special-chars.html

Shall高级编程

>stdout</TT
> [dash]. </B
><A
NAME="COXEX"
></A
></P
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat -</B
></TT
>
 <TT
CLASS="USERINPUT"
><B
>abc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>abc</TT
>
 
 <TT
CLASS="COMPUTEROUTPUT"
>...</TT
>
 
 <TT
CLASS="USERINPUT"
><B
>Ctl-D</B
></TT
></PRE
></TD
></TR
></TABLE
>
	    </P
><P
>As expected, <TT
CLASS="USERINPUT"
><B
>cat -</B
></TT
> echoes
	    <TT
CLASS="FILENAME"
>stdin</TT
>, in this case keyboarded user input,
	    to <TT
CLASS="FILENAME"
>stdout</TT
>. But, does I/O redirection using
	    <B
CLASS="COMMAND"
>-</B
> have real-world applications?</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;(cd /source/directory &#38;&#38; tar cf - . ) | (cd /dest/directory &#38;&#38; tar xpvf -)
   2&nbsp;# Move entire file tree from one directory to another
   3&nbsp;# [courtesy Alan Cox &#60;[e-mail]&#62;, with a minor change]
   4&nbsp;
   5&nbsp;# 1) cd /source/directory
   6&nbsp;#    Source directory, where the files to be moved are.
   7&nbsp;# 2) &#38;&#38;
   8&nbsp;#   "And-list": if the 'cd' operation successful,
   9&nbsp;#    then execute the next command.
  10&nbsp;# 3) tar cf - .
  11&nbsp;#    The 'c' option 'tar' archiving command creates a new archive,
  12&nbsp;#    the 'f' (file) option, followed by '-' designates the target file
  13&nbsp;#    as stdout, and do it in current directory tree ('.').
  14&nbsp;# 4) |
  15&nbsp;#    Piped to ...
  16&nbsp;# 5) ( ... )
  17&nbsp;#    a subshell
  18&nbsp;# 6) cd /dest/directory
  19&nbsp;#    Change to the destination directory.
  20&nbsp;# 7) &#38;&#38;
  21&nbsp;#   "And-list", as above
  22&nbsp;# 8) tar xpvf -
  23&nbsp;#    Unarchive ('x'), preserve ownership and file permissions ('p'),
  24&nbsp;#    and send verbose messages to stdout ('v'),
  25&nbsp;#    reading data from stdin ('f' followed by '-').
  26&nbsp;#
  27&nbsp;#    Note that 'x' is a command, and 'p', 'v', 'f' are options.
  28&nbsp;#
  29&nbsp;# Whew!
  30&nbsp;
  31&nbsp;
  32&nbsp;
  33&nbsp;# More elegant than, but equivalent to:
  34&nbsp;#   cd source/directory
  35&nbsp;#   tar cf - . | (cd ../dest/directory; tar xpvf -)
  36&nbsp;#
  37&nbsp;#     Also having same effect:
  38&nbsp;# cp -a /source/directory/* /dest/directory
  39&nbsp;#     Or:
  40&nbsp;# cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
  41&nbsp;#     If there are hidden files in /source/directory.</PRE
></TD
></TR
></TABLE
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf -
   2&nbsp;#  --uncompress tar file--    | --then pass it to "tar"--
   3&nbsp;#  If "tar" has not been patched to handle "bunzip2",
   4&nbsp;#+ this needs to be done in two discrete steps, using a pipe.
   5&nbsp;#  The purpose of the exercise is to unarchive "bzipped" kernel source.</PRE
></TD
></TR
></TABLE
></P
><P
>Note that in this context the <SPAN
CLASS="QUOTE"
>"-"</SPAN
> is not
            itself a Bash operator, but rather an option recognized by
	    certain UNIX utilities that write to
	    <TT
CLASS="FILENAME"
>stdout</TT
>, such as <B
CLASS="COMMAND"
>tar</B
>,
	    <B
CLASS="COMMAND"
>cat</B
>, etc.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo "whatever" | cat -</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>whatever</TT
> </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Where a filename is expected,
	      <TT
CLASS="REPLACEABLE"
><I
>-</I
></TT
> redirects output to
	      <TT
CLASS="FILENAME"
>stdout</TT
> (sometimes seen with
	      <TT
CLASS="USERINPUT"
><B
>tar cf</B
></TT
>), or accepts input from
	      <TT
CLASS="FILENAME"
>stdin</TT
>, rather than from a file.
	      <A
NAME="FILTERDASH"
></A
>
	      This is a method of using a file-oriented utility as a
	      filter in a pipe.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>file</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...</TT
>
 	      </PRE
></TD
></TR
></TABLE
>

