linux xargs详解

xargs  [-0prtx] [-E  eof-str] [-e[eof-str]] [--eof[=eof-str]] [--null]

       [-d delimiter] [--delimiter delimiter]  [-I  replace-str]  [-i[replace-
       str]] [--replace[=replace-str]]   [-l[max-lines]]   [-L   max-lines]
       [--max-lines[=max-lines]] [-n max-args] [--max-args=max-args] [-s  max-
       chars]  [--max-chars=max-chars] [-P max-procs] [--max-procs=max-procs]
       [--interactive]     [--verbose]     [--exit]   [--no-run-if-empty]
       [--arg-file=file]   [--show-limits]   [--version]   [--help]   [command
       [initial-arguments]]

最经典应用模式：  somecommand |xargs -item  command   

不带command ，默认的使用echo 输出

用途：
    1.构造参数列表并运行命令，即将接收的参数传递给后面的command 命令执行

    2.将多行输入转换为单行 （特殊功效）

优点：

       1. 将输入参数整理后，去除<newline>换行符，以一个列表形式处理
       2. 避免参数过长引发的问题，使用xargs -n 参数适当控制，对于经常产生大量输出的命令如find、locate和grep来说非常有用

XARGS 一般是和管道一起使用： 

XXcomand  | xargs  -x  comand initial-args

-x 代表选项

选项：

-p 操作具有可交互性，每次执行comand都交互式提示用户选择

-i -i 选项告诉 xargs 可以使用{}代替传递过来的参数， 建议使用-I，其符合POSIX标准

-I 

 格式： xargs  -I  rep-str  comand  rep-srt             rep-str 为代替传递给xargs参数， 可以使 {} $ @ 等符号 ，其主要作用是当xargs   command 后有多个参数时，调整参数位置。例如：

          find  . -name  "*.txt "  |xargs -I {}  cp {} /tmp   

-t    启用命令行输出模式：其先回显要运行的命令，然后执行命令，打印出命令结果，跟踪与调试xargs的利器，也是研究xargs运行原理的好办法；

-r

   如果没有要处理的参数传递给xargsxargs 默认是带 空参数运行一次，如果你希望无参数时，停止 xargs，直接退出，使用 -r 选项即可，其可以防止xargs 后面命令带空参数运行报错。If the standard input does not contain any nonblanks, do not run the command, exit

-s size

   设置每次构造Command行的长度总大小,包括 command +init-param +传递参数,Size 参数必须是正整数 

-L num

   从标准输入一次读取num行送给Command命令 ，-l和-L功能一样，不建议使用。

-n
  xargs 的-n选项设置每次送给command命令的参数个数，参数以空白字符或<newline>换行符分割
   -L 和 -n 标志是互相排斥的；最后指定的标志生效。

  -x 如果有任何 Command 行大于 -s Size 标志指定的字节数，停止运行 xargs 命令，-L -I -n 默认打开-x参数

常见的经典用法：

    find . -name "*.txt" |xargs rm {} 

帮助理解各个参数的实例

-t  参数，打开调试功能，显示每次所组的命令，在调试中非常有用，可以看出xargs的执行原理

-L num 参数控制每次输入的行数，如下是控制每次输入2行

[root@andes.com ~/tmp/dir]#ls -l
total 44
-rwx------ 1 root root 1026 Sep 27 05:28 data.txt
-rw-r--r-- 1 root root 1047 Sep 27 05:28 d.txt
-rwx------ 1 root root 2009 Sep 27 05:28 env2.txt
-rwx------ 1 root root 2009 Sep 27 05:28 env.txt
-rwx------ 1 root root 1998 Sep 27 05:28 export2.txt
-rwx------ 1 root root 1998 Sep 27 05:28 export.txt
-rwx------ 1 root root   28 Sep 27 05:28 fuck.txt
-rwx------ 1 root root 5373 Sep 27 05:28 set.txt
-rw-r--r-- 1 root root   21 Sep 27 05:28 s.txt
-rw-r--r-- 1 root root   35 Sep 27 10:13 t.txt
[root@andes.com ~/tmp/dir]#ls -l |xargs -t -L 2
/bin/echo total 44 -rwx------ 1 root root 1026 Sep 27 05:28 data.txt
total 44 -rwx------ 1 root root 1026 Sep 27 05:28 data.txt
/bin/echo -rw-r--r-- 1 root root 1047 Sep 27 05:28 d.txt -rwx------ 1 root root 2009 Sep 27 05:28 env2.txt
-rw-r--r-- 1 root root 1047 Sep 27 05:28 d.txt -rwx------ 1 root root 2009 Sep 27 05:28 env2.txt
/bin/echo -rwx------ 1 root root 2009 Sep 27 05:28 env.txt -rwx------ 1 root root 1998 Sep 27 05:28 export2.txt
-rwx------ 1 root root 2009 Sep 27 05:28 env.txt -rwx------ 1 root root 1998 Sep 27 05:28 export2.txt
/bin/echo -rwx------ 1 root root 1998 Sep 27 05:28 export.txt -rwx------ 1 root root 28 Sep 27 05:28 fuck.txt
-rwx------ 1 root root 1998 Sep 27 05:28 export.txt -rwx------ 1 root root 28 Sep 27 05:28 fuck.txt
/bin/echo -rwx------ 1 root root 5373 Sep 27 05:28 set.txt -rw-r--r-- 1 root root 21 Sep 27 05:28 s.txt
-rwx------ 1 root root 5373 Sep 27 05:28 set.txt -rw-r--r-- 1 root root 21 Sep 27 05:28 s.txt
/bin/echo -rw-r--r-- 1 root root 35 Sep 27 10:13 t.txt
-rw-r--r-- 1 root root 35 S

