Is `xargs -t` the output of stderr or stdout, and can you control it?
Let's say I have a directory with hi.txt and blah.txt and I execute the following command on the linux-ish command line
ls *.* | xargs -t -i{} echo {}
the output you see is
echo blah.txt
blah.txt
echo hi.txt
hi.txt
I would like to redirect the output of stderr (say "echo blah.txt" doesn't work ...) leaving only the output of the xargs -t command written to std out, but that looks the same as stderr.
ls *.* | xargs -t -i{} echo {} 2> /dev/null
Is there a way to control it so that it is output to standard output?
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So, I believe you want to have as stdout
- stdout from a utility executed by xargs
- list of commands generated by xargs -t
You want to ignore the stderr stream generated by the executed utility.
Please correct me if I am wrong.
First, 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 for both stdout and stderr, so we can be sure that we only get what we want.
The tangential way to do this is not to use xargs, but rather make. Repeat the command and then do it however. This is his 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 are tied to using xargs, you need to change your utility that xargs uses to suppress stderr. Then you can use the trick 2>&1
others mentioned to move the command list generated by xargs -t from stderr to standard output.
% 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 folds whatever you want to output to stdout (apart from what you don't want).
If you do that much, you can write a generic utility to stop stderr:
% cat surpress_stderr
#!/bin/sh
$@ 2>/dev/null
% ./surpress_stderr ./myecho hello world
STDOUT hello world
% ls *.* | xargs -t -i{} ./surpress_stderr ./myecho {} 2>&1
./surpress_stderr ./myecho blah.txt 2>/dev/null
STDOUT blah.txt
./surpress_stderr ./myecho hi.txt 2>/dev/null
STDOUT hi.txt
%
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Using:
ls | xargs -t -i{} echo {} 2>&1 >/dev/null
2>&1
sends standard error to xargs
wherever standard output is currently going; >/dev/null
sends the original standard output to /dev/null
. Thus, the net result is that standard output contains echo commands, but /dev/null
file names. We can discuss spaces in filenames and whether it would be easier to use a sed
script to place an "echo" at the front of each line (no option -t
) or you can use:
ls | xargs -i{} echo echo {}
(Tested: Solaris 10, Korn Shell, should work on other shells and Unix platforms.)
If you don't mind seeing the inner workings of commands, I've managed to separate the error output from xargs
and the error output of the command being executed.
al * zzz | xargs -t 2>/tmp/xargs.stderr -i{} ksh -c "ls -dl {} 2>&1"
Command (non-standard) al
lists its arguments, one per line:
for arg in "$@"; do echo "$arg"; done
The first redirect ( 2>/tmp/xargs.stderr
) sends the error output from xargs
to a file /tmp/xargs.stderr
. The command executed is: " ksh -c "ls -dl {} 2>&1"
", which uses the Korn shell to run ls -ld
on a filename, with any error output being printed to standard output.
The output in /tmp/xargs.stderr
looks like this:
ksh -c ls -dl x1 2>&1
ksh -c ls -dl x2 2>&1
ksh -c ls -dl xxx 2>&1
ksh -c ls -dl zzz 2>&1
I used ' ls -ld
' instead echo
, to make sure I tested errors - files x1
, x2
and xxx
there, but zzz
does not work.
The standard output looked like this:
-rw-r--r-- 1 jleffler rd 1020 May 9 13:05 x1
-rw-r--r-- 1 jleffler rd 1069 May 9 13:07 x2
-rw-r--r-- 1 jleffler rd 87 May 9 20:42 xxx
zzz: No such file or directory
When run without a command wrapped in ' ksh -c "..."
', the I / O redirection was passed as a command argument (' ls -ld
'), and so it reported that it could not find the file ' 2>&1
". That is, xargs
it did not use the shell to redirect the I / O itself.
Various other redirects could be arranged, but the main problem is that it xargs
does not provide for separating its own error output from the command it is executing, so this is difficult to do.
Another fairly obvious option is to use a xargs
script to write a shell and then execute the shell. This is the variation I showed earlier:
ls | xargs -i{} echo echo {} >/tmp/new.script
Then you can see the commands with:
cat /tmp/new.script
You can run commands to throw errors:
sh /tmp/new.script 2>/dev/null
And, if you don't want to see standard output from commands, add 1>&2
to the end of the command.
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It looks like xargs -t goes to stderr and you can't help it.
You can do:
ls | xargs -t -i{} echo "Foo: {}" >stderr.txt | tee stderr.txt
to only display stderr data in your terminal as you run your command, then grep through stderr.txt after seeing if anything unexpected happened, on the lines grep -v Foo: stderr.txt
Also note that on Unix, ls *.*
this is not how you display everything. If you want to view all files, just run it ls
yourself.
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As I understand, your problem using GNU Parallel http://www.gnu.org/software/parallel/ will do the right thing:
ls *.* | parallel -v echo {} 2> /dev/null
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