STANDARD INPUT

The standard input shall be used only if no file operands are specified, or if a file operand is −. See Input Files.

INPUT FILES

The input files shall be text files.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of lp:

LANG

This variable shall determine the locale to use for the locale categories when both LC_ALL and the corresponding environment variable (beginning with LC_ ) do not specify a locale. See 2.6.

LC_ALL

This variable shall determine the locale to be used to override any values for locale categories specified by the settings of LANG or any environment variables beginning with LC_.

LC_CTYPE

This variable shall determine the locale for the interpretation of sequences of bytes of text data as characters (e.g., single- versus multibyte characters in arguments and input files).

LC_MESSAGES

This variable shall determine the language in which messages should be written.

LPDEST

This variable shall be interpreted as a string that names the output device or destination. If the LPDEST environment variable is not set, the PRINTER environment variable shall be used. The −d dest option shall take precedence over LPDEST. Results are undefined when −d is not specified and LPDEST contains a value that is not a valid device or destination name.

PRINTER

This variable shall be interpreted as a string that names the output device or destination. If the LPDEST and PRINTER environment variables are not set, an unspecified output device is used. The −d dest option and the LPDEST environment variable shall take precedence over PRINTER. Results are undefined when −d is not specified, LPDEST is unset, and PRINTER contains a value that is not a valid device or destination name.

ASYNCHRONOUS EVENTS

Default.

STANDARD OUTPUT

A message concerning the identification or status of the print request may be written, in an unspecified format.

STANDARD ERROR

Used only for diagnostic messages.

OUTPUT FILES

None.

EXAMPLES

Since the default destination, device type, queueing mechanisms, and acceptable forms of input are all unspecified, usage guidelines for what a portable application can do are as follows:

1. 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.
2. 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 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 implementation’s default page size.

Alternatively, the application can arrange its installation in such a way that 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 the standard tells the industry that portable 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 clearly shows the working group’s intent, even if it cannot mandate that all systems (such as laptops) have printers. Examples: To print file file: lp −c file To print multiple files with headers: pr file1 file2 | lp On most existing implementations of lp, an option is provided to pass printer specific options to the daemon handling the printer. It is not specified here

because the printer-specific options are widespread and in conflict, the lp specified here is not required to even have a queueing mechanism, and the choice of options varies widely from printer to printer. Nonetheless, implementors are encouraged to use this mechanism where appropriate: −o option Specifies an implementation-defined option that controls the specific operation of the printer. The following options could be used for the meanings below if the hardware is capable of supporting the option.

option Meaning lp2 two logical pages per physical page lp4 four logical pages per physical page d double sided

POSIX. 2 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 will have 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.

HISTORY OF DECISIONS MADE

The lp utility was designed to be a basic version of a utility that is already available in many historical implementations. The working group felt 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 working group 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. The standard makes no representations concerning the format of the printed output, except that it must be ‘‘human-readable’’ and ‘‘nonvolatile.’’ 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.) A portable application will use one of the file operands only with the −c option or if the file is publicly readable and guaranteed to be available at the time of printing. This is because the standard gives the implementation the freedom to queue up the request for printing at some later time by a different process that might not be able to access the file. The standard 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 jumbled up 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 simply wait until the printer is ready and then wait until it’s finished. Because of that, there is no attempt to define a queueing mechanism (priorities, classes of output, etc.). The −n and −d options were added in response to balloting objections that too little historical value was being provided. 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 PostScript, dimpress, hp, and lineprint formats

Given that a POSIX. 2 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 POSIX. 2 compliant systems. The term ‘‘unspecified’’ is used in this clause in lieu of ‘‘implementation defined’’ as most known implementations would not be able to say anything fully useful in their conformance documents: the existence and usage of printers is very dependent on how the system administrator configures each individual system.