cat “input file is output file” while trying to combine multiple files into one

Question

The plan was to collect all source lines of Java into one file, recursively:

$ find . -name '*.java' | xargs cat >> all.java

But there was an error:

cat: ./all.java: input file is output file

The file all.java didn't exist before this command, so I thought that maybe xargs was trying to run cat >> all.java file1 file2 file3 ...

Answer 1

First of all, you can safely ignore that error. The command will run successfully and it will, correctly, ignore all.java itself. It is simply letting you know that it did so.

Anyway, to avoid the error you could use tee and find's exec option:

$ find . -name '*.java' -exec cat {} + | tee all.java

From man find:

   -exec command ;
          Execute  command;  true  if 0 status is returned.  All following
          arguments to find are taken to be arguments to the command until
          an  argument  consisting of `;' is encountered.  The string `{}'
          is replaced by the current file name being processed  everywhere
          it occurs in the arguments to the command, not just in arguments
          where it is alone, as in some versions of find. 

   -exec command {} +
          This  variant  of the -exec action runs the specified command on
          the selected files, but the command line is built  by  appending
          each  selected file name at the end; the total number of invoca‐
          tions of the command will  be  much  less  than  the  number  of
          matched  files. 

So, you can use -exec to tell find to run a command on each of its results. The {} is replaced with the actual file/directory name found. The + is just the marker that tells find that the command ends here. I use it instead of \; because it will run fewer commands since it will attempt to combine them into as few runs as possible.

Or use find's ! to exclude all.java:

$ find . -name '*.java' ! -name all.java -exec cat {} +  >> all.java

Or globbing:

$ shopt -s globstar
$ cat **/*.java > all.java

Answer 2

The problem was that first the file all.java was created, then find found it and fed it to cat, which didn't like it because it was outputting there.

Possible solution:

$ find . -name '*.java' | xargs cat >> all.j

Or:

$ find . -name '*.java' | xargs cat >> ../all.java