在背景列印一個檔案稱作 Spooling,緩衝程式 (Spooler) 讓使用者能夠繼續執行電腦的其他程式而不需要等候印表機緩慢的完成列印工作。
FreeBSD 內含的緩衝程式 (Spooler) 稱作 lpd(8),而列印工作會使用 lpr(1) 來提交。
建立要用來儲存列印工作的目錄、設定擁有關係以及權限來避免其他使用者可以檢視這些檔案的內容:
#
mkdir -p /var/spool/lpd/lp
#
chown daemon:daemon /var/spool/lpd/lp
#
chmod 770 /var/spool/lpd/lp
印表機會定義在 /etc/printcap
,每台印表機項目所包含的詳細資料有名稱、連接的接頭以及各種其他設定。建立 /etc/printcap
使用以下內容:
lp:\
:lp=/dev/unlpt0:\
:sh:\
:mx#0:\
:sd=/var/spool/lpd/lp:\
:lf=/var/log/lpd-errs: 
印表機的名稱。 lpr(1) 會傳送列印工作到 | |||||||||||
印表機所連接到裝置。替換此行為正確的連線類型,如此處所示。
| |||||||||||
在列印工作開始時不列印首頁。 | |||||||||||
不限制列印工作的最大尺寸。 | |||||||||||
此印表機的緩衝 (Spooling) 目錄路徑,每台印表機會自己使用一個獨立的緩衝 (Spooling) 目錄。 | |||||||||||
回報此印表機的錯誤的日誌檔。 |
在建立 /etc/printcap
之後,使用 chkprintcap(8) 測試印表機是否有錯誤:
#
chkprintcap
在繼續之前修正任何回報的問題。
開啟 /etc/rc.conf
中的 lpd(8):
lpd_enable="YES"
啟動服務:
#
service lpd start
Documents are sent to the printer with
lpr
. A file to be printed can be named on
the command line or piped into lpr
. These
two commands are equivalent, sending the contents of
doc.txt
to the default printer:
%
lpr doc.txt
%
cat doc.txt | lpr
Printers can be selected with -P
. To
print to a printer called
laser
:
%
lpr -Plaser doc.txt
The examples shown so far have sent the contents of a text file directly to the printer. As long as the printer understands the content of those files, output will be printed correctly.
Some printers are not capable of printing plain text, and the input file might not even be plain text.
Filters allow files to be translated or processed. The typical use is to translate one type of input, like plain text, into a form that the printer can understand, like PostScript® or PCL. Filters can also be used to provide additional features, like adding page numbers or highlighting source code to make it easier to read.
The filters discussed here are
input filters or
text filters. These filters convert the
incoming file into different forms. Use su(1) to become
root
before
creating the files.
Filters are specified in
/etc/printcap
with the
if=
identifier. To use
/usr/local/libexec/lf2crlf
as a filter,
modify /etc/printcap
like this:
lp:\
:lp=/dev/unlpt0:\
:sh:\
:mx#0:\
:sd=/var/spool/lpd/lp:\
:if=/usr/local/libexec/lf2crlf:\
:lf=/var/log/lpd-errs:
The backslash line continuation
characters at the end of the lines in
printcap
entries reveal that an entry
for a printer is really just one long line with entries
delimited by colon characters. An earlier example can be
rewritten as a single less-readable line:
lp:lp=/dev/unlpt0:sh:mx#0:sd=/var/spool/lpd/lp:if=/usr/local/libexec/lf2crlf:lf=/var/log/lpd-errs:
Typical FreeBSD text files contain only a single line feed character at the end of each line. These lines will “stairstep” on a standard printer:
A printed file looks
like the steps of a staircase
scattered by the wind
A filter can convert the newline characters into
carriage returns and newlines. The carriage returns make
the printer return to the left after each line. Create
/usr/local/libexec/lf2crlf
with these
contents:
#!/bin/sh
CR=$'\r'
/usr/bin/sed -e "s/$/${CR}/g"
Set the permissions and make it executable:
#
chmod 555 /usr/local/libexec/lf2crlf
Modify /etc/printcap
to use the
new filter:
:if=/usr/local/libexec/lf2crlf:\
Test the filter by printing the same plain text file. The carriage returns will cause each line to start at the left side of the page.
