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xargs - construct argument lists and invoke utility
xargs
[-t][-p]][-E eofstr][-I
replstr][-L number][-n number
[-x]]
[-s size][utility
[argument...]]
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}.
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:
The command line length accumulated exceeds the size specified by the -s option (or {LINE_MAX} if there is no -s option).
The last iteration has fewer than number, but not zero, operands remaining.
The total number of arguments exceeds that specified by the -n option.
The total number of lines exceeds that specified by the -L option.
End-of-file is encountered on standard input before size bytes are accumulated.
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.
The following operands shall be supported:
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>.
The file /dev/tty shall be used to read responses required by the -p option.
The following environment variables shall affect the execution of xargs:
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.
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.
The following exit values shall be returned:
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.
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.
The following command combines the output of the parenthesised commands onto one line, which is then written to the end-of-file log:
(logname; date; printf "%s\n" "$0 $*") | xargs >>log
The following command invokes diff with successive pairs of arguments originally typed as command line arguments (assuming there are no embedded <blank>s in the elements of the original argument list):
printf "%s\n" "$*" | xargs -n 2 -x diff
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.
a. ls | xargs -p -L 1 ar -r arch b. ls | xargs -p -L 1 | xargs ar -r arch
The following executes with successive pairs of arguments originally typed as command line arguments:
echo $* | xargs -n 2 diff
On XSI-conformant systems, the following moves all files from directory $1 to directory $2, and echoes each move command just before doing it:
ls $1 | xargs -I {} -t mv $1/{} $2/{}
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.
Shell Command Language , echo , find , the System Interfaces volume of IEEE Std 1003.1-2001, exec
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