The lp utility shall copy the input files to an output destination
in an unspecified manner. The default output
destination should be to a hardcopy device, such as a printer or microfilm
recorder, that produces non-volatile, human-readable
documents. If such a device is not available to the application, or
if the system provides no such device, the lp utility
shall exit with a non-zero exit status.
The actual writing to the output device may occur some time after
the lp utility successfully exits. During the portion
of the writing that corresponds to each input file, the implementation
shall guarantee exclusive access to the device.
The lp utility shall associate a unique request ID with
each request.
Normally, a banner page is produced to separate and identify each
print job. This page may be suppressed by
implementation-defined conditions, such as an operator command or
one of the -o option values.
The lp utility was designed to be a basic version of a utility
that is already available in many historical
implementations. The standard developers considered that it should
be implementable simply as:
cat "$@" > /dev/lp
after appropriate processing of options, if that is how the implementation
chose to do it and if exclusive access could be
granted (so that two users did not write to the device simultaneously).
Although in the future the standard developers may add
other options to this utility, it should always be able to execute
with no options or operands and send the standard input to an
unspecified output device.
This volume of IEEE Std 1003.1-2001 makes no representations concerning
the format of the printed output, except that
it must be "human-readable" and "non-volatile". Thus, writing by default
to a disk or tape drive or a display terminal would
not qualify. (Such destinations are not prohibited when -d dest,
LPDEST , or PRINTER are used,
however.)
This volume of IEEE Std 1003.1-2001 is worded such that a "print
job" consisting of multiple input files, possibly
in multiple copies, is guaranteed to print so that any one file is
not intermixed with another, but there is no statement that all
the files or copies have to print out together.
The -c option may imply a spooling operation, but this is not
required. The utility can be implemented to wait until the
printer is ready and then wait until it is finished. Because of that,
there is no attempt to define a queuing mechanism
(priorities, classes of output, and so on).
On some historical systems, the request ID reported on the STDOUT
can be used to later cancel or find the status of a request
using utilities not defined in this volume of IEEE Std 1003.1-2001.
Although the historical System V lp and BSD lpr utilities
have provided similar functionality, they used different
names for the environment variable specifying the destination printer.
Since the name of the utility here is lp,
LPDEST (used by the System V lp utility) was given precedence
over PRINTER (used by the BSD lpr
utility). Since environments of users frequently contain one or the
other environment variable, the lp utility is required
to recognize both. If this was not done, many applications would send
output to unexpected output devices when users moved from
system to system.
Some have commented that lp has far too little functionality
to make it worthwhile. Requests have proposed additional
options or operands or both that added functionality. The requests
included:
|
|
|
*
|
Wording requiring the output to be "hardcopy"
|
|
*
|
A requirement for multiple printers
|
|
*
|
Options for supporting various page-description languages
|
|
Given that a compliant system is not required to even have a printer,
placing further restrictions upon the behavior of the
printer is not useful. Since hardcopy format is so application-dependent,
it is difficult, if not impossible, to select a
reasonable subset of functionality that should be required on all
compliant systems.
The term unspecified is used in this section in lieu of implementation-defined
as most known implementations would
not be able to make definitive statements in their conformance documents;
the existence and usage of printers is very dependent on
how the system administrator configures each individual system.
Since the default destination, device type, queuing mechanisms, and
acceptable forms of input are all unspecified, usage
guidelines for what a conforming application can do are as follows:
|
|
|
*
|
Use the command in a pipeline, or with -c, so that there are
no permission problems and the files can be safely deleted
or modified.
|
|
*
|
Limit output to text files of reasonable line lengths and printable
characters and include no device-specific formatting
information, such as a page description language. The meaning of "reasonable"
in this context can only be answered as a
quality-of-implementation issue, but it should be apparent from historical
usage patterns in the industry and the locale. The pr and fold
utilities can be used to achieve
reasonable formatting for the default page size of the implementation.
|
|
Alternatively, the application can arrange its installation in such
a way that it requires the system administrator or operator
to provide the appropriate information on lp options and environment
variable values.
At a minimum, having this utility in this volume of IEEE Std 1003.1-2001
tells the industry that conforming
applications require a means to print output and provides at least
a command name and LPDEST routing mechanism that can be
used for discussions between vendors, application writers, and users.
The use of "should" in the DESCRIPTION of lp clearly
shows the intent of the standard developers, even if they cannot mandate
that all systems (such as laptops) have printers.
This volume of IEEE Std 1003.1-2001 does not specify what the ownership
of the process performing the writing to the
output device may be. If -c is not used, it is unspecified whether
the process performing the writing to the output device
has permission to read file if there are any restrictions in
place on who may read file until after it is printed.
Also, if -c is not used, the results of deleting file
before it is printed are unspecified.
