How to evaluate results of pipes in a bash script

Question

I need help for the following problem: I'd like to kill all instances of a program, let's say xpdf. At the prompt the following works as intended:

$ ps -e | grep xpdf | sed -n -e "s/^[^0-9]*\([0-9]*\)[^0-9]\{1,\}.*$/\1/p" | xargs kill -SIGTERM

(the sed-step is required to extract the PID).

However, there might be the case that no xpdf-process is running. Then it would be difficult, to embed the line into a script, because it aborts after it immediately with a message from kill. What can I do about it?

I tried (in a script)

#!/bin/bash
#
set -x
test=""
echo "test = < $test >"
test=`ps -e | grep xpdf | sed -n -e "s/^[^0-9]*\([0-9]*\)[^0-9]\{1,\}.*$/\1/p"`
echo "test = < $test >"
if [ -z "$test" ]; then echo "xpdf läuft nicht";
else echo "$test" | xargs -d" " kill -SIGTERM
fi

When running the script above I get

$ Kill_ps
+ test=
+ echo 'test = <  >'
test = <  >
++ ps -e
++ grep xpdf
++ sed -n -e 's/^[^0-9]*\([0-9]*\)[^0-9]\{1,\}.*$/\1/p'
+ test='21538
24654
24804
24805'
+ echo 'test = < 21538
24654
24804
24805 >'
test = < 21538
24654
24804
24805 >
+ '[' -z '21538
24654
24804
24805' ']'
+ xargs '-d ' kill -SIGTERM
+ echo '21538
24654
24804
24805'
kill: failed to parse argument: '21538
24654
24804
24805

Some unexpected happens: In test there are more PIDs then processes

At the prompt:

$ ps -e | grep xpd
21538 pts/3    00:00:00 xpdf.real
24654 pts/2    00:00:00 xpdf.real

When running the script again, the 24* PIDs change.

So here are my questions:

1. Where do the additional PIDs come from?
2. What can I do to handle the situation, in which no process I want to kill is running (or why does xargs not accept echo "$test" as input)? (I want my script not to be aborted)

Thanks in advance.

Answer 1

You can use pkill(1) or killall(1) instead of parsing ps output. (Parsing human-readable output for scripting is not recommended. This is an example.)

Usage:

 pkill xpdf
 killall xpdf # Note that on solaris, it kills all the processes that you can kill. https://unix.stackexchange.com/q/252349#comment435182_252356
 pgrep xpdf # Lists pids
 ps -C xpdf -o pid=  # Lists pids

Note that tools like pkill, killall, pgrep will perform similar to ps -e | grep. So, if you do a pkill sh, it will try to kill sh, bash, ssh etc, since they all match the pattern.

But to answer your question about skipping xargs from running when there is nothing on the input, you can use -r option for (GNU) xargs.

   -r, --no-run-if-empty
          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.

Answer 2

This answer is for the case where killall or pkill (suggested in this answer) are not enough for you. For example if you really want to print "xpdf läuft nicht" if there is no pid to kill or applying kill -SIGTERM because you want to be sure of the signal you send to your pids or whatever.

You could use a bash loop instead of xargs and sed. It's pretty simple to iterate over CSV/column outputs:

count=0
while read -r uid pid ppid trash; do
  kill -SIGTERM $pid
  (( count++ ))
done < <(ps -ef|grep xpdf)
[[ $count -le 0 ]] && echo "xpdf läuft nicht"

There is a quicker way using pgrep (the previous one was for illustrate how to iterate over column-based outputs of commands with bash):

count=0
while read -r; do
  kill -SIGTERM $REPLY
  (( count++ ))
done < <(pgrep xpdf)
[[ $count -le 0 ]] && echo "xpdf läuft nicht"

If you're version of xargs can provide you the --no-run-if-empty you can still use it with pgrep (like suggested in this answer), but it's not available on the BSD or POSIX version. It is my case on macOS for example...

awk could also do the trick (with only one command after the pipe):

ps -ef|awk 'BEGIN {c="0"} {if($0 ~ "xpdf" && !($0 ~ "awk")){c++; system("kill -SIGTERM "$2)}} END {if(c <= 0){print "xpdf läuft nicht"}}'