moreadv.html

Shall高级编程

><P
>The default command for <B
CLASS="COMMAND"
>xargs</B
> is
	      <A
HREF="internal.html#ECHOREF"
>echo</A
>. This means that input
	      piped to <B
CLASS="COMMAND"
>xargs</B
> may have linefeeds and
	      other whitespace characters stripped out.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>total 0
 -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file1
 -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file2</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l | xargs</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan...</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>find ~/mail -type f | xargs grep "Linux"</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>./misc:User-Agent: slrn/0.9.8.1 (Linux)
 ./sent-mail-jul-2005: hosted by the Linux Documentation Project.
 ./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)
 ./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article
 ./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem
 . . .</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
><TT
CLASS="USERINPUT"
><B
>ls | xargs -p -l gzip</B
></TT
> <A
HREF="filearchiv.html#GZIPREF"
>gzips</A
> every file in current
	      directory, one at a time, prompting before each
	      operation.</P
><P
><A
NAME="XARGSONEATATIME"
></A
></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
>Note that <I
CLASS="FIRSTTERM"
>xargs</I
> processes the
	      arguments passed to it sequentially, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>one at
	      a time</I
></SPAN
>.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>find /usr/bin | xargs file</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/usr/bin:          directory
 /usr/bin/foomatic-ppd-options:          perl script text executable
 . . .</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="XARGSLIMARGS"
></A
></P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>An interesting <I
CLASS="FIRSTTERM"
>xargs</I
>
	      option is <TT
CLASS="OPTION"
>-n <TT
CLASS="REPLACEABLE"
><I
>NN</I
></TT
></TT
>,
	      which limits to <TT
CLASS="REPLACEABLE"
><I
>NN</I
></TT
> the number
	      of arguments passed.</P
><P
><TT
CLASS="USERINPUT"
><B
>ls | xargs -n 8 echo</B
></TT
> lists the files in the
	      current directory in <TT
CLASS="LITERAL"
>8</TT
> columns.</P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="XARGSWS"
></A
></P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Another useful option is
	      <TT
CLASS="OPTION"
>-0</TT
>, in combination with <TT
CLASS="USERINPUT"
><B
>find
	      -print0</B
></TT
> or <TT
CLASS="USERINPUT"
><B
>grep -lZ</B
></TT
>. This
	      allows handling arguments containing whitespace or
	      quotes.</P
><P
>	    <TT
CLASS="USERINPUT"
><B
>find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f</B
></TT
>
	    </P
><P
>	    <TT
CLASS="USERINPUT"
><B
>grep -rliwZ GUI / | xargs -0 rm -f</B
></TT
>
	    </P
><P
>Either of the above will remove any file containing <SPAN
CLASS="QUOTE"
>"GUI"</SPAN
>.
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>(Thanks, S.C.)</I
></SPAN
></P
><P
>Or:
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat /proc/"$pid"/"$OPTION" | xargs -0 echo
   2&nbsp;#  Formats output:         ^^^^^^^^^^^^^^^
   3&nbsp;#  From Han Holl's fixup of "get-commandline.sh"
   4&nbsp;#+ script in "/dev and /proc" chapter.</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX41"
></A
><P
><B
>Example 15-5. Logfile: Using <I
CLASS="FIRSTTERM"
>xargs</I
> to monitor system log</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Generates a log file in current directory
   4&nbsp;# from the tail end of /var/log/messages.
   5&nbsp;
   6&nbsp;# Note: /var/log/messages must be world readable
   7&nbsp;# if this script invoked by an ordinary user.
   8&nbsp;#         #root chmod 644 /var/log/messages
   9&nbsp;
  10&nbsp;LINES=5
  11&nbsp;
  12&nbsp;( date; uname -a ) &#62;&#62;logfile
  13&nbsp;# Time and machine name
  14&nbsp;echo ---------------------------------------------------------- &#62;&#62;logfile
  15&nbsp;tail -n $LINES /var/log/messages | xargs | fmt -s &#62;&#62;logfile
  16&nbsp;echo &#62;&#62;logfile
  17&nbsp;echo &#62;&#62;logfile
  18&nbsp;
  19&nbsp;exit 0
  20&nbsp;
  21&nbsp;#  Note:
  22&nbsp;#  ----
  23&nbsp;#  As Frank Wang points out,
  24&nbsp;#+ unmatched quotes (either single or double quotes) in the source file
  25&nbsp;#+ may give xargs indigestion.
  26&nbsp;#
  27&nbsp;#  He suggests the following substitution for line 15:
  28&nbsp;#  tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s &#62;&#62;logfile
  29&nbsp;
  30&nbsp;
  31&nbsp;
  32&nbsp;#  Exercise:
  33&nbsp;#  --------
  34&nbsp;#  Modify this script to track changes in /var/log/messages at intervals
  35&nbsp;#+ of 20 minutes.
