xargs学习笔记

[root@localhost ~]# echo "hello world" | echo

[root@localhost ~]# echo "hello world" | xargs echo
hello world

#大多数命令都不接受标准输入作为参数，只能直接在命令行输入参数

[root@localhost ~]# man xargs | tee -a man.xargs
XARGS(1) General Commands Manual XARGS(1)

NAME
xargs - build and execute command lines from standard input

#从标准输入构建和执行命令行

 xargs命令的作用，是将标准输入转为命令行参数。

[root@localhost ~]# touch a.js b.js c.js
[root@localhost ~]# find /root -name "*.js" | xargs ls -la
-rw-r--r--. 1 root root 0 Dec 23 21:57 /root/a.js
-rw-r--r--. 1 root root 0 Dec 23 21:57 /root/b.js
-rw-r--r--. 1 root root 0 Dec 23 21:57 /root/c.js

 

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 new‐
lines, 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.

#本手册页记录了GNU版本的xargs。xargs从标准输入中读取项目，用空格（可以用双引号、单引号或反斜杠保护）或新行分隔，并使用任何初始参数和从标准输入读取的项目执行一次或多次命令（默认值为/bin/echo）。标准输入上的空白行将被忽略。

 

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.

#命令的命令行会一直构建到达到系统定义的限制（除非使用-n和-L选项）。指定的命令将根据需要多次调用，以用完输入项列表。一般来说，命令的调用次数会比输入中的项目少得多。这通常会带来显著的性能优势。一些命令也可以并行执行；请参见-P选项。

 

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.

#因为Unix文件名可以包含空格和换行符，所以这种默认行为通常是有问题的；xargs会错误地处理包含空格和/或换行符的文件名。在这种情况下，最好使用-0选项，这样可以防止出现此类问题。使用此选项时，您需要确保为xargs生成输入的程序也使用空字符作为分隔符。例如，如果该程序是GNU find，则-print0选项可以为您完成此操作。

[root@localhost ~]# find /root -type f -print0 | xargs -0 ls

 

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.

#如果命令的任何调用以255的状态退出，xargs将立即停止，而不读取任何进一步的输入。发生这种情况时，stderr上会发出错误消息。

 

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.

#输入项以空字符而不是空格终止，引号和反斜杠也不特殊（每个字符都按字面意思）。禁用文件结尾字符串，该字符串与任何其他参数一样处理。当输入项可能包含空格、引号或反斜杠时很有用。GNU find-print0选项生成适合此模式的输入。

 

-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. Otherwise, stdin is redirected from /dev/null.

#从文件中读入作为 stdin

 

--delimiter=delim, -d delim
Input items are terminated by the specified character. 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. 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 con‐sists of simply newline-separated items,although it is almost always better to design your program to use --null where this is possible.

#默认情况下，xargs将换行符和空格作为分隔符，把标准输入分解成一个个命令行参数

#-d参数可以更改分隔符

[root@localhost ~]# echo -e "a\tb\tc\t"
a       b       c
[root@localhost ~]# echo -e "a\tb\tc\t" | xargs -d "\t" echo
a b c

 

-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 ignored. 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 in‐stead.

-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.

#如果标准输入包含多行，-L参数指定多少行作为一个命令行参数。

 

-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 spec‐ifies -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.

#-n num 后面加次数，表示命令在执行的时候一次用的argument的个数，默认是用所有的。

[root@localhost ~]# ls *.js | xargs -t -n2 ls -al
ls -al a.js b.js
-rw-r--r--. 1 root root 0 Dec 24 02:28 a.js
-rw-r--r--. 1 root root 0 Dec 24 02:28 b.js
ls -al c.js d.js
-rw-r--r--. 1 root root 0 Dec 24 02:28 c.js
-rw-r--r--. 1 root root 0 Dec 24 02:28 d.js

 

-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; other‐wise 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 SI‐GUSR2 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.

#-p参数打印出要执行的命令，询问用户是否要执行

[root@localhost ~]# echo 'one two three' | xargs -p rm
rm one two three ?...y

 

--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 spec‐
ify --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.

#-t 表示先打印命令，然后再执行

 

-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

#有些命令（比如rm）一旦参数过多会报错"参数列表过长"，而无法执行，改用xargs就没有这个问题，因为它对每个参数执行一次命令。
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.

#在/tmp目录或其下找到名为core的文件并将其删除。请注意，如果有任何文件名包含换行符或空格，这将无法正常工作。

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 actu‐
al limits 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 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 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 general can be sent to the bug-findutils mailing list. To join the list, send email to bug-findutils-request@gnu.org.

XARGS(1)