xargs: sestavit seznamy argumentů a vyvolat utilitu

Originální popis anglicky: xargs - construct argument lists and invoke utility

Návod, kniha: POSIX Programmer's Manual

STRUČNĚ

xargs [-t][-p]][-E eofstr ][-I replstr][-L number ][-n number [-x]]        [-s size][utility [argument...]]

POPIS / INSTRUKCE

The xargs utility shall construct a command line consisting of the utility and argument operands specified followed by as many arguments read in sequence from standard input as fit in length and number constraints specified by the options. The xargs utility shall then invoke the constructed command line and wait for its completion. This sequence shall be repeated until one of the following occurs:

*

An end-of-file condition is detected on standard input.

*

The logical end-of-file string (see the -E eofstr option) is found on standard input after double-quote processing, apostrophe processing, and backslash escape processing (see next paragraph).

*

An invocation of a constructed command line returns an exit status of 255.

The application shall ensure that arguments in the standard input are separated by unquoted <blank>s, unescaped <blank>s, or <newline>s. A string of zero or more non-double-quote ( ' )' characters and non- <newline>s can be quoted by enclosing them in double-quotes. A string of zero or more non-apostrophe ( '" ) characters and non- <newline>s can be quoted by enclosing them in apostrophes. Any unquoted character can be escaped by preceding it with a backslash. The utility named by utility shall be executed one or more times until the end-of-file is reached or the logical end-of file string is found. The results are unspecified if the utility named by utility attempts to read from its standard input. The generated command line length shall be the sum of the size in bytes of the utility name and each argument treated as strings, including a null byte terminator for each of these strings. The xargs utility shall limit the command line length such that when the command line is invoked, the combined argument and environment lists (see the exec family of functions in the System Interfaces volume of IEEE Std 1003.1-2001) shall not exceed {ARG_MAX}-2048 bytes. Within this constraint, if neither the -n nor the -s option is specified, the default command line length shall be at least {LINE_MAX}.

OPTIONS

The xargs utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines. The following options shall be supported:

-E  eofstr

Use eofstr as the logical end-of-file string. If -E is not specified, it is unspecified whether the logical end-of-file string is the underscore character ( '_' ) or the end-of-file string capability is disabled. When eofstr is the null string, the logical end-of-file string capability shall be disabled and underscore characters shall be taken literally.

-I  replstr

Insert mode: utility is executed for each line from standard input, taking the entire line as a single argument, inserting it in arguments for each occurrence of replstr. A maximum of five arguments in arguments can each contain one or more instances of replstr. Any <blank>s at the beginning of each line shall be ignored. Constructed arguments cannot grow larger than 255 bytes. Option -x shall be forced on.

-L  number

The utility shall be executed for each non-empty number lines of arguments from standard input. The last invocation of utility shall be with fewer lines of arguments if fewer than number remain. A line is considered to end with the first <newline> unless the last character of the line is a <blank>; a trailing <blank> signals continuation to the next non-empty line, inclusive. The -L and -n options are mutually-exclusive; the last one specified shall take effect.

-n  number

Invoke utility using as many standard input arguments as possible, up to number (a positive decimal integer) arguments maximum. Fewer arguments shall be used if:

-p

Prompt mode: the user is asked whether to execute utility at each invocation. Trace mode ( -t) is turned on to write the command instance to be executed, followed by a prompt to standard error. An affirmative response read from /dev/tty shall execute the command; otherwise, that particular invocation of utility shall be skipped.

-s  size

Invoke utility using as many standard input arguments as possible yielding a command line length less than size (a positive decimal integer) bytes. Fewer arguments shall be used if:

Values of size up to at least {LINE_MAX} bytes shall be supported, provided that the constraints specified in the DESCRIPTION are met. It shall not be considered an error if a value larger than that supported by the implementation or exceeding the constraints specified in the DESCRIPTION is given; xargs shall use the largest value it supports within the constraints.

-t

Enable trace mode. Each generated command line shall be written to standard error just prior to invocation.

