Use xargs and kill in script with optional signal

Question

A script for terminating a server on a certain port:

#!/bin/bash

PORT="$1"
SIGNAL="$2"

if [ "${SIGNAL}" != "" ]; then SIGNAL=" -${SIGNAL}"; fi

lsof -i:"${PORT}" |\
  grep -e "localhost:${PORT}" -e TCP -e LISTEN |\
  tr -s ' ' |\
  cut -d' ' -f2 |\
  tee /dev/tty |\
  xargs --no-run-if-empty kill "$SIGNAL"

Works: killbyport 4242

But if I want to do a kill -9 I'd do: killbyport 4242 9, and that errors:

kill: (-9): No such process

The xargs and kill aren't cooperating – how do I fix it?

(PS: I want to fix this script, rather than change it to something else. It's almost working.)

Always paste your script into `https://shellcheck.net`, a syntax checker, or install `shellcheck` locally. Make using `shellcheck` part of your development process. — waltinator, Mar 13 '22 at 02:26
@waltinator Thanks. Note shellcheck gives "No issues detected!" :) — lonix, Mar 13 '22 at 02:30
You do not need to escape the newline after the pipe character. IMHO it is best to avoid escaping newline when possible. — pabouk - Ukraine stay strong, Mar 13 '22 at 16:10
@pabouk Interesting, why do you suggest not to escape newlines? — lonix, Mar 14 '22 at 02:49
Backslash used for line continuation has multiple problems: * You have to be careful that the backslash is not followed by a whitespace (invisible in most editors by default) or any other character. * You cannot add a comment to such a line. --- See for example this answer for Python (almost the same situation as in a shell): https://stackoverflow.com/a/61933/320437 — pabouk - Ukraine stay strong, Mar 14 '22 at 08:36

G-Man Says 'Reinstate Monica' · Accepted Answer · 2022-03-13T02:51:01.830

12

The problem is that you’re explicitly adding a space to SIGNAL:

SIGNAL=" -${SIGNAL}"

and then referencing it in quotes:

kill "$SIGNAL"

Since kill is seeing an argument that doesn’t begin with dash (because it begins with space, and then dash), it isn’t seen as an option, but as an ordinary argument — in this case, a PID. A quick fix is to not add the space to SIGNAL:

SIGNAL="-${SIGNAL}"

But it doesn’t make sense that this is working when SIGNAL is null. See But what if …?. The first example (ignorecase) almost exactly matches your situation.

edited Mar 13 '22 at 02:51

answered Mar 13 '22 at 02:35

G-Man Says 'Reinstate Monica'

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Your link led me to the answer, thanks. I added my fix below. – lonix Mar 13 '22 at 03:06

score 3 · Answer 2 · answered Mar 13 '22 at 03:05

3

@G-Man's answer led me in the right direction. For completeness, the fix is to use this instead:

xargs --no-run-if-empty kill ${SIGNAL:+"-$SIGNAL"}

answered Mar 13 '22 at 03:05

lonix

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score 2 · Answer 3 · answered Mar 13 '22 at 02:31

2

The result of this is to run the command kill " -9" ... which is wrong. Kill will interpret " -9" as a pid instead of "-9" which would be signal 9.

You are being too clever with your quoting.

answered Mar 13 '22 at 02:31

user10489

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isn't it the leading whitespace that's the real problem here, rather than the quoting? – steeldriver Mar 13 '22 at 02:34
@steeldriver: It's both. Adding the space is an error, but, if `$2` was a valid non-negative integer, `kill $SIGNAL` (without quotes) would have worked (but for the wrong reason). – G-Man Says 'Reinstate Monica' Mar 13 '22 at 02:38
Without the quotes, the extra space is stripped. With the quotes, the kill works, but doesn't use the specified signal because it thinks it is a pid that it can't find. – user10489 Mar 13 '22 at 02:58
I've never been accused of being too clever... thanks! ;) – lonix Mar 13 '22 at 06:05

Use xargs and kill in script with optional signal

3 Answers3