How to ignore xargs commands if stdin input is empty?

Question

Consider this command:

ls /mydir/*.txt | xargs chown root

The intention is to change owners of all text files in mydir to root

The issue is that if there are no .txt files in mydir then xargs thows an error saying there is no path specified. This is a harmless example because an error is being thrown, but in some cases, like in the script that i need to use here, a blank path is assumed to be the current directory. So if I run that command from /home/tom/ then if there is no result for ls /mydir/*.txt and all files under /home/tom/ have their owners changed to root.

So how can I have xargs ignore an empty result?

Boatwright

Aside: Never pipe output from ls for programmatic use; see mywiki.wooledge.org/ParsingLs – Gopherwood

My use case is git branch --merged | grep -v '^* ' | xargs git branch -d, which also fails on empty input – Alcaeus

I think you can run git branch --merged | grep -v '^* ' | xargs --no-run-if-empty git branch -d – Elum

Answer 1

For GNU xargs, you can use the -r or --no-run-if-empty option:

--no-run-if-empty
-r
If the standard input does not contain any nonblanks, do not run the command. Normally, the command is run once even if there is no input. This option is a GNU extension.

The BSD version of xargs, which is used on macOS, does not run the command for an empty input, so the -r option is not needed (nor is it accepted by that version of xargs).

Pilkington

I honestly searched in google first and found this (but wouldn't have asked without reading the manual). I kind of thought this hould be the default behavior, not needing a switch to enable it. – Jehoash

Unfortunately this does not work on a Mac. Neither the short nor the long option. – Vauban

@Vauban - OSX has BSD userspace, so this sort of thing will happen frequently. You can work around it by using homebrew to install the GNU fileutils. – Arlina

You mean brew install findutils. findutils, not fileutils. – Choate

so like: foo | xargs --no-run-if-empty bar --baz – Registered

On macOS, not providing the flag results in not running the command anyway, so I guess it just isn't needed. – Huntsman

@Huntsman just acknowledging your helpful comment—I was tripped up by the macOS/Linux differences myself. – Gorrian

Answer 2

Users of non-GNU xargs may take advantage of -L <#lines>, -n <#args>, -i, and -I <string>:

ls /empty_dir/ | xargs -n10 chown root # chown executed every 10 args or fewer
ls /empty_dir/ | xargs -L10 chown root # chown executed every 10 lines or fewer
ls /empty_dir/ | xargs -i cp {} {}.bak # every {} is replaced with the args from one input line
ls /empty_dir/ | xargs -I ARG cp ARG ARG.bak # like -i, with a user-specified placeholder

Keep in mind that xargs splits the line at whitespace but quoting and escaping are available; RTFM for details.

Also, as Doron Behar mentions, this workaround isn't portable so checks may be needed:

$ uname -is
SunOS sun4v
$ xargs -n1 echo blah < /dev/null

$ uname -is
Linux x86_64
$ xargs --version | head -1
xargs (GNU findutils) 4.7.0-git
$ xargs -n1 echo blah < /dev/null
blah

Stig

Does not seem to answer the question? – Marduk

@Nicolas: it's the way to prevent execution if your version of xargs does not support the --no-run-if-empty switch and attempts to run the command argument without STDIN input (this is the case in Solaris 10). The versions in other unixes may just ignore an empty list (e.g. AIX). – Stig

Yes! On MAC OSX, echo '' | xargs -n1 echo blah does nothing; whereas echo 'x' | xargs -n1 echo blah prints "blah". – Akene

For both GNU and BSD/OSX support I end up using something like: ls /mydir/*.txt | xargs -n 1 -I {} chown root {}, as this answer suggests. – Truthvalue

@LuísBianchin, -I and -i already imply -n 1 since they substitute one argument per placeholder. Also your placeholder can be any string (after shell processing); {} is just the default for -i. – Stig

Great point @arielCo! In that case, one can do: ls /mydir/*.txt | xargs -n 1 chown root {} – Truthvalue

It should be noted that echo '' | xargs -n1 echo blah prints blah with GNU's xargs. – Wesson

@DoronBehar yikes, so this isn't portable. Thanks - I'll edit accordingly. – Stig

Answer 3

man xargs says --no-run-if-empty.

Lempres

If you are on OSX it will not. edk750 has explained this in their comment if you are curious why. – Apicella

Answer 4

In terms of xargs, you can use -r as suggested, however it's not supported by BSD xargs.

