【整理】Linux命令xargs的用法和参数详解

【背景】

折腾:

【已解决】利用linux管道或重定向将find出来的文件送给rm删除

期间,需要去搞清楚xargs的用法,参数的含义。

【解决过程】

1.去看看help的输出:

Administrator@PC-20130611GART /cygdrive/e/Dev_Root/docbook/dev/books
$ xargs --help
用法: xargs [-0prtx] [--interactive] [--null] [-d|--delimiter=delim]
       [-E eof-str] [-e[eof-str]]  [--eof[=eof-str]]
       [-L max-lines] [-l[max-lines]] [--max-lines[=max-lines]]
       [-I replace-str] [-i[replace-str]] [--replace[=replace-str]]
       [-n max-args] [--max-args=max-args]
       [-s max-chars] [--max-chars=max-chars]
       [-P max-procs]  [--max-procs=max-procs] [--show-limits]
       [--verbose] [--exit] [--no-run-if-empty] [--arg-file=file]
       [--version] [--help] [command [initial-arguments]]

将错误报告发送到 <bug-findutils@gnu.org>。

但是还是不知道0,p,r等参数的含义。

2.去试试help某个参数,是否可以看到帮助信息:

Administrator@PC-20130611GART /cygdrive/e/Dev_Root/docbook/dev/books
$ xargs help 0

结果没用。

3.然后看了:

wiki xargs

大概明白为何要有xargs了:

正常情况下,linux中的很多命令和工具,比如grep和awk,是可以通过管道,获得内容,当做输入参数的

但是对于cp,echo等命令,不识别从(终端中的)标准输入所获得的内容,

而识别,命令后面的内容的,当做参数。

且Linux 2.6.23之前,任意长(足够长,太长)的参数命令,是无法输入到命令中的

所以,才有了xargs:

将(足够长的,太长的)输入,分割成一个参数子列表

子列表中的参数,长度就很短,就可以被接受了。

人家举的例子:

比如:

rm /path/*

或:

rm `find /path -type f`

当文件太多时,就会报错:

Argument list too long

剩下的,就不去多研究了。

4.关于xargs,后来想到,应该可以通过man查看到帮助。

果然可以:

输入

man xargs

可以找到参数含义的说明:

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.

       The command line for command is built up until it reaches a system-defined limit (unless  the  -n  and  -L  options  are
       used).  The specified command will be invoked as many times as necessary to use up the list of input items.  In general,
       there will be many fewer invocations of command than there were items in the input.  This will normally have significant
       performance benefits.  Some commands can usefully be executed in parallel too; see the -P option.

       Because  Unix filenames can contain blanks and newlines, this default behaviour is often problematic; filenames contain-
       ing blanks and/or newlines 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 in-
       put.  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.  Otherwise, stdin is redirected from /dev/null.

       --null
       -0     Input  items  are  terminated  by a null character instead of by whitespace, and the quotes and backslash are not
              special (every character is taken literally).  Disables the end of file string, which is treated like  any  other
              argument.   Useful when input items might contain white space, quote marks, or backslashes.  The GNU find -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 input consists of simply newline-separated items, although it is almost always better to de-
              sign  your program to use --null where this is possible.  The specified delimiter may be a single character, a C-
              style character escape such as \n, 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 in-
              put is ignored.  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, because 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 is a synonym for -Ireplace-str if replace-str is specified.  If the replace-str argument is missing,
              the effect is the same as -I{}.  This option is deprecated; use -I instead.

       -L max-lines
              Use at most max-lines nonblank 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 de-
              faults 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-args arguments 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 response 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 terminat-
              ing nulls at the ends of the argument strings.  The largest allowed value is system-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.  xargs automatically adapts to tighter constraints.

       --verbose
       -t     Print the command line on the standard error output before executing 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 or the -L 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 containing 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  or
       directory names containing 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 extra 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 Edition) 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 scripts to 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 option 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), Finding Files (on-line in Info, or printed)

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

       It is not possible for xargs to be used securely, since there will always 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 Consid-
       erations'' chapter in the findutils Texinfo documentation.  The -execdir option of find can often be used as a more  se-
       cure 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 invoca-
       tion 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 because it doesn't use the -i option.  The second in-
       vocation  of  xargs does have such a limit, but we have ensured that the it never encounters 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 limit, 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://savannah.gnu.org/bugs/?group=findutils.  The reason  for  this
       is  that  you  will  then be able to track progress in fixing the problem.   Other comments 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-findu-
       tils-request@gnu.org.

                                                                                                                       XARGS(1)

从中可以看出各个参数的含义。

和xargs的功能:

(1)xargs从标准输入(standard input)中读取内容项,然后根据空格或换行将内容分割成子项;

并且执行对应的命令(默认执行的命令是echo:/bin/echo)

当然,给命令传递的参数,就是上面经过分割后所获得子项,即参数列表了;

空行,会被忽略,不会被当做参数。

(2)The command line for command is built up until it reaches a system-defined limit (unless  the  -n  and  -L  options  are

used).  The specified command will be invoked as many times as necessary to use up the list of input items.  In general,

there will be many fewer invocations of command than there were items in the input.  This will normally have significant

performance benefits.  Some commands can usefully be executed in parallel too; see the -P option.

(3)Because  Unix filenames can contain blanks and newlines, this default behaviour is often problematic; filenames contain-

ing blanks and/or newlines 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.

(4)If any invocation of the command exits with a status of 255, xargs will stop immediately without reading any further in-

put.  An error message is issued on stderr when this happens.

 

【总结】

  • -0==–null:输入项,是以null字符结尾的,而不是以白空格(whitespace)结尾的
    • 且引号和反斜杠,都不是特殊字符 –> 每个输入的字符,都视为普通字符
    • 禁止掉文件结束符 –> 被视为别的参数
    • 当输入项可能包含白空格,引号,反斜杠等情况时,才适合用此参数
    • 使用find命令加上-print0时,所产生的输出,正好适合和此-0参数  -> 往往是用find带-print0时,加上此xargs -0,两者搭配使用

其他参数,就不多解释了。

还是看英文解释吧。



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