Unix - “xargs” - output “in the middle” (not at th

Question

example use of xargs application in Unix can be something like this:

ls | xargs echo

which is the same as (let's say I have someFile and someDir/ in the working directory):

echo someFile someDir

so xargs take its input and place it at the end of the next command (here at the end of echo).

But sometimes I want xargs to place its input somewhere in the middle of next command.

For example:

find . -type f -name "*.cpp" -print | xargs g++ -o outputFile

so if I had in the current directory files a.cpp, b.cpp, c.cpp the output would be the same as with the command:

g++ -o outputFile a.cpp b.cpp c.cpp

but I want to have something like this:

g++ a.cpp b.cpp c.cpp -o outputFile

Is there a way to do it?

P.S.: I need it in some cases, because e.g.:

i586-mingw32msvc-g++ -o outputFile `pkg-config --cflags --libs gtkmm-2.4` a.cpp b.cpp c.cpp

doesn't work but this one works fine:

i586-mingw32msvc-g++ a.cpp b.cpp c.cpp -o outputFile `pkg-config --cflags --libs gtkmm-2.4`

Answer 1

GNU Parallel http://www.gnu.org/software/parallel/ would be a solution too:

find . -type f -name "*.cpp" -print | parallel -X g++ {} -o outputFile

Answer 2

To answer the original question asked in the title of how to use xargs with the input in the middle rather than the end:

$ echo a b c | xargs -I {} echo before {} after
before a b c after

This replaces {} in the command with the piped output. There are some subtle differences between BSD and GNU xargs described below:

BSD xargs (e.g. on MacOS/Darwin, freebsd)

Use -I REPLACE, which will replace the string REPLACE (or whatever you pass) in the command. For example:

$ echo a b c | xargs -I {} echo before {} after
before a b c after
$ echo a b c | xargs -I REPLACE echo before REPLACE after
before a b c after
$ echo 'a
> b
> c' | xargs -L1 -I {} echo before {} after
before a after
before b after
before c after

The man page describes the option:

 -I replstr
     Execute utility for each input line, replacing one or more occur-
     rences of replstr in up to replacements (or 5 if no -R flag is
     specified) arguments to utility with the entire line of input.
     The resulting arguments, after replacement is done, will not be
     allowed to grow beyond replsize (or 255 if no -S flag is speci-
     fied) bytes; this is implemented by concatenating as much of the
     argument containing replstr as possible, to the constructed argu-
     ments to utility, up to replsize bytes.  The size limit does not
     apply to arguments to utility which do not contain replstr, and
     furthermore, no replacement will be done on utility itself.
     Implies -x.

GNU xargs (e.g. on Linux)

$ echo a b c | xargs -i echo before {} after
before a b c after
$ echo a b c | xargs -I THING echo before THING after
before a b c after

Use either the -I REPLACE or the the -i argument, which the man page describes:

   -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.

The -L 1 on -I means that it will execute each of the input in a separate command:

$ echo "a
> b
> c" | xargs -I THING echo before THING after
before a after
before b after
before c after

(-i does not have this effect, though is apparently deprecated.)

Answer 3

If your version of xargs doesn't include the -I feature, an alternative is to write a little shell script containing the command you want to execute:

#!/bin/sh
exec i586-mingw32msvc-g++ "$@" -o outputFile...

Then use xargs to run that:

find . -type f -name "*.cpp" -print | xargs my_gcc_script

Answer 4

You do not need xargs for this. Just use:

g++ `find . -type f -name '*.cpp'` -o outputFile