создавать и выполнять командные строки из стандартного ввода (build and execute command lines from standard input)
Имя (Name)
xargs - build and execute command lines from standard input
Синопсис (Synopsis)
xargs [options] [command [initial-arguments]]
Описание (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 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 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 error message is issued on stderr when this happens.
Параметры (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. Otherwise, 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 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
consists of simply newline-separated items, although it is
almost always 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
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 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 output 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).
-o, --open-tty
Reopen stdin as /dev/tty in the child process before
executing the command. This is useful if you want xargs
to run an interactive application.
-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.
The options --max-lines (-L, -l), --replace (-I, -i) and --max-
args (-n) are mutually exclusive. If some of them are specified
at the same time, then xargs will generally use the option
specified last on the command line, i.e., it will reset the value
of the offending option (given before) to its default value.
Additionally, xargs will issue a warning diagnostic on stderr.
The exception to this rule is that the special max-args value 1
('-n1') is ignored after the --replace option and its aliases -I
and -i, because it would not actually conflict.
Примеры (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.
Статус выхода (Exit)
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.
Стандарты (Conforming to)
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 -o option is an extension to the POSIX standard for better
compatibility with BSD.
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.
Ошибки (баги) (Bugs)
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 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.
Сообщение об ошибках (Reporting bugs)
GNU findutils online help:
<https://www.gnu.org/software/findutils/#get-help>
Report any translation bugs to
<https://translationproject.org/team/>
Report any other issue via the form at the GNU Savannah bug
tracker:
<https://savannah.gnu.org/bugs/?group=findutils>
General topics about the GNU findutils package are discussed at
the bug-findutils mailing list:
<https://lists.gnu.org/mailman/listinfo/bug-findutils>
Смотри также (See also)
find(1), kill(1), locate(1), updatedb(1), fork(2), execvp(3),
locatedb(5), signal(7)
Full documentation <https://www.gnu.org/software/findutils/xargs>
or available locally via: info xargs
