screenshots on mac

1. Command + Shift 3 (Full screen), or
2. Command + Shift 4 (User selection)

The #2 is good but it wouldn’t allow us to select a specific window. To printscreen a specific window in Mac, try this following.

Command + Shift 4, then hit Space

http://www.hongkiat.com/blog/how-to-screen-capture-printscreen-selective-windows-in-mac-os-x/

SVN ignore

$ svn propedit svn:ignore .

edit that file like following:

  1 *.out

  2 sftp

  3 sftp-server

  4 ssh

  5 ssh-add

  6 ssh-agent

  7 ssh-keygen

  8 ssh-keyscan

  9 ssh-keysign

 10 ssh-pkcs11-helper

 11 sshd

 12 scp

http://stackoverflow.com/questions/122313/how-do-you-include-exclude-a-certain-type-of-files-under-subversion

Crypto 101

Secret key: Diffie–Hellman, 
Two parties don't know each other can establish a secret key

Alice				Bob
Secret	Public	Calculates	Sends	Calculates	Public	Secret
a	p, g		p,g			b
a	p, g, A	ga mod p = A	A		p, g	b
a	p, g, A		B	gb mod p = B	p, g, A, B	b
a, s	p, g, A, B	Ba mod p = s		Ab mod p = s	p, g, A, B	b, s

Public-private key: RSA

https://www.cs.utexas.edu/~mitra/honors/soln.html

RSA Algorithm Example

• Choose p = 3 and q = 11
• Compute n = p * q = 3 * 11 = 33
• Compute φ(n) = (p - 1) * (q - 1) = 2 * 10 = 20
• Choose e such that 1 < e < φ(n) and e and n are coprime. Let e = 7
• Compute a value for d such that (d * e) % φ(n) = 1. One solution is d = 3 [(3 * 7) % 20 = 1]
• Public key is (e, n) => (7, 33)
• Private key is (d, n) => (3, 33)
• The encryption of m = 2 is c = 2⁷ % 33 = 29
• The decryption of c = 29 is m = 29³ % 33 = 2

sudo keeps PYTHONPATH

http://superuser.com/questions/363813/sudo-does-not-preserve-pythonpath


$visudo

Add Defaults env_keep += PYTHONPATH

Add user to the sudo list

$ visudo

It basically use vi to open /etc/sudoers with write permission

Add following:

root    ALL=(ALL) ALL

username  ALL=(ALL) ALL

If you directly open /etc/sudoers using vi, you need to change the file permission and then edit the file. Assume you system setup only read permission for /etc/sudoers

Send messages to other login users through terminal

$ w
show all of the login users
$ last
show last login users

$ write username
type message
Ctr+D: end of message

unlock a deny host

1.
$ service denyhosts stop

2.
$ vim /etc/hosts.deny
delete the IP from the file

3.
$ cd /va/lib/denyhosts
$ grep -iRH IP ./*

[root@server denyhosts]# grep -iRH IP ./*

./hosts:IP:0:Sun Feb  9 11:27:57 2014

./hosts-restricted:IP:0:Sun Feb  9 11:27:57 2014

./hosts-root:IP:0:Sun Feb  9 11:27:57 2014

./hosts-valid:IP:11:Sun Feb  9 11:31:57 2014

./users-hosts:username - IP:11:Sun Feb  9 11:31:57 2014

delete the IPs from all of the files. 

4. 

$ service denyhosts start