How to use > in an xargs command?
I want to find a bash command that will let me grep every file in a directory and write the output of that grep to a separate file. My guess would have been to do something like this
ls -1 | xargs -I{} "grep ABC '{}' > '{}'.out"
but, as far as I know, xargs doesn't like the double-quotes. If I remove the double-quotes, however, then the command redirects the output of the entire command to a single file called '{}'.out instead of to a series of individual files.
Does anyone know of a way to do this using xargs? I just used this grep scenario as an example to illustrate my problem with xargs so any solutions that don't use xargs aren't as applicable for me.
Do not make the mistake of doing this:
sh -c "grep ABC {} > {}.out"
This will break under a lot of conditions, including funky filenames and is impossible to quote right. Your {} must always be a single completely separate argument to the command to avoid code injection bugs. What you need to do, is this:
xargs -I{} sh -c 'grep ABC "$1" > "$1.out"' -- {}
Applies to xargs as well as find .
By the way, never use xargs without the -0 option (unless for very rare and controlled one-time interactive use where you aren't worried about destroying your data).
Also don't parse ls . Ever. Use globbing or find instead: http://mywiki.wooledge.org/ParsingLs
Use find for everything that needs recursion and a simple loop with a glob for everything else:
find /foo -exec sh -c 'grep "$1" > "$1.out"' -- {} ;
or non-recursive:
for file in *; do grep "$file" > "$file.out"; done
Notice the proper use of quotes.
A solution without xargs is the following:
find . -mindepth 1 -maxdepth 1 -type f -exec sh -c "grep ABC '{}' > '{}.out'" ;
...and the same can be done with xargs , it turns out:
ls -1 | xargs -I {} sh -c "grep ABC '{}' > '{}.out'"
Edit: single quotes added after remark by lhunath.
I assume your example is just an example and that you may need > for other things. GNU Parallel http://www.gnu.org/software/parallel/ may be your rescue. It does not need additional quoting as long as your filenames do not contain n:
ls | parallel "grep ABC {} > {}.out"
If you have filenames with n in it:
find . -print0 | parallel -0 "grep ABC {} > {}.out"
As an added bonus you get the jobs run in parallel.
Edit.
You can install GNU Parallel simply by:
wget http://git.savannah.gnu.org/cgit/parallel.git/plain/src/parallel
chmod 755 parallel
Watch the intro videos to learn more: http://pi.dk/1
10 seconds installation:
wget pi.dk/3 -qO - | sh -x
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