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I can't wrap my head around this. Why would /dev/null be used as input to an if statement? What is the use of < /dev/null in the following?
if ( $PROG --version ) < /dev/null > /dev/null 2>&1; then
$PROG
else
echo "failed"
exit 1
fi
I (think) I understand that > /dev/null 2>&1 is just used to suppress any output from both stdout and stderr.
737
/dev/null is a special filesystem object that discards everything written into it. Redirecting a stream into it means hiding your program's output. The 2>&1 part means "redirect the error stream into the output stream", so when you redirect the output stream, error stream gets redirected as well.
/dev/null in Linux is a null device file. This will discard anything written to it, and will return EOF on reading. This is a command-line hack that acts as a vacuum, that sucks anything thrown to it.
By combining redirection with the /dev/null device, we can silence error output, normal output, or both.
You write to /dev/null every time you use it in a command such as touch file 2> /dev/null. You read from /dev/null every time you empty an existing file using a command such as cat /dev/null > bigfile or just > bigfile. Because of the file's nature, you can't change it in any way; you can only use it.
If $PROG is a program that expects input from stdin, your script might stall forever waiting for you to type something in. The use of < /dev/null is to provide an empty input to such a program. You're right about the > and 2>&1.
This snippet of script is checking to see if $PROG --version exits with a 0 status (success), and then runs that program without any flags. If $PROG --version fails, the script echoes "failed" and exits.
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