	    By itself on the command line, <A
HREF="filearchiv.html#FILEREF"
>file</A
> fails with an error message.
	    </P
><P
>	    Add a <SPAN
CLASS="QUOTE"
>"-"</SPAN
> for a more useful result. This causes the
	      shell to await user input.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>file -</B
></TT
>
 <TT
CLASS="USERINPUT"
><B
>abc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>standard input:              ASCII text</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>file -</B
></TT
>
 <TT
CLASS="USERINPUT"
><B
>#!/bin/bash</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>standard input:              Bourne-Again shell script text executable</TT
>
 	      </PRE
></TD
></TR
></TABLE
>

	      Now the command accepts input from <TT
CLASS="FILENAME"
>stdin</TT
>
	        and analyzes it.
	    </P
><P
>The <SPAN
CLASS="QUOTE"
>"-"</SPAN
> can be used to pipe
	      <TT
CLASS="FILENAME"
>stdout</TT
> to other commands. This permits
	      such stunts as <A
HREF="assortedtips.html#PREPENDREF"
>prepending lines
	      to a file</A
>.</P
><P
>Using <A
HREF="filearchiv.html#DIFFREF"
>diff</A
> to
	      compare a file with a <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>section</I
></SPAN
>
	      of another:</P
><P
><TT
CLASS="USERINPUT"
><B
>grep Linux file1 | diff file2 -</B
></TT
></P
><P
>Finally, a real-world example using
	      <TT
CLASS="REPLACEABLE"
><I
>-</I
></TT
> with <A
HREF="filearchiv.html#TARREF"
>tar</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX58"
></A
><P
><B
>Example 3-4. Backup of all files changed in last day</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Backs up all files in current directory modified within last 24 hours
   4&nbsp;#+ in a "tarball" (tarred and gzipped file).
   5&nbsp;
   6&nbsp;BACKUPFILE=backup-$(date +%m-%d-%Y)
   7&nbsp;#                 Embeds date in backup filename.
   8&nbsp;#                 Thanks, Joshua Tschida, for the idea.
   9&nbsp;archive=${1:-$BACKUPFILE}
  10&nbsp;#  If no backup-archive filename specified on command line,
  11&nbsp;#+ it will default to "backup-MM-DD-YYYY.tar.gz."
  12&nbsp;
  13&nbsp;tar cvf - `find . -mtime -1 -type f -print` &#62; $archive.tar
  14&nbsp;gzip $archive.tar
  15&nbsp;echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."
  16&nbsp;
  17&nbsp;
  18&nbsp;#  Stephane Chazelas points out that the above code will fail
  19&nbsp;#+ if there are too many files found
  20&nbsp;#+ or if any filenames contain blank characters.
  21&nbsp;
  22&nbsp;# He suggests the following alternatives:
  23&nbsp;# -------------------------------------------------------------------
  24&nbsp;#   find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"
  25&nbsp;#      using the GNU version of "find".
  26&nbsp;
  27&nbsp;
  28&nbsp;#   find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' \;
  29&nbsp;#         portable to other UNIX flavors, but much slower.
  30&nbsp;# -------------------------------------------------------------------
  31&nbsp;
  32&nbsp;
  33&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Filenames beginning with
	      <SPAN
CLASS="QUOTE"
>"-"</SPAN
> may cause problems when coupled with the
	      <SPAN
CLASS="QUOTE"
>"-"</SPAN
> redirection operator. A script should
	      check for this and add an appropriate prefix to such
	      filenames, for example <TT
CLASS="FILENAME"
>./-FILENAME</TT
>,
	      <TT
CLASS="FILENAME"
>$PWD/-FILENAME</TT
>, or
	      <TT
CLASS="FILENAME"
>$PATHNAME/-FILENAME</TT
>.</P
><P
>If the value of a variable begins with a
	        <TT
CLASS="REPLACEABLE"
><I
>-</I
></TT
>, this may likewise create
		problems.
		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;var="-n"
   2&nbsp;echo $var		
   3&nbsp;# Has the effect of "echo -n", and outputs nothing.</PRE
></TD
></TR
></TABLE
>
              </P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>previous working directory. </B
>A <B
CLASS="COMMAND"
>cd -</B
> command changes to the
		previous working directory. This uses the
		<A
HREF="variables2.html#OLDPWD"
>$OLDPWD</A
> <A
HREF="othertypesv.html#ENVREF"
>environmental variable</A
