extmisc.html

Shall高级编程

><B
>yes different
	      string</B
></TT
>, which would continually output
	      <TT
CLASS="COMPUTEROUTPUT"
>different string</TT
> to
	      <TT
CLASS="FILENAME"
>stdout</TT
>.</P
><P
>One might well ask the purpose of this. From the
	      command line or in a script, the output of
	      <B
CLASS="COMMAND"
>yes</B
> can be redirected or piped into a
	      program expecting user input. In effect, this becomes a sort
	      of poor man's version of <B
CLASS="COMMAND"
>expect</B
>.</P
><P
><TT
CLASS="USERINPUT"
><B
>yes | fsck /dev/hda1</B
></TT
> runs
	      <B
CLASS="COMMAND"
>fsck</B
> non-interactively (careful!).</P
><P
><TT
CLASS="USERINPUT"
><B
>yes | rm -r dirname</B
></TT
> has same effect as
	     <TT
CLASS="USERINPUT"
><B
>rm -rf dirname</B
></TT
> (careful!).</P
><DIV
CLASS="WARNING"
><TABLE
CLASS="WARNING"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/warning.png"
HSPACE="5"
ALT="Warning"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Caution advised when piping <B
CLASS="COMMAND"
>yes</B
>
	     to a potentially dangerous system command, such
	     as <A
HREF="system.html#FSCKREF"
>fsck</A
> or <A
HREF="system.html#FDISKREF"
>fdisk</A
>. It might have unintended
	     consequences.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>yes</B
> command parses variables.
	       For example:</P
><P
>	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>yes $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>3.1.17(1)-release
 3.1.17(1)-release
 3.1.17(1)-release
 3.1.17(1)-release
 3.1.17(1)-release
 . . .</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	        </P
><P
>This <SPAN
CLASS="QUOTE"
>"feature"</SPAN
> may not be particularly useful.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="BANNERREF"
></A
><B
CLASS="COMMAND"
>banner</B
></DT
><DD
><P
>Prints arguments as a large vertical banner to
	      <TT
CLASS="FILENAME"
>stdout</TT
>, using an ASCII character
	      (default '#'). This may be redirected to a printer for
	      hardcopy.</P
></DD
><DT
><A
NAME="PRINTENVREF"
></A
><B
CLASS="COMMAND"
>printenv</B
></DT
><DD
><P
>Show all the <A
HREF="othertypesv.html#ENVREF"
>environmental
	      variables</A
> set for a particular user.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>printenv | grep HOME</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>HOME=/home/bozo</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="LPREF"
></A
><B
CLASS="COMMAND"
>lp</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>lp</B
> and <B
CLASS="COMMAND"
>lpr</B
>
	      commands send file(s) to the print queue, to be printed as
	      hard copy.

	      <A
NAME="AEN12974"
HREF="#FTN.AEN12974"
>[2]</A
>

	      These commands trace the origin of their names to the
	      line printers of another era.</P
><P
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>lp file1.txt</B
></TT
>
	        or <TT
CLASS="PROMPT"
>bash </TT
><TT
CLASS="USERINPUT"
><B
>lp
		&#60;file1.txt</B
></TT
></P
><P
>It is often useful to pipe the formatted output from
	        <B
CLASS="COMMAND"
>pr</B
> to <B
CLASS="COMMAND"
>lp</B
>.</P
><P
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>pr -options file1.txt | lp</B
></TT
>
	        </P
><P
>Formatting packages, such as <A
HREF="textproc.html#GROFFREF"
>groff</A
> and
		<I
CLASS="FIRSTTERM"
>Ghostscript</I
> may send their output
		directly to <B
CLASS="COMMAND"
>lp</B
>.</P
><P
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>groff -Tascii file.tr | lp</B
></TT
>
	        </P
><P
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>gs -options | lp file.ps</B
></TT
>
	        </P
><P
>Related commands are <B
CLASS="COMMAND"
>lpq</B
>, for viewing
	        the print queue, and <B
CLASS="COMMAND"
>lprm</B
>, for removing
		jobs from the print queue.</P
></DD
><DT
><A
NAME="TEEREF"
></A
><B
CLASS="COMMAND"
>tee</B
></DT
><DD
><P
>[UNIX borrows an idea from the plumbing trade.]</P
><P
>This is a redirection operator, but with a difference. Like the
	      plumber's <I
CLASS="FIRSTTERM"
>tee,</I
> it permits <SPAN
CLASS="QUOTE"
>"siponing
              off"</SPAN
> <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>to a file </I
></SPAN
>the output of a command 
	      or commands within a pipe, but without affecting the result. This is
	      useful for printing an ongoing process to a file or paper, perhaps to
	      keep track of it for debugging purposes.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
>                              (redirection)
                             |----&#62; to file
                             |
   ==========================|====================
   command ---&#62; command ---&#62; |tee ---&#62; command ---&#62; ---&#62; output of pipe
   ===============================================
 	      </PRE
></TD
></TR
></TABLE
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat listfile* | sort | tee check.file | uniq &#62; result.file</PRE
></TD
></TR
></TABLE
></P
><P
>(The file <TT
CLASS="FILENAME"
>check.file</TT
> contains
	        the concatenated sorted <SPAN
CLASS="QUOTE"
>"listfiles,"</SPAN
>
	        before the duplicate lines are removed by <A
HREF="textproc.html#UNIQREF"
>uniq</A
>.)</P
></DD
><DT
><A
NAME="MKFIFOREF"
></A
><B
CLASS="COMMAND"
>mkfifo</B
></DT
><DD
><P
><A
NAME="NAMEDPIPEREF"
></A
>This obscure command
	      creates a <I
CLASS="FIRSTTERM"
>named pipe</I
>, a temporary
	      <I
CLASS="FIRSTTERM"
>first-in-first-out buffer</I
> for
	      transferring data between processes.