GNU Enscript converts plain text files into nicely-formatted PostScript® for printing on PostScript® printers. It adds page numbers, wraps long lines, and provides numerous other features to make printed text files easier to read. Depending on the local paper size, install either print/enscript-letter or print/enscript-a4 from the Ports Collection.
Create /usr/local/libexec/enscript
with these contents:
#!/bin/sh
/usr/local/bin/enscript -o -
Set the permissions and make it executable:
#
chmod 555 /usr/local/libexec/enscript
Modify /etc/printcap
to use the
new filter:
:if=/usr/local/libexec/enscript:\
Test the filter by printing a plain text file.
Many programs produce PostScript® documents. However, inexpensive printers often only understand plain text or PCL. This filter converts PostScript® files to PCL before sending them to the printer.
Install the Ghostscript PostScript® interpreter, print/ghostscript9, from the Ports Collection.
Create /usr/local/libexec/ps2pcl
with these contents:
#!/bin/sh
/usr/local/bin/gs -dSAFER -dNOPAUSE -dBATCH -q -sDEVICE=ljet4 -sOutputFile=- -
Set the permissions and make it executable:
#
chmod 555 /usr/local/libexec/ps2pcl
PostScript® input sent to this script will be rendered and converted to PCL before being sent on to the printer.
Modify /etc/printcap
to use this
new input filter:
:if=/usr/local/libexec/ps2pcl:\
Test the filter by sending a small PostScript® program to it:
%
printf "%%\!PS \n /Helvetica findfont 18 scalefont setfont \
72 432 moveto (PostScript printing successful.) show showpage \004" | lpr
A filter that detects the type of input and
automatically converts it to the correct format for the
printer can be very convenient. The first two characters of
a PostScript® file are usually %!
. A
filter can detect those two characters. PostScript® files
can be sent on to a PostScript® printer unchanged. Text
files can be converted to PostScript® with
Enscript as shown earlier.
Create /usr/local/libexec/psif
with
these contents:
#!/bin/sh
#
# psif - Print PostScript or plain text on a PostScript printer
#
IFS="" read -r first_line
first_two_chars=`expr "$first_line" : '\(..\)'`
case "$first_two_chars" in
%!)
# %! : PostScript job, print it.
echo "$first_line" && cat && exit 0
exit 2
;;
*)
# otherwise, format with enscript
( echo "$first_line"; cat ) | /usr/local/bin/enscript -o - && exit 0
exit 2
;;
esac
Set the permissions and make it executable:
#
chmod 555 /usr/local/libexec/psif
Modify /etc/printcap
to use this
new input filter:
:if=/usr/local/libexec/psif:\
Test the filter by printing PostScript® and plain text files.
Writing a filter that detects many different types of input and formats them correctly is challenging. print/apsfilter from the Ports Collection is a smart “magic” filter that detects dozens of file types and automatically converts them to the PDL understood by the printer. See http://www.apsfilter.org for more details.
The entries in /etc/printcap
are
really definitions of queues. There can
be more than one queue for a single printer. When combined
with filters, multiple queues provide users more control over
how their jobs are printed.
As an example, consider a networked PostScript® laser
printer in an office. Most users want to print plain text,
but a few advanced users want to be able to print PostScript®
files directly. Two entries can be created for the same
printer in /etc/printcap
:
textprinter:\
:lp=9100@officelaser:\
:sh:\
:mx#0:\
:sd=/var/spool/lpd/textprinter:\
:if=/usr/local/libexec/enscript:\
:lf=/var/log/lpd-errs:
psprinter:\
:lp=9100@officelaser:\
:sh:\
:mx#0:\
:sd=/var/spool/lpd/psprinter:\
:lf=/var/log/lpd-errs:
Documents sent to textprinter
will be
formatted by the
/usr/local/libexec/enscript
filter shown
in an earlier example. Advanced users can print PostScript®
files on psprinter
, where no filtering is
done.
This multiple queue technique can be used to provide direct access to all kinds of printer features. A printer with a duplexer could use two queues, one for ordinary single-sided printing, and one with a filter that sends the command sequence to enable double-sided printing and then sends the incoming file.
