Nohup: ignoring input and redirecting stderr to stdout

io-redirectionnohupstderrstdin

I am starting my application in the background using nohup as mentioned below –

root@phx5qa01c:/bezook# nohup java -jar ./exhibitor-1.5.1/lib/exhibitor-1.5.1-jar-with-dependencies.jar -c file --fsconfigdir /opt/exhibitor/conf --hostname phx5qa01c.phx.qa.host.com > exhibitor.out &
[1] 30781
root@phx5qa01c:/bezook# nohup: ignoring input and redirecting stderr to stdout

But every time I see this message –

nohup: ignoring input and redirecting stderr to stdout

Will there be any problem if I see this message? What does it mean and how can I avoid it?

Best Answer

To make sure that your application is disassociated from its terminal - so that it will not interfere with foreground commands and will continue to run after you logout - nohup ensures that neither stdin nor stdout nor stderr are a terminal-like device. The documentation describes what actions it takes:

If the standard output is a terminal, all output written by the named utility to its standard output shall be appended to the end of the file nohup.out in the current directory. If nohup.out cannot be created or opened for appending, the output shall be appended to the end of the file nohup.out in the directory specified by the HOME environment variable. If neither file can be created or opened for appending, utility shall not be invoked.

If the standard error is a terminal, all output written by the named utility to its standard error shall be redirected to the same file descriptor as the standard output.

You redirected stdout to a file when you typed > exhibitor.out in your command line. If you're OK with having your application's stderr be directed to the same file as its stdout, you don't need to do anything else. Or you can redirect stderr to a different file by adding an argument such as 2> exhibitor.err. (Thanks to an unknown user - my notifications didn't show a name - for suggesting inclusion of this alternative.)

Update

As mikemaccana notes, the systemd journal is now the standard logging device for most distros. To view the stdout and stderr of a systemd unit use the journalctl command.

sudo journalctl -u [unit]

Original Answer

By default stdout and stderr of a systemd unit are sent to syslog.

If you're using the full systemd, this will be accesible via journalctl. On Fedora, it should be /var/log/messages but syslog will put it where your rules say.

Due to the date of the post, and assuming most people that are exposed to systemd are via fedora, you were probably hit by the bug described here: https://bugzilla.redhat.com/show_bug.cgi?id=754938 It has a good explanation of how it all works too =) (This was a bug in selinux-policy that caused error messages to not be logged, and was fixed in selinux-policy-3.10.0-58.fc16)

Shell – Can redirecting stdout and stderr to the same file mangle lines

Yes, exactly that can happen if lines to stdout are long enough.

#!/usr/bin/perl
use strict;
use warnings;

for (1..10) {
    print "START"; print "-" x 100000; print "END\n";
    warn "!\n";
}

Running:

./writer.pl > out 2>&1

Checking: Open file out in an editor and find stderr ! in between -, not always between END and START

This will vary between OS:s, languages and systems in various ways, but your basic assumption is correct. Experiment by varying length of stdout-lines: print "-" x 100, print "-" x 10000 etc.

Best Answer

Related Solutions

Systemd – How to View stdout/stderr of a Service

Update

Shell – Can redirecting stdout and stderr to the same file mangle lines

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