For watching some command output after time intervals

watch -n 1 "cat /proc/meminfo | grep MemFree"

ssh login without a passwd

Let say you want to access to a machine with IP 10.0.0.4, and make sure you have command ssh, ssh-keygen, ssh-copy-id.

First, generate the key, the key will be used to open the remote machines door.

ssh-keygenYou will see something like that

Enter file in which to save the key (/home/myname/.ssh/id_rsa):Whatever it appears just press enter until it ends, press enter for passphase as well.

Okay, the key will be generated, something looks like ~/.ssh/id_rsa.pub

Copy over the key to remote machine, and enter your password

ssh-copy-id -i ~/.ssh/id_rsa.pub mysurface@10.0.0.4Done. Now you can ssh 10.0.0.4 with username mysurface without password.

ssh mysurface@10.0.0.4

Taglist info in vim

1) Download taglist plugin from http://vim-taglist.sourceforge.net/
2)Unzip in $HOME/.vim directory
3)Open a vim editor and type :TListToggle

You should see a Tag List Wiindows at your left side. This will contain info about all the functions , variables, macros etc. Very useful

Make your vim competent with other IDE's

Simple File Sharing Mechanism between machines

cd _directory_to_share_
python -m SimpleHTTPServer _port_no_

where port_no could be anything like 1234

From the other machine, http://_ip_addr_:_port_no_/

Setting up private network

Assumption:

----> M2 <--------------------> M3--
| -----------------------------------------|
M1 ------------------------------------M4


M2 and M3 are servers
M1 and M4 are clients

M1 - 10.18.71.238
M2 - 10.18.71.1
M3 - 10.18.207.1
M4 - 10.18.207.180


Hardware connections
1) All are single homed (They have only one ethernet slot)
2) There are 2 network switches (M1 and M2 are connected to one switch; M3 and M4 are connected to the other switch. These hubs are directly connected so that M2 and M3 are connected
3) All of these switches are just accept and forward type of switches

Configurations

On Clients
M1
1) Assign the ip
ifconfig eth0 10.18.71.238/24
2) Set the default gw as 10.18.71.1 (M2)
ip route flush all
route add net 10.18.71.0/24 dev eth0 ( This means, if you need connection to the local n/w, try anywhere where you find eth0)
route add default gw 10.18.71.1


M4
1) Assign the ip
ifconfig eth0 10.18.207.180/24
2) Set the default gw as 10.18.207.1 (M3)
ip route flush all
route add net 10.18.207.0/24 dev eth0 ( This means, if you need connection to the local n/w, try anywhere where you find eth0)
route add default gw 10.18.207.1

Check: Now M1 should be pingable to/from M2 and M3 should be pingable to/from M4. If these are NOT pinging, something else might be wrong. Resolve this before proceeding further

On servers
M2
1) Assign the ip
ifconfig eth0 10.18.71.1/24
2) Make this as a router
ip route flush all
route add -net 10.18.71.0/24 dev eth0
route add -host 10.18.207.1 dev eth0
route add -host 10.18.207.180 gw 10.18.207.1
echo 1 > /proc/sys/net/ipv4/ip_forward

M3
1) Assign the ip
ifconfig eth0 10.18.207.1/24
2) Make this as a router
ip route flush all
route add -net 10.18.207.0/24 dev eth0
route add -host 10.18.71.1 dev eth0
route add -host 10.18.71.238 gw 10.18.71.1
echo 1 > /proc/sys/net/ipv4/ip_forward

Check : Now M2 --> M3 should be pingable

Now the private network is establised. All machines should be pingable from/to all machines

P.S- Make sure to disable Firewalls of all machines (iptables --flush; iptables -t nat --flush; verify iptables -L and iptables -t nat -L. This should NOT have any entries in them

Viewing the contents of ext2/3 filesystem

If you have created a filesystem with .ext2 OR .ext3 extensions, in other words, if they are either of ext2/3 filesystems, they could be viewed with the following command
debugfs xxx.ext2

Creating and analysing coredump messages

1) Creating a core dump

a) Run the command ulimit -c unlimited

b) Run the executable that is crashing

c) You should see a message like "Segmentation fault (core dumped)"

2) Analysing core dump using gdb

a) Run the command

gdb

b) You should see the point of the crash (in the back trace)

Reading symbols from /lib/libcrypto.so.0.9.8...done.Loaded symbols for /lib/libcrypto.so.0.9.8Reading symbols from /lib/libresolv.so.2...done.Loaded symbols for /lib/libresolv.so.2Reading symbols from /lib/libc.so.6...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/libdl.so.2...done.Loaded symbols for /lib/libdl.so.2Reading symbols from /lib/ld.so.1...done.Loaded symbols for /lib/ld.so.1

#0 0x0fd60954 in strlen () from /lib/libc.so.6

This means it is some problem with strlen .