Linux list all processes based on memory consumption .

list top 5 processes
ps -eo pmem,pcpu,vsize,pid,cmd | sort -k 1 -nr | head -5

list all processes in assending order .
ps -e -orss=,args= | sort -b -k1,1n | pr -TW$COLUMNS

easy ntp time sync between servers

Choose one server from which time need to be synced .

then on clinet open  /etc/ntp.conf

#server 0.rhel.pool.ntp.org ( comment these three lines )
#server 1.rhel.pool.ntp.org
#server 2.rhel.pool.ntp.org
server 10.10.10.180  ( this is the server which i choose so all other servers will be in sync with this server )

 /usr/sbin/ntpdate -s -b -p 8 -u 10.10.10.180

/etc/init.d/ntpd restart

Other commands that are used for NTP are 

ntpdate -u 10.10.10.180

output : ntpdate[13786]: adjust time server 10.10.10.180 offset 0.000221 sec

ntpdate -d 10.10.10.180 

output : output will be more in detail

ntpdate[13806]: ntpdate 4.2.2p1@1.1570-o Thu Nov 26 11:34:35 UTC 2009 (1)

Looking for host 10.10.10.180 and service ntp

host found : 10.10.10.180

transmit(10.10.10.180)

receive(10.10.10.180)

transmit(10.10.10.180)

receive(10.10.10.180)

transmit(10.10.10.180)

receive(10.10.10.180)

transmit(10.10.10.180)

receive(10.10.10.180)

transmit(10.10.10.180)

server 10.10.10.180, port 123

stratum 3, precision -20, leap 00, trust 000

refid [10.10.10.180], delay 0.02583, dispersion 0.00000

transmitted 4, in filter 4

reference time:    d5b8497a.219236b6  Fri, Aug 16 2013 12:17:22.131

originate timestamp: d5b849c8.3769d786  Fri, Aug 16 2013 12:18:40.216

transmit timestamp:  d5b849c8.37669489  Fri, Aug 16 2013 12:18:40.216

filter delay:  0.02588  0.02585  0.02585  0.02583

         0.00000  0.00000  0.00000  0.00000

filter offset: -0.00006 -0.00007 -0.00006 -0.00007

         0.000000 0.000000 0.000000 0.000000

delay 0.02583, dispersion 0.00000

offset -0.000070

16 Aug 12:18:40 ntpdate[13806]: adjust time server 10.10.10.180 offset -0.000070 sec

ntpq -p

How to copy directory structure in linux

Sometimes we need to copy only directory structure leaving content ( files )

find . -type d >dirs.txt

xargs mkdir -p

Script to import csv into dhcpd.conf import multiple mac bindings

make sure your csv file is in this order MAC,IP,NAME and file name of csv is ips.csv
Once that is done just run the script and check mac_out.txt which is file to be imported in dhcpd.conf

!/bin/bash

INPUT=ips.csv
OUTPUT=mac_out.txt
OLDIFS=$IFS
IFS=","
[ ! -f $INPUT ] && { echo "$INPUT file not found"; exit 99; }
[ -f $OUTPUT ] && { rm $OUTPUT; }
[ ! -f $OUTPUT ] && { touch $OUTPUT; }
while read mac ip uname
do
echo "host $uname {" >> $OUTPUT
echo -e "\thardware ethernet $mac;" >> $OUTPUT
echo -e "\tfixed-address $ip;" >> $OUTPUT
echo "}" >> $OUTPUT
done < $INPUT

IFS=$OLDIFS

How to drop cache or in-memory cache on linux and free up some memory ?

$ echo 1 | sudo tee /proc/sys/vm/drop_caches  # drop pagecache
$ echo 2 | sudo tee /proc/sys/vm/drop_caches  # drop dentries and inodes
$ echo 3 | sudo tee /proc/sys/vm/drop_caches  # drop pagecache, dentries and inod

Also we can use 

echo 1 > /proc/sys/vm/drop_caches

echo 2 > /proc/sys/vm/drop_caches

echo 3 > /proc/sys/vm/drop_caches

This is also used when we take benchmarks of server.

Command to get linux dhcp client list

cat /var/log/messages | grep DHCPACK