xargs with stdin/stdout redirection

First of all, do not use ls output as a file list. Use shell expansion or find. See below for potential consequences of ls+xargs misuse and an example of proper xargs usage.

1. Simple way: for loop

If you want to process just the files under A/, then a simple for loop should be enough:

for file in A/*.dat; do ./a.out < "$file" > "${file%.dat}.ans"; done

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2._pre1 Why not   ls | xargs ?

Here's an example of how bad things may turn if you use ls with xargs for the job. Consider a following scenario:

• first, let's create some empty files:

  $ touch A/mypreciousfile.dat\ with\ junk\ at\ the\ end.dat
  $ touch A/mypreciousfile.dat
  $ touch A/mypreciousfile.dat.ans
  

• see the files and that they contain nothing:

  $ ls -1 A/
  mypreciousfile.dat
  mypreciousfile.dat with junk at the end.dat
  mypreciousfile.dat.ans
  
  $ cat A/*
  

• run a magic command using xargs:

  $ ls A/*.dat | xargs -I file sh -c "echo TRICKED > file.ans"
  

• the result:

  $ cat A/mypreciousfile.dat
  TRICKED with junk at the end.dat.ans
  
  $ cat A/mypreciousfile.dat.ans
  TRICKED
  

So you've just managed to overwrite both mypreciousfile.dat and mypreciousfile.dat.ans. If there were any content in those files, it'd have been erased.

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2. Using  xargs : the proper way with  find 

If you'd like to insist on using xargs, use -0 (null-terminated names) :

find A/ -name "*.dat" -type f -print0 | xargs -0 -I file sh -c './a.out < "file" > "file.ans"'

Notice two things:

1. this way you'll create files with .dat.ans ending;
2. this will break if some file name contains a quote sign (").

Both issues can be solved by different way of shell invocation:

find A/ -name "*.dat" -type f -print0 | xargs -0 -L 1 bash -c './a.out < "$0" > "${0%dat}ans"'

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3. All done within find ... -exec

 find A/ -name "*.dat" -type f -exec sh -c './a.out < "{}" > "{}.ans"' \;

This, again, produces .dat.ans files and will break if file names contain ". To go about that, use bash and change the way it is invoked:

 find A/ -name "*.dat" -type f -exec bash -c './a.out < "$0" > "${0%dat}ans"' {} \;

Use GNU Parallel:

parallel ./a.out "<{} >{.}.ans" ::: A/*.dat

Added bonus: You get the processing done in parallel.

Watch the intro videos to learn more: http://www.youtube.com/watch?v=OpaiGYxkSuQ

Try doing something like this (syntax may vary a bit depending on the shell you use):

$ for i in $(find A/ -name \*.dat); do ./a.out < ${i} > ${i%.dat}.ans; done