Cluster SSH (cssh) is one of mankind’s greatest achievements. Really. Or at least, from a sysadmin point of view. :-) It’s one of those programs that, once you know it and start using it, you wonder how were you able to survive all those dark years without it.
cssh takes a username and several IP addresses on the command line, and opens a SSH session against each of those servers on an independent xterm window. Then, you can click on any of these windows and work independently with that particular server, or go back to the cssh console and write there, and ssh will retransmit each keypress to every SSH session.
This is a very useful tool when working with server farms, clusters, etc. For example lately I’ve been doing some monitoring and maintenance tasks on a 32-server farm on a major ISP: one by one each config modification would have take hours, with cssh it is a matter of minutes. :) Besides, as it replicates every keypress, it even works with text editors like vi: you can edit a file on every server at the same time, navigate through it, modify it… in parallel on every server. You have to be very careful when doing this, though. ;)
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A one-liner function that, when piped with the output of another command, prepends each line of that command’s output with a timestamp.
Very useful with commands that output a series of lines periodically but without a timestamp (like vmstat), so that you can’t just send their output to a file and go back to it later without a timeframe.
$ function timestamp { while read l; do d=`date +%H:%M:%S`; echo -e "$d $l"; done; }
$ vmstat 1 | timestamp
12:17:03 procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
12:17:03 r b swpd free buff cache si so bi bo in cs us sy id wa
12:17:03 1 0 4 16740 42816 395164 0 0 6 5 174 11 3 1 96 0
12:17:04 0 0 4 16656 42816 395184 0 0 0 0 393 510 1 1 98 0
12:17:05 0 0 4 16656 42816 395184 0 0 0 0 391 781 2 1 98 0
12:17:06 1 0 4 16656 42824 395176 0 0 0 84 462 976 3 1 95 0
12:17:07 0 0 4 16656 42824 395184 0 0 0 0 433 1545 11 3 86 0
12:17:08 0 0 4 16656 42824 395184 0 0 0 0 356 807 1 2 97 0
I’m almost ashamed to admit I didn’t knew of this command until very recently. :-)
Everybody knows the kill command, and almost everybody knows killall too (like kill using the process’ name instead of its PID). The problem with killall is that it only signals the processes matching the exact name you specify, and sometimes you want to work over a series of related processes that begin with the same word, or have a given word on their command line. pkill does just that:
pkill -9 -f foo
As a side note, what I used before when I wanted to do this was:
kill $SIGNAL $(ps aux | grep $PROG | awk '{print $2}')
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ifstat is like some kind of vmstat for network interfaces. Every X seconds, it prints two columns per network interface monitoring the input and output bandwidth.
Quite useful when you need to diagnose a slow network, identify bottlenecks, monitor bandwidth consumption, etc. When invoked with the -t parameter it adds a timestamp on each line, very important if you plan to left it running for hours dumping its output to a file.
# ifstat -t
Time eth0 eth1
HH:MM:SS KB/s in KB/s out KB/s in KB/s out
12:40:25 0.05 0.19 0.00 0.00
12:40:26 0.26 0.24 0.00 0.00
12:40:27 6.87 30.14 0.00 0.00
12:40:28 0.17 0.26 0.00 0.00
12:40:29 0.05 0.22 0.00 0.00
12:40:30 0.05 0.14 0.00 0.00
12:40:31 0.05 0.14 0.00 0.00
12:40:32 0.29 0.38 0.00 0.00
12:40:33 0.29 0.38 0.00 0.00
12:40:34 0.13 0.22 0.00 0.00
12:40:35 2.85 8.70 0.00 0.00
12:40:36 9.01 38.84 0.00 0.00
12:40:37 0.55 0.24 0.00 0.00
12:40:38 0.05 0.14 0.00 0.00
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I start here a new section on my blog, Command Line (feed), where I will comment on UNIX commands (Linux and MacOS X mainly) that I find useful on my daily work. The articles will range from a very obscure command that I didn’t knew of until some day I needed something like it and found it and saved my day, new or creative ways to use old commands, or small shell-scripts that can make your life easier.
Unless otherwise specified, all these commands are just an “aptitude install” away on Debian and derivate distros (on Ubuntu they may be on universe or multiverse). For RedHat, CentOS, etc. there’s a good chance that you’ll find RPM packages on Dag Wieërs’ repository.
So, without further addo, the first command on the next entry. :)
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