Is `xargs -t` output stderr or stdout, and can you control it?

Question

say i have a directory with hi.txt and blah.txt and i execute the following command on a linux-ish command line

ls *.* | xargs -t -i{} echo {}

the output you will see is

echo blah.txt
blah.txt
echo hi.txt
hi.txt

i'd like to redirect the stderr output (say 'echo blah.txt' fails...), leaving only the output from the xargs -t command written to std out, but it looks as if it's stderr as well.

ls *.* | xargs -t -i{} echo {} 2> /dev/null

Is there a way to control it, to make it output to stdout?

Answer 1

So I believe what you want is to have as stdout is

• the stdout from the utility that xargs executes
• the listing of commands generated by xargs -t

You want to ignore the stderr stream generated by the executed utility.

Please correct me if I'm wrong.

First, let's create a better testing utility:

% cat myecho
#!/bin/sh
echo STDOUT $@
echo STDERR $@ 1>&2
% chmod +x myecho
% ./myecho hello world
STDOUT hello world
STDERR hello world
% ./myecho hello world >/dev/null
STDERR hello world
% ./myecho hello world 2>/dev/null
STDOUT hello world
%

So now we have something that actually outputs to both stdout and stderr, so we can be sure we're only getting what we want.

A tangential way to do this is not to use xargs, but rather, make. Echoing a command and then doing it is kind of what make does. That's its bag.

% cat Makefile
all: $(shell ls *.*)

$(shell ls): .FORCE
  ./myecho $@ 2>/dev/null

.FORCE:
% make
./myecho blah.txt 2>/dev/null
STDOUT blah.txt
./myecho hi.txt 2>/dev/null
STDOUT hi.txt
% make >/dev/null
% 

If you're tied to using xargs, then you need to modify your utility that xargs uses so it surpresses stderr. Then you can use the 2>&1 trick others have mentioned to move the command listing generated by xargs -t from stderr to stdout.

% cat myecho2
#!/bin/sh
./myecho $@ 2>/dev/null
% chmod +x myecho2
% ./myecho2 hello world
STDOUT hello world
% ls *.* | xargs -t -i{} ./myecho2 {} 2>&1
./myecho blah.txt 2>/dev/null
STDOUT blah.txt
./myecho hi.txt 2>/dev/null
STDOUT hi.txt
% ls *.* | xargs -t -i{} ./myecho2 {} 2>&1 | tee >/dev/null
%

So this approach works, and collapses everything you want to stdout (leaving out what you don't want).

If you find yourself doing this a lot, you can write a general utility to surpress stderr:

% cat surpress_stderr
#!/bin/sh
$@ 2>/dev/null
% ./surpress_stderr ./myecho hello world
STDOUT hello world
% ls *.* | xargs -t -i{} ./surpress_stderr ./myecho {} 2>&1
./surpress_stderr ./myecho blah.txt 2>/dev/null
STDOUT blah.txt
./surpress_stderr ./myecho hi.txt 2>/dev/null
STDOUT hi.txt
%