linux: xargs docs

 

xargs  docs

 

 

 

 

[root@rockylinux shell]# man  xargs

 

XARGS(1)                                                  General Commands Manual                                                  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]  [--process-slot-var=name] [--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 spec‐
       ified  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 containing 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 input.  An  er‐
       ror message is issued on stderr when this happens.

OPTIONS
       -0, --null
              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.

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

       --delimiter=delim, -d delim
              Input  items are terminated by the specified character.  The specified delimiter may be a single character, a C-style charac‐
              ter 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.  When processing the input, quotes and backslash are not special;
              every character in the input is taken literally.  The -d option disables any end-of-file string, which is  treated  like  any
              other argument.  You can use this option when the input consists of simply newline-separated items, although it is almost al‐
              ways better to design your program to use --null where this is possible.

       -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.

       -e[eof-str], --eof[=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.

       -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.

       -i[replace-str], --replace[=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.

       -l[max-lines], --max-lines[=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.

       -n max-args, --max-args=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.

       -P max-procs, --max-procs=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.   While  xargs
              is  running, you can send its process a SIGUSR1 signal to increase the number of commands to run simultaneously, or a SIGUSR2
              to decrease the number.  You cannot increase it above an implementation-defined limit (which is  shown  with  --show-limits).
              You  cannot  decrease it below 1.  xargs never terminates its commands; when asked to decrease, it merely waits for more than
              one existing command to terminate before starting another.

              Please note that it is up to the called processes to properly manage parallel access to shared resources.   For  example,  if
              more  than one of them tries to print to stdout, the ouptut will be produced in an indeterminate order (and very likely mixed
              up) unless the processes collaborate in some way to prevent this.  Using some kind of locking scheme is one  way  to  prevent
              such problems.  In general, using a locking scheme will help ensure correct output but reduce performance.  If you don't want
              to tolerate the performance difference, simply arrange for each process to produce a separate output file (or  otherwise  use
              separate resources).

       -p, --interactive
              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.

       --process-slot-var=name
              Set the environment variable name to a unique value in each running child process.  Values are reused  once  child  processes
              exit.  This can be used in a rudimentary load distribution scheme, for example.

       -r, --no-run-if-empty
              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.

       -s max-chars, --max-chars=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 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.

       --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.

       -t, --verbose
              Print the command line on the standard error output before executing it.

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

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

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

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 file‐
       names 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.   Howev‐
       er,  I know of no implementation whose actual limit is that small.  The --show-limits option can be used to discover the actual lim‐
       its in force on the current system.

SEE ALSO
       find(1), locate(1), locatedb(5), updatedb(1), fork(2), execvp(3), kill(1), signal(7),

       The  full documentation for xargs is maintained as a Texinfo manual.  If the info and xargs programs are properly installed at  your
       site, the command info xargs should give you access to the complete manual.

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 de‐
       tailed 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 because it doesn't use the -i option.  The second  invocation  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 gener‐
       al can be sent to the bug-findutils mailing list.  To join the list, send email to bug-findutils-request@gnu.org.

                                                                                                                                   XARGS(1)

 

 

 

 

一、特别说明：

 

 

　　1、 -d x：表示以“x”为分隔符，将传入的“字符串”重新分割为若干个“字符串”。默认分隔符为（空格/制表符/回车）。

 

 

　　2、 -n  k：（分组参数）将xargs分割后的“u”个字符串重新分为若干组，每组最多为“k”个字符串；分组数量v=[u/k]+1；"k"本质为执行一次后续命令需要的参数个数；后续命令会执行v次（v为分组数）。说明： 若前序命令传入"6"个字符串，每组做多为“4”个字符串，分组数量为v=[6/4]+1=1+1=2([]，为取整运算)，则前面命令传入6个参数则后续命令执行2次。

 

 

　　3、举例和说明：

　　　　3.1、"xargs -d -"：将"cat tmp.txt"传入的字符串重新分割，以“-”为分隔符；分割为6个字符串（nnnn，uuuu，mmmm，bbbb，eeee，rrrr）；

　　　　3.2、"-n 4"：将“xargs分割后的字符串”分组，每4个字符串为一组，共计2组（nnnn，uuuu，mmm，bbbb；     eeee，rrrr）；

　　　　3.3、ep.sh：将两组参数分别传给“bash脚本 ep.sh”

 

 

[root@rockylinux tmp]# cat ep.sh 
#!/usr/bin/bash


for v in "$@"
do
        echo "input_para=" $v
done

echo "-- input_para --"
echo "0=" $0
echo "1=" $1
echo "2=" $2
echo "3=" $3
echo "4=" $4
echo "5=" $5
echo "6=" $6
echo "7=" $7
echo "8=" $8
echo "9=" $9
echo "10=" ${10}
echo "-- end --"
[root@rockylinux tmp]# cat tmp.txt 
nnnn-uuuu-mmmm-bbbb-eeee-rrrr
[root@rockylinux tmp]# cat tmp.txt | xargs -d - -n 4  ./ep.sh 
input_para= nnnn
input_para= uuuu
input_para= mmmm
input_para= bbbb
-- input_para --
0= ./ep.sh
1= nnnn
2= uuuu
3= mmmm
4= bbbb
5=
6=
7=
8=
9=
10=
-- end --
input_para= eeee
input_para= rrrr
-- input_para --
0= ./ep.sh
1= eeee
2= rrrr
3=
4=
5=
6=
7=
8=
9=
10=
-- end --
[root@rockylinux tmp]#