-x

Terminate if a command line containing number arguments (see the -n option above)  or number lines (see the -L option above) will not fit in the implied or specified size (see the -s option above).

 

OPERANDS

The following operands shall be supported:

utility

The name of the utility to be invoked, found by search path using the PATH environment variable, described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables. If utility is omitted, the default shall be the echo utility. If the utility operand names any of the special built-in utilities in Special Built-In Utilities , the results are undefined.

argument

An initial option or operand for the invocation of utility.

 

STDIN

The standard input shall be a text file. The results are unspecified if an end-of-file condition is detected immediately following an escaped <newline>.

INPUT FILES

The file /dev/tty shall be used to read responses required by the -p option.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of xargs:

LANG

Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL

If set to a non-empty string value, override the values of all the other internationalization variables.

LC_COLLATE

 

Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements used in the extended regular expression defined for the yesexpr locale keyword in the LC_MESSAGES category.

LC_CTYPE

Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments and input files) and the behavior of character classes used in the extended regular expression defined for the yesexpr locale keyword in the LC_MESSAGES category.

LC_MESSAGES

Determine the locale for the processing of affirmative responses and that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH

Determine the location of message catalogs for the processing of LC_MESSAGES .

PATH

Determine the location of utility, as described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables.

 

ASYNCHRONOUS EVENTS

Default.

STDOUT

Not used.

STDERR

The standard error shall be used for diagnostic messages and the -t and -p options. If the -t option is specified, the utility and its constructed argument list shall be written to standard error, as it will be invoked, prior to invocation. If -p is specified, a prompt of the following format shall be written (in the POSIX locale): at the end of the line of the output from -t.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

    0

All invocations of utility returned exit status zero.

1-125

A command line meeting the specified requirements could not be assembled, one or more of the invocations of utility returned a non-zero exit status, or some other error occurred.

  126

The utility specified by utility was found but could not be invoked.

  127

The utility specified by utility could not be found.

 

CONSEQUENCES OF ERRORS

If a command line meeting the specified requirements cannot be assembled, the utility cannot be invoked, an invocation of the utility is terminated by a signal, or an invocation of the utility exits with exit status 255, the xargs utility shall write a diagnostic message and exit without processing any remaining input. The following sections are informative.

APPLICATION USAGE

The 255 exit status allows a utility being used by xargs to tell xargs to terminate if it knows no further invocations using the current data stream will succeed. Thus, utility should explicitly exit with an appropriate value to avoid accidentally returning with 255. Note that input is parsed as lines; <blank>s separate arguments. If xargs is used to bundle output of commands like find dir -print or ls into commands to be executed, unexpected results are likely if any filenames contain any <blank>s or <newline>s. This can be fixed by using find to call a script that converts each file found into a quoted string that is then piped to xargs. Note that the quoting rules used by xargs are not the same as in the shell. They were not made consistent here because existing applications depend on the current rules and the shell syntax is not fully compatible with it. An easy rule that can be used to transform any string into a quoted form that xargs interprets correctly is to precede each character in the string with a backslash. On implementations with a large value for {ARG_MAX}, xargs may produce command lines longer than {LINE_MAX}. For invocation of utilities, this is not a problem. If xargs is being used to create a text file, users should explicitly set the maximum command line length with the -s option. The command, env, nice, nohup, time, and xargs utilities have been specified to use exit code 127 if an error occurs so that applications can distinguish "failure to find a utility" from "invoked utility exited with an error indication". The value 127 was chosen because it is not commonly used for other meanings; most utilities use small values for "normal error conditions'' and the values above 128 can be confused with termination due to receipt of a signal. The value 126 was chosen in a similar manner to indicate that the utility could be found, but not invoked. Some scripts produce meaningful error messages differentiating the 126 and 127 cases. The distinction between exit codes 126 and 127 is based on KornShell practice that uses 127 when all attempts to exec the utility fail with [ENOENT], and uses 126 when any attempt to exec the utility fails for any other reason.

PŘÍKLADY POUŽITÍ

1.

The following command combines the output of the parenthesised commands onto one line, which is then written to the end-of-file log:

 

2.