So as workaround you may pass some extra temporary file, for example:

find /mydir -type f -name "*.txt" -print0 | xargs -0 chown root $(mktemp)

or redirect its stderr into null (2> /dev/null), e.g.

find /mydir -type f -name "*.txt" -print0 | xargs -0 chown root 2> /dev/null || true

---

Another better approach is to iterate over found files using while loop:

find /mydir -type f -name "*.txt" -print0 | while IFS= read -r -d '' file; do
  chown -v root "$file"
done

See also: Ignore empty results for xargs in Mac OS X

---

Also please note that your method of changing the permissions isn't great and it's discouraged. Definitely you shouldn't parse output of ls command (see: Why you shouldn't parse the output of ls). Especially when you're running your command by root, because your files can consist special characters which may be interpreted by the shell or imagine the file having a space character around /, then the results can be terrible.

Therefore you should change your approach and use find command instead, e.g.

find /mydir -type f -name "*.txt" -execdir chown root {} ';'

Dilapidate

Sorry, I don't understand how the while IFS= read -r -d '' file is valid. Can you explain? – Dulcedulcea

The latest macOS BSD version accepts -r, but ignores it, as it does nothing on no inputs anyway. – Roxana

Answer 5

A cross-platform (Mac and Linux) alternative to using -r/--no-run-if-empty xargs' parameter:

Example with empty parameters (same result on Ubuntu 18.04 and Big Sur):

$ echo | xargs -I {}  echo "This is a test with '{}'"
$
$
$ cat /dev/null | xargs -I {}  echo "This is a test with '{}'"
$

Example with multiple lines:

$ seq 1 5  | xargs -I {}  echo "This is a test with '{}'"
This is a test with '1'
This is a test with '2'
This is a test with '3'
This is a test with '4'
This is a test with '5'
$

Aile

xargs: warning: options --replace and -L are mutually exclusive, ignoring previous --replace value that's what I get with GNU xargs – Kamerman

As I referred this is a cross-platform alternative. xargs for MacOS catalina doesn't provide the -r / --replace parameter. – Aile

Nope. That's wrong. Just try cat /dev/null | xargs -I {} -L1 echo {} ...and while it sort of works on macOS it does not on Linux. – Langue

@Langue Indeed, my answer was wrong. Updated it with the correct parameters and checked my solution against ubuntu 18.04 and Big Sur. Thanks. – Aile

You example is still wrong. It should be cat /dev/null | xargs -I {} echo "This is a test with '{}'" and echo hello | xargs -I {} echo "This is a test with '{}'" This is a test with 'hello' – Langue

@tcurdtJust updated my answer with your suggestion. Thanks – Aile

Good answer but I'd suggest a slightly more complex example that more fully captures the nuances of xargs. seq 1 7 | sed -n '1{N;N;s@\n@ @g};${p;q};2,${N;s@\n@ @};p' | xargs -I % echo % – Yuu

Answer 6

This is a behaviour of GNU xargs which can be supressed by using -r, --no-run-if-empty.

The *BSD variant of xargs has this behavoir on default, so -r is not needed. Since FreeBSD 7.1 (released in january 2009) an -r argument is accepted (witch does nothing) for compatiblity reasons.

I personally prefer using longopts in scripts but since the *BSD xargs does not uses longopts just use "-r" and xargs will act the same on *BSD an on linux systems

xargs on MacOS (currently MacOS Mojave) sadly don't supports the "-r" argument.

Yakut

Answer 7

On OSX: Bash reimplementation of xargs dealing with the -r argument, put that e.g. in $HOME/bin and add it to the PATH:

#!/bin/bash
stdin=$(cat <&0)
if [[ $1 == "-r" ]] || [[ $1 == "--no-run-if-empty" ]]
then
    # shift the arguments to get rid of the "-r" that is not valid on OSX
    shift
    # wc -l return some whitespaces, let's get rid of them with tr
    linecount=$(echo $stdin | grep -v "^$" | wc -l | tr -d '[:space:]') 
    if [ "x$linecount" = "x0" ]
    then
      exit 0
    fi
fi

# grep returns an error code for no matching lines, so only activate error checks from here
set -e
set -o pipefail
echo $stdin | /usr/bin/xargs $@

Khaddar