can use the %HT format:
$ stat -f %HT
Character Device
$ stat -f %HT < foo.txt
Regular File
$ echo foo | stat -f %HT
Fifo File