Interpretations

Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-2001 IEEE Standard Standard for Information Technology -- Portable Operating System Interface (POSIX®)

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Interpretation Request #104
Topic: nohup standard input Relevant Sections: XCU nohup

Users of the popular security suite OpenSSH have been reporting a problem with "nohup" that suggests a correction to the POSIX specification for "nohup".

Here's the problem. The user logs in to a host via ssh, and types the command "nohup make &" at the shell prompt, and then logs out. Unfortunately the ssh session does not terminate: it hangs, useless, until the "make" finishes.

The problem is that "make" still has standard input open. Standard input won't be used, but ssh doesn't know that, and it keeps the connection open so long as any process has access to the input file description.

A simple improvement is to have "nohup" redirect standard input from /dev/null if standard input is a terminal. GNU nohup now does this. Another possible improvement would be to redirect standard input from an invalid file (e.g., "/") so that all reads fail.

Unfortunately, POSIX currently prohibits these improvements. This is leading to practical problems in practice. There is no real utility to having nohup hold on to standard input if it is a terminal, so the POSIX requirement is counterproductive.

The standard should be changed to allow these improvements. At the very least, the problem should be documented in the standard, so that users are warned about it.

Append the following after XCU page 674 line 26132:

If the standard input is a terminal, the file that the standard input of the utility refers to is implementation defined.

Change XCU page 677 line 26215 from: nohup sh -c 'complex-command-line'

to: nohup sh -c 'complex-command-line' ‹/dev/null

Append the following after XCU page 677 line 26126:

Historical versions of nohup did not affect standard input, but that causes problems in the common scenario where the user logs into a system, types the command "nohup make &" at the prompt, and then logs out. If standard input is not affected by nohup, the login session may not terminate for quite some time, since standard input remains open until "make" exits. To avoid this problem the current standard allows implementations to redirect standard input if it is a terminal. Since the behavior is implementation defined, portable programs that may run into the problem should redirect standard input themselves. For example, instead of "nohup make &" an application can invoke "nohup make ‹//dev/null &".

Interpretation Response
The standards states the requirements for nohup and conforming implementations must conform to this. However, concerns have been raised about this which are being referred to the sponsor.

Rationale for Interpretation
None.