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xargsconstruct and execute command lines |
Command |
xargs
[-I placeholder]
[-i[placeholder]]
[-L number]
[-l[number]]
[-n number]
[-ptx]
[-E [eofstr]]
[-e[eofstr]]
[-s size]
[command [argument ...] ]
xargs command line typically contains the
skeleton or template of another command. This
template looks like a normal command, except that it is lacking some arguments.
xargs adds arguments from the standard input to complete the
command, then executes the resulting command. If more input remains, it repeats
this process.
xargs gets the needed arguments from the standard input.
Different options tell how the standard input is to be interpreted to obtain
these arguments.
-I placeholdercauses xargs to consider each full line in the
standard input to be a single argument. The placeholder following the
-I is a string that can appear multiple times in the command
template. xargs strips the input line of any leading white
space characters and inserts it in place of the placeholder string. For
example, with
xargs -I '{}' mv dir1/'{}' dir2/'{}'
the standard input should consist of lines giving names of files that
you want moved from dir1 to dir2.
xargs substitutes these names for the {}
placeholder in each place that it appears in the command template.
When xargs creates arguments for the template command, no
single argument can be longer than 255 characters after the input has replaced
the placeholders. The -x option (described below) is
automatically in effect if -I or -i is
used.
-i[placeholder]behaves like -I, except that the placeholder
is optional; if you omit the placeholder string, it defaults to the
string {}. Thus, the previous example could be written as
xargs -i mv dir1/'{}' dir2/'{}'
xargs -i'{}'/ mv dir1/'{}' dir2/'{}'
-L numberreads number lines from the standard input and concatenates them
into one long string (with a blank separating each of the original lines).
xargs then appends this string to the command template and
executes the resulting command. This process is repeated until
xargs reaches to the end of the standard input. If there are
fewer than number lines left in the file the last time the command is
executed, xargs uses whatever is available.
With this option, a line must contain at least one non-blank character; blank
lines are skipped and do not count towards the number of lines being added to
the template. xargs considers a line to end at the first
newline character, unless the last character of the line is a blank or a tab,
in which case, the current line is considered to extend to the end of the next
non-empty line.
If you omit the -L option, the default number of lines read
from standard input is 1. The -x option (described in a
following paragraph) is automatically in effect if -L or
-l is used.
-lnumberacts like the -L option, but the number
argument is optional. number defaults to 1.
-n numberreads a maximum of number arguments from the standard input and puts them on the end of the command template. For example, in
xargs -n 2 diff
obtains two arguments from the standard input, appends them to the
diff command, and executes the
command. When you use this option, xargs considers arguments
to be strings of characters separated from each other by white space characters
(blanks, horizontal tabs, or newlines). Empty lines are always skipped (that is,
they don't count as arguments).
xargs reads fewer than number arguments if:
-s option (or {LINE_MAX} if you did not
specify -s)"), you must enclose the argument in apostrophes.
Conversely, if the argument contains an apostrophe ('), you must
enclose the argument in double quotes. You can also put a backslash
(\) in front of a character to tell xargs to
ignore any special meaning the character may have (for example, white space
characters, or quotes).
Typically, an xargs command must use exactly one of the
preceding options. If you specify more than one, xargs uses
the one that appears last on the command line. If you do not specify any of
these options, xargs keeps reading input until it fills up
its internal buffer, concatenating arguments onto the end of the command
template. When the buffer is full, xargs executes the
resulting command, then starts constructing a new command. For example,
prints the names of files in the current directory as one long line. When you invokels | xargs echo
xargs in this way, the total length of all arguments
must be less than the size specified by the -s option.
If no command template appears on the command line, xargs
uses echo by default. When
xargs executes a command, it uses your search rules to find
the command; this means that you can run shell scripts as well as normal
programs.
Note that the command you want to execute should be in your search
PATH.
xargs terminates prematurely if it cannot execute a
constructed command or if an executed command returns a non-zero status value.
If an executed command is a shell program, it should explicitly contain an
exit command to avoid returning a
non-zero status value by accident; see
sh for details.
You can use the following options with any of the three main options.
-E eofstrdefines eofstr to represent end-of-file on the standard input. For example,
-E :::
tells xargs that ::: represents the
end of the standard input, even if it is not at the end of the file. If there is
no -E or -e option, a single underscore
(_) marks the end of the input.
-e[eofstr]acts like -E, but the eofstr argument is
optional. If you specify -e without eofstr, there is
no end-of-file marker string, and _ is taken literally instead of
as an end-of-file marker. xargs stops reading input when it
reaches the specified end-of-file marker or the true end of the file.
-pprompts you before each command. This turns on the
-t option so that you see each constructed command before it
is executed. Then, xargs displays ?..., asking
if you really want to executed this command. If you type a string beginning with
y, xargs goes ahead and executes the command as
displayed; otherwise, the command is not executed, and xargs
goes on to construct a new command.
-s sizesets the maximum allowable size of an argument list to size
characters (where size is an integer). The value of size must be
less than or equal to the system variable LINE_MAX. The length of
the argument list is the length of the entire constructed command; this includes
the length of the command name, the length of each argument, plus one blank for
separating each item on the line.
-twrites each constructed command to the standard error just before executing the command.
-xterminates xargs if it creates a command that is
longer than the size given by the -s option (or
{LINE_MAX} if you did not specify -s). This
option comes into effect automatically if you specify -i or
-l.
ls | xargs -n 3 echo
PATHcontains a list of directories that constitute your search path.
0Successful completion of all commands.
1-125Failure due to any of the following:
xargs could not assemble a command line126xargs found command but could not invoke
it.
127xargs could not find command.
-E, -e, -I,
-i, -l, -L, and
-p options are extensions to the POSIX standard.
The x/OPEN standard states that the -I and
-i options are limited to five arguments, and that the total
size of the constructed arguments cannot exceed 255 bytes. As an extension to
the standard, these limits do not apply to this implementation of
xargs. There is no limit on the number of arguments for
-I; this is an extension to the x/OPEN standard.
LINE_MAX characters.