-n num  控制每次输入的参数个数

假设你希望使用 rm 命令（该命令将作为 xargs 命令的参数）删除文件。然而，rm 只能接受有限数量的参数。如果你的参数列表超出该限制怎么办？xargs 的 -n 选项限制单个命令行的参数个数。

[root@andes.com ~/tmp/dir]#ls
data.txt  d.txt  env2.txt  env.txt  export2.txt  export.txt  fuck.txt  set.txt  s.txt  t.txt
[root@andes.com ~/tmp/dir]#ls |xargs -t -n 2 file
file data.txt d.txt
data.txt: ISO-8859 text
d.txt:    ISO-8859 text
file env2.txt env.txt
env2.txt: ASCII text, with very long lines
env.txt:  ASCII text, with very long lines
file export2.txt export.txt
export2.txt: ASCII text, with very long lines
export.txt:  ASCII text, with very long lines
file fuck.txt set.txt
fuck.txt: ASCII text
set.txt:  ASCII text, with very long lines
file s.txt t.txt
s.txt: ASCII text
t.txt: ASCII text
[root@andes.com ~/tmp/dir]#

-E  EOF 指定输入结束符

[root@andes.com ~/tmp/dir]#cat t.txt
tata-hi -fuck - ok
fuck _you _ you
[root@andes.com ~/tmp/dir]#xargs -a t.txt -E _
tata-hi -fuck - ok fuck _you
[root@andes.com ~/tmp/dir]#xargs -a t.txt -E -
tata-hi -fuck

-r  xargs 默认是空参数comand也要执行一次，如使用-r参数遇到空参数则直接退出，不会再执行一次，避免程序执行错误。

[root@andes.com ~/tmp/dir]#ls
data.txt  d.txt  env2.txt  env.txt  export2.txt  export.txt  fuck.txt  set.txt  s.txt  t.txt
[root@andes.com ~/tmp/dir]#ls |grep Tata
[root@andes.com ~/tmp/dir]#ls |grep Tata |xargs -t
/bin/echo

[root@andes.com ~/tmp/dir]#ls |grep Tata |xargs -t -r
[root@andes.com ~/tmp/dir]#

------------------------------------------------------------------------------------------------

MAN 手册

XARGS(1)       XARGS(1)


NAME
       xargs - build and execute command lines from standard input


SYNOPSIS
       xargs  [-0prtx] [-E  eof-str] [-e[eof-str]] [--eof[=eof-str]] [--null]
       [-d delimiter] [--delimiter delimiter]  [-I  replace-str]  [-i[replace-
       str]] [--replace[=replace-str]]   [-l[max-lines]]   [-L   max-lines]
       [--max-lines[=max-lines]] [-n max-args] [--max-args=max-args] [-s  max-
       chars]  [--max-chars=max-chars] [-P max-procs] [--max-procs=max-procs]
       [--interactive]     [--verbose]     [--exit]   [--no-run-if-empty]
       [--arg-file=file]   [--show-limits]   [--version]   [--help]   [command
       [initial-arguments]]


DESCRIPTION
       This manual page documents the GNU version of xargs.  xargs reads items
       from  the  standard  input, delimited by blanks (which can be protected
       with double or single quotes or a backslash) or newlines, and  executes
       the  command (default is /bin/echo) one or more times with any initial-
       arguments followed by items read from standard input.  Blank  lines  on
       the standard input are ignored.


       Because Unix  filenames can contain blanks and newlines, this default
       behaviour is often problematic; filenames containing blanks and/or new-
       lines  are  incorrectly processed by xargs.  In these situations it is
       better to use the -0 option, which prevents such problems.   When using
       this option you will need to ensure that the program which produces the
       input for xargs also uses a null character as  a separator.   If  that
       program is GNU find for example, the -print0 option does this for you.


       If any invocation of the command exits with a status of 255, xargs will
       stop  immediately  without reading any further input.  An error message
       is issued on stderr when this happens.

OPTIONS
       --arg-file=file
       -a file
      Read items from file instead of standard input.  If you use this
      option,  stdin  remains unchanged when commands are run.Other-
      wise, stdin is redirected from /dev/null.


       --null
       -0     Input items are terminated by a null  character  insteadof  by
      whitespace,  and the quotes and backslash are not special (every
      character is taken literally).  Disables the end of file string,
      which  is treated  likeany other argument.  Useful when input
      items might contain white space, quote  marks,  or  backslashes.
      The  GNUfind  -print0  option produces input suitable for this
      mode.

       --delimiter=delim
       -d delim
      Input items are terminated by the specified  character.Quotes
      and  backslash  are not special; every character in the input is
      taken literally.Disables  the  end-of-file  string,  which  is
      treated  like any other argument. This can be used when the in-
      put consists of simply newline-separated items, althoughit  is
      almost  always better to design your program to use --null where
      this is possible. The specified delimiter may be a single char-
      acter,  a C-style  character  escape such as , or an octal or
      hexadecimal escape code.Octal and hexadecimal escape codes are
      understood as for the printf command.   Multibyte characters are
      not supported.

       -E eof-str
      Set the end of file string to  eof-str.If  the end  of  file
      string  occurs  as a line of input, the rest of the input is ig-
      nored.  If neither -E nor -e is used, no end of file  string  is
      used.


       --eof[=eof-str]
       -e[eof-str]
      This option is a synonym for the -E option.  Use -E instead, be-
      cause it is POSIX compliant while this option is not.   If  eof-
      str  is  omitted, there is no end of file string. If neither -E
      nor -e is used, no end of file string is used.


       --help Print a summary of the options to xargs and exit.

       -I replace-str
      Replace occurrences of replace-str in the initial-arguments with
      names  read  from standard input.  Also, unquoted blanks do not
      terminate input items; instead  the  separator  is  the  newline
      character.  Implies -x and -L 1.


       --replace[=replace-str]
       -i[replace-str]
      This  option  isa  synonym for -Ireplace-str if replace-str is
      specified, and for -I{} otherwise.  This option  is  deprecated;
      use -I instead.

       -L max-lines
      Use  at  most  max-linesnonblank input lines per command line.
      Trailing blanks cause an input line to be logically continued on
      the next input line.  Implies -x.

       --max-lines[=max-lines]
       -l[max-lines]
      Synonym for the -L option.  Unlike -L, the max-lines argument is
      optional. If max-lines is not specified, it  defaults  to  one.
      The  -l  option is deprecated since the POSIX standard specifies
      -L instead.


       --max-args=max-args
       -n max-args
      Use at most max-args arguments per  command  line.   Fewer  than
      max-argsarguments will be used if the size (see the -s option)
      is exceeded, unless the -x option is given, in which case xargs
      will exit.


       --interactive
       -p     Prompt  the user about whether to run each command line and read
      a line from the terminal. Only run the command line if the  re-
      sponse starts with ‘y’ or ‘Y’.  Implies -t.


       --no-run-if-empty
       -r     If the standard input does not contain any nonblanks, do not run
      the command.  Normally, the command is run once even if there is
      no input. This option is a GNU extension.


       --max-chars=max-chars
       -s max-chars
      Use at most max-chars characters per command line, including the
      command and initial-arguments and the terminating nulls  at  the
      ends of the argument strings.  The largest allowed value is sys-
      tem-dependent, and is calculated as the  argument length limit
      for  exec, less the size of your environment, less 2048 bytes of
      headroom. If this value is more than 128KiB, 128Kib is used  as
      the  default value; otherwise, the default value is the maximum.
      1KiB is 1024 bytes.


       --verbose
       -t     Print the command line on the standard error output before  exe-
      cuting it.

       --version
      Print the version number of xargs and exit.

       --show-limits
      Display  the limits on the command-line length which are imposed
      by the operating system, xargs’ choice of buffer size and the -s
      option. Pipe  the  input  from /dev/null (and perhaps specify
      --no-run-if-empty) if you don’t want xargs to do anything.

       --exit
       -x     Exit if the size (see the -s option) is exceeded.


       --max-procs=max-procs
       -P max-procs
      Run up to max-procs processes at a time; the default is  1.   If
      max-procs is 0, xargs will run as many processes as possible at
      a time.  Use the -n option with -P; otherwise chances  are  that
      only one exec will be done.

EXAMPLES
       find /tmp -name core -type f -print | xargs /bin/rm -f


       Find  files  named core in or below the directory /tmp and delete them.
       Note that this will work incorrectly if there are  any  filenames  con-
       taining newlines or spaces.


       find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f


       Find  files  named core in or below the directory /tmp and delete them,
       processing filenames in such a way that file ordirectory  names  con-
       taining spaces or newlines are correctly handled.

       find /tmp -depth -name core -type f -delete


       Find  files  named core in or below the directory /tmp and delete them,
       but more efficiently than in the previous example (because we avoid the
       need  to use fork(2) and exec(2) to launch rm and we don’t need the ex-
       tra xargs process).

       cut -d: -f1 < /etc/passwd | sort | xargs echo

       Generates a compact listing of all the users on the system.

       xargs sh -c ’emacs "$@" < /dev/tty’ emacs


       Launches the minimum number of copies of Emacs needed,  one  after  the
       other, to edit the files listed on xargs’ standard input.  This example
       achieves the same effect as BSD’s -o option, but in a more flexible and
       portable way.



EXIT STATUS
       xargs exits with the following status:
       0 if it succeeds
       123 if any invocation of the command exited with status 1-125
       124 if the command exited with status 255
       125 if the command is killed by a signal
       126 if the command cannot be run
       127 if the command is not found
       1 if some other error occurred.


       Exit  codes  greater  than 128 are used by the shell to indicate that a
       program died due to a fatal signal.


STANDARDS CONFORMANCE
       As of GNU xargs version 4.2.9, the default behaviour of xargs is not to
       have  a logical end-of-file marker.  POSIX (IEEE Std 1003.1, 2004 Edi-
       tion) allows this.


       The -l and -i options appear in the 1997 version of the POSIX standard,
       but  do not appear in the 2004 version of the standard. Therefore you
       should use -L and -I instead, respectively.


       The POSIX standard allows implementations to have a limit on  the  size
       of arguments to the exec functions.  This limit could be as low as 4096
       bytes including the  size  of  the  environment.  For  scriptsto  be
       portable,  they must not rely on a larger value. However, I know of no
       implementation whose actual limit is that small. The --show-limits op-
       tion  can be used to discover the actual limits in force on the current
       system.






SEE ALSO
       find(1), locate(1), locatedb(5), updatedb(1), fork(2), execvp(3), Find-
       ing Files (on-line in Info, or printed)


BUGS
       The  -L optionis incompatible with the -I option, but perhaps should
       not be.


       It is not possible for xargs to be used securely, since there will  al-
       ways  be  a  time gap between the production of the list of input files
       and their use in the commands that xargs issues. If other  users  have
       access  to  the system, they can manipulate the filesystem during this
       time window to force the action of the commands xargs runs to apply  to
       files  that  you didn’t intend. For a more detailed discussion of this
       and related problems, please refer to the  ‘‘Security  Considerations’’
       chapter in the findutils Texinfo documentation.The -execdir option of
       find can often be used as a more secure alternative.


       When you use the -I option, each line read from the input  is  buffered
       internally.    This means that there is an upper limit on the length of
       input line that xargs will accept when used with the  -I  option.   To
       work  around this limitation, you can use the -s option to increase the
       amount of buffer space that xargs uses, and you can also use  an extra
       invocation  of  xargs to ensure that very long lines do not occur.  For
       example:


       somecommand | xargs -s 50000 echo | xargs -I ’{}’ -s 100000 rm ’{}’


       Here, the first invocation of xargs has no input line length limit  be-
       cause  it  doesn’t  use the -i option.The second invocation of xargs
       does have such a limit, but we have ensured that the itnever  encoun-
       ters  a line which is longer than it can handle.  This is not an ideal
       solution.  Instead, the -i option should not impose a line length  lim-
       it,  which  is  why  this  discussion appears in the BUGS section.  The
       problem doesn’t occur with the output of find(1) because it emits  just
       one filename per line.


       The  best  way  to  report  a  bug  is to use the form at http://savan-
       nah.gnu.org/bugs/?group=findutils.  The reason for  this is  that  you
       will then be able to track progress in fixing the problem.   Other com-
       ments about xargs(1) and about the findutils package in general can  be
       sent  to  the bug-findutils mailing list.  To join the list, send email
       to bug-findutils-request@gnu.org.






      XARGS(1)