In a cleanup script the line `grep -l OBSOLETE *.conf | xargs rm --` occasionally fails with `rm: missing operand`, because grep matches nothing and GNU xargs runs rm once anyway with no file names. Which GNU xargs option stops the command from being run at all when the input is empty?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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The options
Correct -r
Correct. -r, long form --no-run-if-empty, suppresses the command entirely if standard input contains nothing but whitespace. Without it, GNU xargs runs the command once with no added arguments.
Not correct -t
Wrong. -t (--verbose) prints each command line to standard error just before running it. It makes the problem visible rather than preventing it.
Not correct -p
Wrong. -p (--interactive) prompts for confirmation before each command line. An operator could refuse the empty run, but the script would then hang waiting for input.
Not correct -x
Wrong. -x (--exit) makes xargs exit if a command line would exceed the size limit set with -s. It has nothing to do with empty input.
Why
GNU xargs runs the command at least once even when it reads no items, which is why pipelines that feed it a possibly empty list need -r. The behaviour is not universal: the BSD implementation does not run the command on empty input, and there -r is accepted only for compatibility. Combine -r with -0 when the producing command supports NUL-separated output.
Where this comes from
- Cited
- manual page xargs(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 103 and space the ones you get wrong.
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