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?

Aside: Never pipe output from `ls` for programmatic use; see http://mywiki.wooledge.org/ParsingLs — Charles Duffy, Dec 23 '15 at 17:01
My use case is `git branch --merged | grep -v '^* ' | xargs git branch -d`, which also fails on empty input — belkka, Apr 08 '20 at 16:23
I think you can run `git branch --merged | grep -v '^* ' | xargs --no-run-if-empty git branch -d` — Beatriz Fonseca, Jan 20 '22 at 22:21

Sven Marnach · Accepted Answer · 2021-10-17T19:00:36.947

485

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).

edited Oct 17 '21 at 19:00

answered Nov 28 '11 at 13:49

Sven Marnach

574,206
118
941
841

15

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. – JonnyJD Jun 25 '13 at 15:55
33

Unfortunately this does not work on a Mac. Neither the short nor the long option. – wedi Jun 24 '15 at 15:50
11

@wedi - 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. – edk750 Jul 20 '16 at 19:41
10

You mean `brew install findutils`. `findutils`, not `fileutils`. – Mitar Dec 04 '16 at 20:38
so like: foo | xargs --no-run-if-empty bar --baz – Alexander Mills May 27 '17 at 00:38
10

On macOS, not providing the flag results in not running the command anyway, so I guess it just isn't needed. – Hakanai Oct 09 '18 at 04:58
@Hakanai just acknowledging your helpful comment—I was tripped up by the macOS/Linux differences myself. – Michael Grant Oct 17 '21 at 00:15

arielCo · Answer 2 · 2019-05-30T22:56:21.117

18

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

edited May 30 '19 at 22:56

answered Sep 26 '13 at 21:08

arielCo

413
5
16

3

Does not seem to answer the question? – Nicolas Raoul Jul 21 '14 at 08:24
2

@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). – arielCo Jul 21 '14 at 18:48
4

Yes! On MAC OSX, `echo '' | xargs -n1 echo blah` does nothing; whereas `echo 'x' | xargs -n1 echo blah` prints "blah". – carlosayam Mar 30 '17 at 02:58
2

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. – Luís Bianchin Jul 17 '18 at 11:12
@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`. – arielCo Jul 17 '18 at 15:41
Great point @arielCo! In that case, one can do: `ls /mydir/*.txt | xargs -n 1 chown root {}` – Luís Bianchin Jul 25 '18 at 12:26
5

It should be noted that `echo '' | xargs -n1 echo blah` prints `blah` with GNU's `xargs`. – Doron Behar May 30 '19 at 15:41
@DoronBehar yikes, so this isn't portable. Thanks - I'll edit accordingly. – arielCo May 30 '19 at 22:56

score 11 · Answer 3 · answered Nov 28 '11 at 13:48

11

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

answered Nov 28 '11 at 13:48

thiton

35,651
4
70
100

2

If you are on OSX it will not. edk750 has explained this in their comment if you are curious why. – jasonleonhard Jan 27 '17 at 23:17

kenorb · Answer 4 · 2017-07-21T15:11:59.420

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 {} ';'

Sorry, I don't understand how the `while IFS= read -r -d '' file` is valid. Can you explain? — Adrian, Dec 27 '17 at 18:52
The latest macOS BSD version accepts `-r`, but ignores it, as it does nothing on no inputs anyway. — nroose, Feb 28 '21 at 07:09

alfredocambera · Answer 5 · 2021-05-09T22:26:37.703

9

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'
$

edited May 09 '21 at 22:26

answered Feb 08 '21 at 17:56

alfredocambera

3,155
34
29

1

`xargs: warning: options --replace and -L are mutually exclusive, ignoring previous --replace value` that's what I get with GNU xargs – xeruf Apr 12 '21 at 15:25
As I referred this is a cross-platform alternative. xargs for MacOS catalina doesn't provide the `-r` / `--replace` parameter. – alfredocambera Apr 12 '21 at 21:21
1

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. – tcurdt May 07 '21 at 23:59
@tcurdt Indeed, my answer was wrong. Updated it with the correct parameters and checked my solution against ubuntu 18.04 and Big Sur. Thanks. – alfredocambera May 08 '21 at 12:58
1

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'` – tcurdt May 09 '21 at 13:19
@tcurdtJust updated my answer with your suggestion. Thanks – alfredocambera May 09 '21 at 22:26
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 %` – CervEd Nov 25 '22 at 11:46

Mirko Steiner · Answer 6 · 2018-11-28T14:51:30.160

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.

score 2 · Answer 7 · answered May 15 '17 at 06:39

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 $@

How to ignore xargs commands if stdin input is empty?

7 Answers7

Linked

Related