  36&nbsp;#  Hint: Use the "watch" command. </PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="XARGSCURLYREF"
></A
></P
><P
><A
HREF="moreadv.html#CURLYBRACKETSREF"
>As in
	      <B
CLASS="COMMAND"
>find</B
></A
>, a curly bracket
	      pair serves as a placeholder for replacement text.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX42"
></A
><P
><B
>Example 15-6. Copying files in current directory to another</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# copydir.sh
   3&nbsp;
   4&nbsp;#  Copy (verbose) all files in current directory ($PWD)
   5&nbsp;#+ to directory specified on command line.
   6&nbsp;
   7&nbsp;E_NOARGS=65
   8&nbsp;
   9&nbsp;if [ -z "$1" ]   # Exit if no argument given.
  10&nbsp;then
  11&nbsp;  echo "Usage: `basename $0` directory-to-copy-to"
  12&nbsp;  exit $E_NOARGS
  13&nbsp;fi  
  14&nbsp;
  15&nbsp;ls . | xargs -i -t cp ./{} $1
  16&nbsp;#            ^^ ^^      ^^
  17&nbsp;#  -t is "verbose" (output command line to stderr) option.
  18&nbsp;#  -i is "replace strings" option.
  19&nbsp;#  {} is a placeholder for output text.
  20&nbsp;#  This is similar to the use of a curly bracket pair in "find."
  21&nbsp;#
  22&nbsp;#  List the files in current directory (ls .),
  23&nbsp;#+ pass the output of "ls" as arguments to "xargs" (-i -t options),
  24&nbsp;#+ then copy (cp) these arguments ({}) to new directory ($1).  
  25&nbsp;#
  26&nbsp;#  The net result is the exact equivalent of
  27&nbsp;#+   cp * $1
  28&nbsp;#+ unless any of the filenames has embedded "whitespace" characters.
  29&nbsp;
  30&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="KILLBYNAME"
></A
><P
><B
>Example 15-7. Killing processes by name</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# kill-byname.sh: Killing processes by name.
   3&nbsp;# Compare this script with kill-process.sh.
   4&nbsp;
   5&nbsp;#  For instance,
   6&nbsp;#+ try "./kill-byname.sh xterm" --
   7&nbsp;#+ and watch all the xterms on your desktop disappear.
   8&nbsp;
   9&nbsp;#  Warning:
  10&nbsp;#  -------
  11&nbsp;#  This is a fairly dangerous script.
  12&nbsp;#  Running it carelessly (especially as root)
  13&nbsp;#+ can cause data loss and other undesirable effects.
  14&nbsp;
  15&nbsp;E_BADARGS=66
  16&nbsp;
  17&nbsp;if test -z "$1"  # No command line arg supplied?
  18&nbsp;then
  19&nbsp;  echo "Usage: `basename $0` Process(es)_to_kill"
  20&nbsp;  exit $E_BADARGS
  21&nbsp;fi
  22&nbsp;
  23&nbsp;
  24&nbsp;PROCESS_NAME="$1"
  25&nbsp;ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&#38;&#62;/dev/null
  26&nbsp;#                                                       ^^      ^^
  27&nbsp;
  28&nbsp;# ---------------------------------------------------------------
  29&nbsp;# Notes:
  30&nbsp;# -i is the "replace strings" option to xargs.
  31&nbsp;# The curly brackets are the placeholder for the replacement.
  32&nbsp;# 2&#38;&#62;/dev/null suppresses unwanted error messages.
  33&nbsp;#
  34&nbsp;# Can  grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"?
  35&nbsp;# ---------------------------------------------------------------
  36&nbsp;
  37&nbsp;exit $?
  38&nbsp;
  39&nbsp;#  The "killall" command has the same effect as this script,
  40&nbsp;#+ but using it is not quite as educational.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="WF2"
></A
><P
><B
>Example 15-8. Word frequency analysis using
	      <I
CLASS="FIRSTTERM"
>xargs</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# wf2.sh: Crude word frequency analysis on a text file.
   3&nbsp;
   4&nbsp;# Uses 'xargs' to decompose lines of text into single words.
   5&nbsp;# Compare this example to the "wf.sh" script later on.
   6&nbsp;
   7&nbsp;
   8&nbsp;# Check for input file on command line.
   9&nbsp;ARGS=1
  10&nbsp;E_BADARGS=65
  11&nbsp;E_NOFILE=66
  12&nbsp;
  13&nbsp;if [ $# -ne "$ARGS" ]
  14&nbsp;# Correct number of arguments passed to script?
  15&nbsp;then
  16&nbsp;  echo "Usage: `basename $0` filename"
  17&nbsp;  exit $E_BADARGS
  18&nbsp;fi
  19&nbsp;
  20&nbsp;if [ ! -f "$1" ]       # Check if file exists.
  21&nbsp;then
  22&nbsp;  echo "File \"$1\" does not exist."
  23&nbsp;  exit $E_NOFILE
  24&nbsp;fi
  25&nbsp;
  26&nbsp;
  27&nbsp;
  28&nbsp;########################################################
  29&nbsp;cat "$1" | xargs -n1 | \
  30&nbsp;#  List the file, one word per line. 
  31&nbsp;tr A-Z a-z | \
  32&nbsp;#  Shift characters to lowercase.
  33&nbsp;sed -e 's/\.//g'  -e 's/\,//g' -e 's/ /\