The following command invokes diff with successive pairs of arguments originally typed as command line arguments (assuming there are no embedded <blank>s in the elements of the original argument list):

 

3.

In the following commands, the user is asked which files in the current directory are to be archived. The files are archived into arch; a, one at a time, or b, many at a time.

 

4.

The following executes with successive pairs of arguments originally typed as command line arguments:

 

5.

On XSI-conformant systems, the following moves all files from directory $1 to directory $2, and echoes each move command just before doing it:

 

RATIONALE

The xargs utility was usually found only in System V-based systems; BSD systems included an apply utility that provided functionality similar to xargs -n number. The SVID lists xargs as a software development extension. This volume of IEEE Std 1003.1-2001 does not share the view that it is used only for development, and therefore it is not optional. The classic application of the xargs utility is in conjunction with the find utility to reduce the number of processes launched by a simplistic use of the find -exec combination. The xargs utility is also used to enforce an upper limit on memory required to launch a process. With this basis in mind, this volume of IEEE Std 1003.1-2001 selected only the minimal features required. Although the 255 exit status is mostly an accident of historical implementations, it allows a utility being used by xargs to tell xargs to terminate if it knows no further invocations using the current data stream shall succeed. Any non-zero exit status from a utility falls into the 1-125 range when xargs exits. There is no statement of how the various non-zero utility exit status codes are accumulated by xargs. The value could be the addition of all codes, their highest value, the last one received, or a single value such as 1. Since no algorithm is arguably better than the others, and since many of the standard utilities say little more (portably) than "pass/fail", no new algorithm was invented. Several other xargs options were withdrawn because simple alternatives already exist within this volume of IEEE Std 1003.1-2001. For example, the -i replstr option can be just as efficiently performed using a shell for loop. Since xargs calls an exec function with each input line, the -i option does not usually exploit the grouping capabilities of xargs. The requirement that xargs never produces command lines such that invocation of utility is within 2048 bytes of hitting the POSIX exec {ARG_MAX} limitations is intended to guarantee that the invoked utility has room to modify its environment variables and command line arguments and still be able to invoke another utility. Note that the minimum {ARG_MAX} allowed by the System Interfaces volume of IEEE Std 1003.1-2001 is 4096 bytes and the minimum value allowed by this volume of IEEE Std 1003.1-2001 is 2048 bytes; therefore, the 2048 bytes difference seems reasonable. Note, however, that xargs may never be able to invoke a utility if the environment passed in to xargs comes close to using {ARG_MAX} bytes. The version of xargs required by this volume of IEEE Std 1003.1-2001 is required to wait for the completion of the invoked command before invoking another command. This was done because historical scripts using xargs assumed sequential execution. Implementations wanting to provide parallel operation of the invoked utilities are encouraged to add an option enabling parallel invocation, but should still wait for termination of all of the children before xargs terminates normally. The -e option was omitted from the ISO POSIX-2:1993 standard in the belief that the eofstr option-argument was recognized only when it was on a line by itself and before quote and escape processing were performed, and that the logical end-of-file processing was only enabled if a -e option was specified. In that case, a simple sed script could be used to duplicate the -e functionality. Further investigation revealed that:

*

The logical end-of-file string was checked for after quote and escape processing, making a sed script that provided equivalent functionality much more difficult to write.

*

The default was to perform logical end-of-file processing with an underscore as the logical end-of-file string.

To correct this misunderstanding, the -E eofstr option was adopted from the X/Open Portability Guide. Users should note that the description of the -E option matches historical documentation of the -e option (which was not adopted because it did not support the Utility Syntax Guidelines), by saying that if eofstr is the null string, logical end-of-file processing is disabled. Historical implementations of xargs actually did not disable logical end-of-file processing; they treated a null argument found in the input as a logical end-of-file string. (A null string argument could be generated using single or double quotes ( '' or "" ). Since this behavior was not documented historically, it is considered to be a bug.

FUTURE DIRECTIONS

None.

SOUVISEJÍCÍ

Shell Command Language , echo , find , the System Interfaces volume of IEEE Std 1003.1-2001, exec

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .