nohupposix - Online in the Cloud

PROGRAM:

NAME

nohup — invoke a utility immune to hangups

SYNOPSIS

nohup utility [argument...]

DESCRIPTION

The nohup utility shall invoke the utility named by the utility operand with arguments
supplied as the argument operands. At the time the named utility is invoked, the SIGHUP
signal shall be set to be ignored.

If standard input is associated with a terminal, the nohup utility may redirect standard
input from an unspecified file.

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 a file is created, the file's permission bits shall be set to
S_IRUSR | S_IWUSR.

If standard error is a terminal and standard output is open but is not a terminal, all
output written by the named utility to its standard error shall be redirected to the same
open file description as the standard output. If standard error is a terminal and standard
output either is a terminal or is closed, the same output shall instead be appended to the
end of the nohup.out file as described above.

OPTIONS

None.

OPERANDS

The following operands shall be supported:

utility The name of a utility that is to be invoked. If the utility operand names any of
the special built-in utilities in Section 2.14, Special Built-In Utilities, the
results are undefined.

argument Any string to be supplied as an argument when invoking the utility named by the
utility operand.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of nohup:

HOME Determine the pathname of the user's home directory: if the output file
nohup.out cannot be created in the current directory, the nohup utility shall
use the directory named by HOME to create the file.

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.

PATH Determine the search path that is used to locate the utility to be invoked. See
the Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables.

ASYNCHRONOUS EVENTS

The nohup utility shall take the standard action for all signals except that SIGHUP shall
be ignored.

STDOUT

If the standard output is not a terminal, the standard output of nohup shall be the
standard output generated by the execution of the utility specified by the operands.
Otherwise, nothing shall be written to the standard output.

STDERR

If the standard output is a terminal, a message shall be written to the standard error,
indicating the name of the file to which the output is being appended. The name of the
file shall be either nohup.out or $HOME/nohup.out.

OUTPUT FILES

Output written by the named utility is appended to the file nohup.out (or
$HOME/nohup.out), if the conditions hold as described in the DESCRIPTION.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

126 The utility specified by utility was found but could not be invoked.

127 An error occurred in the nohup utility or the utility specified by utility could
not be found.

Otherwise, the exit status of nohup shall be that of the utility specified by the utility
operand.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

The command, env, nice, nohup, time, and xargs utilities have been specified to use exit
code 127 if an error occurs so that applications can distinguish ``failure to find a
utility'' from ``invoked utility exited with an error indication''. The value 127 was
chosen because it is not commonly used for other meanings; most utilities use small values
for ``normal error conditions'' and the values above 128 can be confused with termination
due to receipt of a signal. The value 126 was chosen in a similar manner to indicate that
the utility could be found, but not invoked. Some scripts produce meaningful error
messages differentiating the 126 and 127 cases. The distinction between exit codes 126 and
127 is based on KornShell practice that uses 127 when all attempts to exec the utility
fail with [ENOENT], and uses 126 when any attempt to exec the utility fails for any other
reason.

EXAMPLES

It is frequently desirable to apply nohup to pipelines or lists of commands. This can be
done by placing pipelines and command lists in a single file; this file can then be
invoked as a utility, and the nohup applies to everything in the file.

Alternatively, the following command can be used to apply nohup to a complex command:

nohup sh −c 'complex-command-line' </dev/null

RATIONALE

The 4.3 BSD version ignores SIGTERM and SIGHUP, and if ./nohup.out cannot be used, it
fails instead of trying to use $HOME/nohup.out.

The csh utility has a built-in version of nohup that acts differently from the nohup
defined in this volume of POSIX.1‐2008.

The term utility is used, rather than command, to highlight the fact that shell compound
commands, pipelines, special built-ins, and so on, cannot be used directly. However,
utility includes user application programs and shell scripts, not just the standard
utilities.

Historical versions of the nohup utility use default file creation semantics. Some more
recent versions use the permissions specified here as an added security precaution.

Some historical implementations ignore SIGQUIT in addition to SIGHUP; others ignore
SIGTERM. An early proposal allowed, but did not require, SIGQUIT to be ignored. Several
reviewers objected that nohup should only modify the handling of SIGHUP as required by
this volume of POSIX.1‐2008.

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, POSIX.1‐2008 allows implementations to redirect
standard input if it is a terminal. Since the behavior is implementation-defined, portable
applications 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 &

FUTURE DIRECTIONS

None.

Use nohupposix online using onworks.net services