special-chars.html

一本完整的描述Unix Shell 编程的工具书的所有范例

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><TR
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# uppercase.sh : Changes input to uppercase.
   3&nbsp;
   4&nbsp;tr 'a-z' 'A-Z'
   5&nbsp;#  Letter ranges must be quoted
   6&nbsp;#+ to prevent filename generation from single-letter filenames.
   7&nbsp;
   8&nbsp;exit 0</PRE
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>
              Now, let us pipe the output of <B
CLASS="COMMAND"
>ls -l</B
> to this
	      script.

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><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l | ./uppercase.sh</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>-RW-RW-R--    1 BOZO  BOZO       109 APR  7 19:49 1.TXT
 -RW-RW-R--    1 BOZO  BOZO       109 APR 14 16:48 2.TXT
 -RW-R--R--    1 BOZO  BOZO       725 APR 20 20:56 DATA-FILE</TT
>
 	      </PRE
></TD
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>
	    </P
><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 <TT
CLASS="FILENAME"
>stdout</TT
> of each process in
	       a pipe must be read as the <TT
CLASS="FILENAME"
>stdin</TT
>
	       of the next. If this is not the case, the data stream
	       will <I
CLASS="EMPHASIS"
>block</I
>, and the pipe will not
	       behave as expected.
	         <TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat file1 file2 | ls -l | sort
   2&nbsp;# The output from "cat file1 file2" disappears.</PRE
></TD
></TR
></TABLE
>
             </P
><P
>A pipe runs as a <A
HREF="othertypesv.html#CHILDREF"
>child
	       process</A
>, and therefore cannot alter script
	       variables.
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;variable="initial_value"
   2&nbsp;echo "new_value" | read variable
   3&nbsp;echo "variable = $variable"     # variable = initial_value</PRE
></TD
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>
             </P
><P
>If one of the commands in the pipe
	       aborts, this prematurely terminates execution of the
	       pipe. Called a <I
CLASS="EMPHASIS"
>broken pipe</I
>, this
	       condition sends a <I
CLASS="EMPHASIS"
>SIGPIPE</I
> <A
HREF="debugging.html#SIGNALD"
>signal</A
>.</P
></TD
></TR
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></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#62;|</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>force redirection (even if
		the <A
HREF="options.html#NOCLOBBERREF"
>noclobber option</A
>
		is set). </B
>This will forcibly overwrite an existing file.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>||</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#ORREF"
>OR logical operator</A
>. </B
>In a <A
HREF="tests.html#TESTCONSTRUCTS1"
>test
		construct</A
>, the <SPAN
CLASS="TOKEN"
>||</SPAN
> operator causes
		a return of <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> (success) if
		<I
CLASS="EMPHASIS"
>either</I
> of the linked test conditions
		is true.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#38;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Run job in background. </B
>A command followed by an <SPAN
CLASS="TOKEN"
>&#38;</SPAN
>
	        will run in the background.</P
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>sleep 10 &#38;</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[1] 850</TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[1]+  Done                    sleep 10</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Within a script, commands and even <A
HREF="loops.html#FORLOOPREF1"
>loops</A
> may run in the
	      background.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BGLOOP"
></A
><P
><B
>Example 3-3. Running a loop in the background</B
></P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# background-loop.sh
   3&nbsp;
   4&nbsp;for i in 1 2 3 4 5 6 7 8 9 10            # First loop.
   5&nbsp;do
   6&nbsp;  echo -n "$i "
   7&nbsp;done &#38; # Run this loop in background.
   8&nbsp;       # Will sometimes execute after second loop.
   9&nbsp;
  10&nbsp;echo   # This 'echo' sometimes will not display.
  11&nbsp;
  12&nbsp;for i in 11 12 13 14 15 16 17 18 19 20   # Second loop.
  13&nbsp;do
  14&nbsp;  echo -n "$i "
  15&nbsp;done  
  16&nbsp;
  17&nbsp;echo   # This 'echo' sometimes will not display.
  18&nbsp;
  19&nbsp;# ======================================================
  20&nbsp;
  21&nbsp;# The expected output from the script:
  22&nbsp;# 1 2 3 4 5 6 7 8 9 10 
  23&nbsp;# 11 12 13 14 15 16 17 18 19 20 
  24&nbsp;
  25&nbsp;# Sometimes, though, you get:
  26&nbsp;# 11 12 13 14 15 16 17 18 19 20 
  27&nbsp;# 1 2 3 4 5 6 7 8 9 10 bozo $
  28&nbsp;# (The second 'echo' doesn't execute. Why?)
  29&nbsp;
  30&nbsp;# Occasionally also:
  31&nbsp;# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
  32&nbsp;# (The first 'echo' doesn't execute. Why?)
  33&nbsp;
  34&nbsp;# Very rarely something like:
  35&nbsp;# 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20 
  36&nbsp;# The foreground loop preempts the background one.
  37&nbsp;
  38&nbsp;exit 0
  39&nbsp;
  40&nbsp;#  Nasimuddin Ansari suggests adding    sleep 1
  41&nbsp;#+ after the   echo -n "$i"   in lines 6 and 14,
  42&nbsp;#+ for some real fun.</PRE
></TD
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><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
>A command run in the background within a
	      script may cause the script to hang, waiting
	      for a keystroke. Fortunately, there is a <A
HREF="internal.html#WAITHANG"
>remedy</A
> for this.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#LOGOPS1"
>AND logical
	    operator</A
>. </B
>In a <A
HREF="tests.html#TESTCONSTRUCTS1"
>test
		construct</A
>, the <SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
> operator causes
		a return of <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> (success) only if
		<I
CLASS="EMPHASIS"
>both</I
> the linked test conditions
		are true.</P
></DIV
></DD
><DT
><A
NAME="DASHREF"
></A
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>option, prefix. </B
>Option flag for a command or filter. Prefix for
	        an operator.</P
></DIV
><P
><TT
CLASS="USERINPUT"
><B
>COMMAND -[Option1][Option2][...]</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>ls -al</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>sort -dfu $filename</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>set -- $variable</B
></TT
></P
><P
>	      <TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ $file1 -ot $file2 ]
   2&nbsp;then
   3&nbsp;  echo "File $file1 is older than $file2."
   4&nbsp;fi
   5&nbsp;
   6&nbsp;if [ "$a" -eq "$b" ]
   7&nbsp;then
   8&nbsp;  echo "$a is equal to $b."
   9&nbsp;fi
  10&nbsp;
  11&nbsp;if [ "$c" -eq 24 -a "$d" -eq 47 ]
  12&nbsp;then
  13&nbsp;  echo "$c equals 24 and $d equals 47."
  14&nbsp;fi</PRE
></TD
></TR
></TABLE
>  
	      </P
></DD
><DT
><A
NAME="DASHREF2"
></A
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>redirection from/to <TT
CLASS="FILENAME"
>stdin</TT
> or <TT
CLASS="FILENAME"
>stdout</TT
> [dash]. </B
><A
NAME="COXEX"
></A
></P
></DIV
><P
><TABLE
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><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    Source directory, where the files to be moved are.
   6&nbsp;# 2) &#38;&#38;                     "And-list": if the 'cd' operation successful, then execute the next command.
   7&nbsp;# 3) tar cf - .              The 'c' option 'tar' archiving command creates a new archive,
   8&nbsp;#                            the 'f' (file) option, followed by '-' designates the target file as stdout,
   9&nbsp;#                            and do it in current directory tree ('.').
  10&nbsp;# 4) |                       Piped to...
  11&nbsp;# 5) ( ... )                 a subshell
  12&nbsp;# 6) cd /dest/directory      Change to the destination directory.
  13&nbsp;# 7) &#38;&#38;                     "And-list", as above
  14&nbsp;# 8) tar xpvf -              Unarchive ('x'), preserve ownership and file permissions ('p'),
  15&nbsp;#                            and send verbose messages to stdout ('v'),
  16&nbsp;#                            reading data from stdin ('f' followed by '-').
  17&nbsp;#
  18&nbsp;#                            Note that 'x' is a command, and 'p', 'v', 'f' are options.
  19&nbsp;# Whew!
  20&nbsp;
  21&nbsp;
  22&nbsp;
  23&nbsp;# More elegant than, but equivalent to:
  24&nbsp;#   cd source/directory
  25&nbsp;#   tar cf - . | (cd ../dest/directory; tar xpvf -)
  26&nbsp;#
  27&nbsp;#     Also having same effect:
  28&nbsp;# cp -a /source/directory/* /dest/directory
  29&nbsp;#     Or:
  30&nbsp;# cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
  31&nbsp;#     If there are hidden files in /source/directory.</PRE
></TD
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></P
><P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;bunzip2 linux-2.6.13.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. 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 <I
CLASS="EMPHASIS"
>section</I
>
	      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