>.</P
></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Do not confuse the <SPAN
CLASS="QUOTE"
>"-"</SPAN
> used in this
		sense with the <SPAN
CLASS="QUOTE"
>"-"</SPAN
> redirection
		operator just discussed. The interpretation of the
		<SPAN
CLASS="QUOTE"
>"-"</SPAN
> depends on the context in which it
		appears.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Minus. </B
>Minus sign in an <A
HREF="operations.html#AROPS1"
>arithmetic
	        operation</A
>.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>=</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Equals. </B
><A
HREF="varassignment.html#EQREF"
>Assignment operator</A
>
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a=28
   2&nbsp;echo $a   # 28</PRE
></TD
></TR
></TABLE
></P
></DIV
><P
>In a <A
HREF="comparison-ops.html#EQUALSIGNREF"
>different context</A
>,
	      the <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>=</SPAN
>"</SPAN
> is a <A
HREF="comparison-ops.html#SCOMPARISON1"
>string comparison</A
>
	      operator.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Plus. </B
>Addition  <A
HREF="operations.html#AROPS1"
>arithmetic
	        operator</A
>.</P
></DIV
><P
>In a <A
HREF="regexp.html#PLUSREF"
>different context</A
>,
	      the <SPAN
CLASS="TOKEN"
>+</SPAN
> is a <A
HREF="regexp.html"
>Regular
	      Expression</A
> operator.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Option. </B
>Option flag for a command or filter.</P
></DIV
><P
>Certain commands and <A
HREF="internal.html#BUILTINREF"
>builtins</A
> use the
	      <TT
CLASS="OPTION"
>+</TT
> to enable certain options and the
	      <TT
CLASS="OPTION"
>-</TT
> to disable them. In <A
HREF="parameter-substitution.html#PARAMSUBREF"
>parameter substitution</A
>,
	      the <TT
CLASS="OPTION"
>+</TT
> prefixes an <A
HREF="parameter-substitution.html#PARAMALTV"
>	      alternate value</A
> that a variable expands to.</P
></DD
><DT
><A
NAME="MODULO00"
></A
><SPAN
CLASS="TOKEN"
>%</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#MODULOREF"
>modulo</A
>. </B
>Modulo (remainder of a division) <A
HREF="operations.html#AROPS1"
>arithmetic
	        operation</A
>.</P
></DIV
><P
>In a <A
HREF="parameter-substitution.html#PCTPATREF"
>different context</A
>,
	      the <SPAN
CLASS="TOKEN"
>%</SPAN
> is a <A
HREF="parameter-substitution.html#PSUB2"
>pattern
	      matching</A
> operator.</P
></DD
><DT
><A
NAME="TILDEREF"
></A
><SPAN
CLASS="TOKEN"
>~</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>home directory [tilde]. </B
>This corresponds to the <A
HREF="variables2.html#HOMEDIRREF"
>$HOME</A
> internal variable.

	      <TT
CLASS="FILENAME"
>~bozo</TT
> is bozo's home directory,
		and <B
CLASS="COMMAND"
>ls ~bozo</B
> lists the contents of it.
		<SPAN
CLASS="TOKEN"
>~/</SPAN
> is the current user's home directory,
		and <B
CLASS="COMMAND"
>ls ~/</B
> lists the contents of it.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~bozo</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~/</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo/</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~:</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/home/bozo:</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ~nonexistent-user</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>~nonexistent-user</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></DIV
></DD
><DT
><A
NAME="WORKINGDIRREF"
></A
><SPAN
CLASS="TOKEN"
>~+</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>current working directory. </B
>This corresponds to the <A
HREF="variables2.html#PWDREF"
>$PWD</A
> internal variable.</P
></DIV
></DD
><DT
><A
NAME="PREVWORKINGDIR"
></A
><SPAN
CLASS="TOKEN"
>~-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>previous working directory. </B
>This corresponds to the <A
HREF="variables2.html#OLDPWD"
>$OLDPWD</A
> internal variable.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>=~</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="bashver3.html#REGEXMATCHREF"
>regular
	  expression match</A
>. </B
>This operator was introduced with <A
HREF="bashver3.html#BASH3REF"
>version 3</A