Some Useful Linux Commands

  1.  Linux Sed command to change values inside file

# sed -i ‘s/SELINUX=enforcing/SELINUX=disabled/g’ /etc/selinux/config


  1. PERL script to check Linux machine type

# perl -MConfig -e ‘print “$Config{myarchname}\n”;’


  1. To check Fibre cable status – Simple script
ls /sys/class/fc_host | while read i; do
echo $i
cat /sys/class/fc_host/$i/port_state

  1. Copy files by giving list as text file

# xargs cp -t /home/example/pictures/jpegdir < filelist.txt


  1. To disable gfs hang lockout messages on /var/log/messages

# echo 0 > /proc/sys/kernel/hung_task_timeout_secs


  1. To disable IPv6 add following lines in /etc/sysctl.conf

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

And reboot the system.

If you want to disable IPv6 without rebooting your system, then you have to run following commands:

echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
echo 1 > /proc/sys/net/ipv6/conf/default/disable_ipv6


sysctl -w net.ipv6.conf.all.disable_ipv6=1
sysctl -w net.ipv6.conf.default.disable_ipv6=1


  1. To kill specific login section

# w

root pts/0 :0.0 10Jan13 38days 0.03s 0.03s bash
root pts/1 ws-zhaider.c 02:13 59.00s 7.64s 7.62s /bin/sh
root pts/2 fedora16.benvin. 08:33 0.00s 0.04s 0.01s w

# skill -KILL -v pts/1


  1. SSH Debugging – To check where the connection is dropping

# ssh -vvv -i$privatekey -oBatchMode=yes user@host


  1. SCSI rescan on RHEL4

# echo “- – -” > /sys/class/scsi_host/host0/scan 
(host0 is just an example.)



To Clear Swap memory on RHEL

/sbin/sysctl vm.drop_caches=3
#sync; echo 3 > /proc/sys/vm/drop_caches



Find Larget File in Linux

# du -k | sort -n | perl -ne ‘if ( /^(\d+)\s+(.*$)/){$l=log($1+.1);$m=int($l/log(1024)); printf  (“%6.1f\t%s\t%25s  %s\n”,($1/(2**(10*$m))),((“K”,”M”,”G”,”T”,”P”)[$m]),”*”x (1.5*$l),$2);}’

the output is like below:




Find FC ID WWNN of a disk/LUN

# systool -c fc_host –v
# systool -c fc_transport -v

2.4.x Kernels
> cat /proc/scsi/[hba_type]/(n)

where hba_type is the driver (e.g. lpfc for Emulex) and (n) is the HBA number.

2.6.x Kernels:

> cat /sys/class/scsi_host/host(n)/[port_name|node_name]

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