I have seen the following line in a bash script for killing a process(in this case started with the command loadgen):

ps xww | grep -i "loadgen" | grep "PATTERNMATCH_FACT.xml" | cut -c1-5 | xargs -i kill {} 2>/dev/null

I would like to understand the reason for piping after the two grep's in the command above.

The way the loadgen command is started is the following. It's a part of the startup script.

./loadgen -XMLFile ${DEMODIR}/bam-103-pattern-match/data/PATTERNMATCH_FACT.xml -duration 0 -frequency 2 

1 Answer 1


ps xww gives the following output

    1 ?        Ss     0:00 init [2]
 1804 pts/0    Ss     0:00 -bash

After the two grep's it pipes the output to cut. This command cuts the character 1-5 out of the output. In the output above it whould be the PID's:


This is piped to xargs. Xargs builds commands that look like this:

kill 1
kill 1804

and executes them. 2>/dev/null means that all error messages are sent to the pseudo device /dev/null.

So your command kills every process that is greped out of the ps command.

Or see explainshell.

  • can you please explain the 2>/dev/null part? I don't get how errors are prevented. What errors are we talking about here?
    – Geek
    Dec 24, 2013 at 17:49
  • Errors of shell commands are always written to channel 2. 2>/dev/null means that this channel is redirected to /dev/null, which is a pseudo device. Errors like when no process with the given PID is found. The 2>/dev/null part is not needed, its just cosmetics.
    – chaos
    Dec 24, 2013 at 17:52
  • can you please explain the xargs -i kill {} part? Especially the {} part of it? Why is that needed?
    – Geek
    Dec 24, 2013 at 18:04
  • 2
    xargs is a tool to expand its arguments into a commands list. Everything piped into xargs is considered a list to expand, and {} is the placeholder for 'list item here'. So sending to ... | xargs kill {} means "for every thing in this list, use the command kill _____, and fill in the blank with the list entry.
    – DopeGhoti
    Dec 24, 2013 at 20:11
  • kill errors are not prevented. They happen as usual. 2>/dev/null just discards the actual error messages.
    – jfs
    Dec 24, 2013 at 20:37

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