Stdin input to a command with nohup

Question

from http://www.gnu.org/software/coreutils/manual/html_node/nohup-invocation.html

nohup runs the given command with hangup signals ignored, so that the command can continue running in the background after you log out.

Synopsis: nohup command [arg]...

If standard input is a terminal, redirect it so that terminal sessions do not mistakenly consider the terminal to be used by the command.

1. Why do we need to do this:

   Make the substitute file descriptor unreadable, so that commands that mistakenly attempt to read from standard input can report an error.

2. Isn't redirection stdin from a file done by nohup command [arg]... 0<myfile? why 0>/dev/null?

   This redirection is a GNU extension; programs intended to be portable to non-GNU hosts can use nohup command [arg]... 0>/dev/null instead.

Answer 1

Imagine you are trying to run a complex script with nohup. You can detect if it tries to read stdin by redirecting stdin to a file descriptor that cannot be read from. Look at these two examples: first 0</dev/null:

rm nohup.out
nohup sh -c 'head -1' 0</dev/null
echo $?
cat nohup.out 

The nohup.out file is empty, and the return code ($?) from the script is 0 ie ok, as the script just read end-of-file. Now try the same script with 0>/dev/null ie 0 opened for output only:

rm nohup.out
nohup sh -c 'head -1' 0>/dev/null
echo $?
cat nohup.out 

This gives the error message in nohup.out of

head: error reading 'standard input': Bad file descriptor

and the exit code is 1, fail. This is presumably more useful to you. You can also get the same effect by closing stdin with 0<&-:

rm nohup.out
nohup sh -c 'head -1' 0<&-
echo $?
cat nohup.out