		<A
NAME="AEN13038"
HREF="#FTN.AEN13038"
>[3]</A
>

	      Typically, one process writes to the FIFO, and the other
	      reads from it. See <A
HREF="contributed-scripts.html#FIFO"
>Example A-15</A
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# This short script by Omair Eshkenazi.
   3&nbsp;# Used in ABS Guide with permission (thanks!).
   4&nbsp;
   5&nbsp;mkfifo pipe1
   6&nbsp;mkfifo pipe2
   7&nbsp;
   8&nbsp;(cut -d' ' -f1 | tr "a-z" "A-Z") &#62;pipe2 &#60;pipe1 &#38;
   9&nbsp;ls -l | tr -s ' ' | cut -d' ' -f3,9- | tee pipe1 |
  10&nbsp;cut -d' ' -f2 | paste - pipe2
  11&nbsp;
  12&nbsp;rm -f pipe1
  13&nbsp;rm -f pipe2
  14&nbsp;
  15&nbsp;# No need to kill background processes when script terminates (why not?).
  16&nbsp;
  17&nbsp;exit $?
  18&nbsp;
  19&nbsp;Now, invoke the script and explain the output:
  20&nbsp;sh mkfifo-example.sh
  21&nbsp;
  22&nbsp;4830.tar.gz          BOZO
  23&nbsp;pipe1   BOZO
  24&nbsp;pipe2   BOZO
  25&nbsp;mkfifo-example.sh    BOZO
  26&nbsp;Mixed.msg BOZO</PRE
></TD
></TR
></TABLE
>
            </P
></DD
><DT
><A
NAME="PATHCHKREF"
></A
><B
CLASS="COMMAND"
>pathchk</B
></DT
><DD
><P
>This command checks the validity of a filename. If the
	      filename exceeds the maximum allowable length (255
	      characters) or one or more of the directories in
	      its path is not searchable, then an error message
	      results.</P
><P
>Unfortunately, <B
CLASS="COMMAND"
>pathchk</B
> does
	      not return a recognizable error code, and it is therefore
	      pretty much useless in a script. Consider instead the
	      <A
HREF="fto.html#RTIF"
>file test operators</A
>.</P
></DD
><DT
><A
NAME="DDREF"
></A
><B
CLASS="COMMAND"
>dd</B
></DT
><DD
><P
>This is the somewhat obscure and much feared <I
CLASS="FIRSTTERM"
>data
	      duplicator</I
> command. Originally a utility
	      for exchanging data on magnetic tapes between UNIX
	      minicomputers and IBM mainframes, this command still
	      has its uses.  The <B
CLASS="COMMAND"
>dd</B
> command simply
	      copies a file (or <TT
CLASS="FILENAME"
>stdin/stdout</TT
>), but
	      with conversions. <A
NAME="DDCONVERSIONS"
></A
>Possible
	      conversions are ASCII/EBCDIC,
	        
		<A
NAME="AEN13075"
HREF="#FTN.AEN13075"
>[4]</A
>

	      upper/lower case, swapping of byte pairs between input
	      and output, and skipping and/or truncating the head or
	      tail of the input file.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Converting a file to all uppercase:
   2&nbsp;
   3&nbsp;dd if=$filename conv=ucase &#62; $filename.uppercase
   4&nbsp;#                    lcase   # For lower case conversion</PRE
></TD
></TR
></TABLE
>
            </P
><P
><A
NAME="DDOPTIONS"
></A
></P
><P
>Some basic options to <B
CLASS="COMMAND"
>dd</B
> are:
                <UL
><LI
><P
>if=INFILE</P
><P
>INFILE is the <I
CLASS="FIRSTTERM"
>source</I
>
		        file.</P
></LI
><LI
><P
>of=OUTFILE</P
><P
>OUTFILE is the <I
CLASS="FIRSTTERM"
>target</I
>
		        file, the file that will have the data written to it.</P
></LI
><LI
><P
>bs=BLOCKSIZE</P
><P
>This is the size of each block of data being read
		        and written, usually a power of 2.</P
></LI
><LI
><P
>skip=BLOCKS</P
><P
>How many blocks of data to skip in INFILE before
		        starting to copy. This is useful when the INFILE has
			<SPAN
CLASS="QUOTE"
>"garbage"</SPAN
> or garbled data in its
			header or when it is desirable to copy only a portion
			of the INFILE.</P
></LI
><LI
><P
>seek=BLOCKS</P
><P
>How many blocks of data to skip in OUTFILE before
		        starting to copy, leaving blank data at beginning
			of OUTFILE.</P
></LI
><LI
><P
>count=BLOCKS</P
><P
>Copy only this many blocks of data, rather than the
		        entire INFILE.</P
></LI
><LI
><P
>conv=CONVERSION</P
><P
>Type of conversion to be applied to INFILE data
                        before copying operation.</P
></LI
></UL
>
            </P
><P
>A <TT
CLASS="USERINPUT"
><B
>dd --help</B
></TT
> lists all the