Several utilities are available to monitor print jobs and check and control printer operation.
lpq(1) shows the status of a user's print jobs. Print jobs from other users are not shown.
Show the current user's pending jobs on a single printer:
%
lpq -Plp
Rank Owner Job Files Total Size
1st jsmith 0 (standard input) 12792 bytes
Show the current user's pending jobs on all printers:
%
lpq -a
lp:
Rank Owner Job Files Total Size
1st jsmith 1 (standard input) 27320 bytes
laser:
Rank Owner Job Files Total Size
1st jsmith 287 (standard input) 22443 bytes
lprm(1) is used to remove print jobs. Normal users
are only allowed to remove their own jobs.
root
can remove
any or all jobs.
Remove all pending jobs from a printer:
#
lprm -Plp
-
dfA002smithy dequeued
cfA002smithy dequeued
dfA003smithy dequeued
cfA003smithy dequeued
dfA004smithy dequeued
cfA004smithy dequeued
Remove a single job from a printer. lpq(1) is used to find the job number.
%
lpq
Rank Owner Job Files Total Size
1st jsmith 5 (standard input) 12188 bytes
%
lprm -Plp
5
dfA005smithy dequeued
cfA005smithy dequeued
lpc(8) is used to check and modify printer status.
lpc
is followed by a command and an
optional printer name. all
can be used
instead of a specific printer name, and the command will be
applied to all printers. Normal users can view status with
lpc(8). Only
class="username">root
can use
commands which modify printer status.
Show the status of all printers:
%
lpc status all
lp:
queuing is enabled
printing is enabled
1 entry in spool area
printer idle
laser:
queuing is enabled
printing is enabled
1 entry in spool area
waiting for laser to come up
Prevent a printer from accepting new jobs, then begin accepting new jobs again:
#
lpc disable lp
lp:
queuing disabled
#
lpc enable lp
lp:
queuing enabled
Stop printing, but continue to accept new jobs. Then begin printing again:
#
lpc stop lp
lp:
printing disabled
#
lpc start lp
lp:
printing enabled
daemon started
Restart a printer after some error condition:
#
lpc restart lp
lp:
no daemon to abort
printing enabled
daemon restarted
Turn the print queue off and disable printing, with a message to explain the problem to users:
#
lpc down lp
Repair parts will arrive on Monday
lp:
printer and queuing disabled
status message is now: Repair parts will arrive on Monday
Re-enable a printer that is down:
#
lpc up lp
lp:
printing enabled
daemon started
See lpc(8) for more commands and options.
Printers are often shared by multiple users in businesses and schools. Additional features are provided to make sharing printers more convenient.
The printer name is set in the first line of the
entry in /etc/printcap
. Additional
names, or aliases, can be added after
that name. Aliases are separated from the name and each
other by vertical bars:
lp|repairsprinter
|salesprinter
:\
Aliases can be used in place of the printer name. For example, users in the Sales department print to their printer with
%
lpr -Psalesprinter
sales-report.txt
Users in the Repairs department print to their printer with
%
lpr -Prepairsprinter
repairs-report.txt
All of the documents print on that single printer. When the Sales department grows enough to need their own printer, the alias can be removed from the shared printer entry and used as the name of a new printer. Users in both departments continue to use the same commands, but the Sales documents are sent to the new printer.
It can be difficult for users to locate their documents in the stack of pages produced by a busy shared printer. Header pages were created to solve this problem. A header page with the user name and document name is printed before each print job. These pages are also sometimes called banner or separator pages.
Enabling header pages differs depending on whether the printer is connected directly to the computer with a USB, parallel, or serial cable, or is connected remotely over a network.
Header pages on directly-connected printers are enabled
by removing the :sh:\
(Suppress Header)
line from the entry in /etc/printcap
.
These header pages only use line feed characters for new
lines. Some printers will need the
/usr/share/examples/printing/hpif
filter to prevent stairstepped text. The filter configures
PCL printers to print both carriage
returns and line feeds when a line feed is received.
Header pages for network printers must be configured on
the printer itself. Header page entries in
/etc/printcap
are ignored. Settings
are usually available from the printer front panel or a
configuration web page accessible with a web browser.
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