Logical AND and OR in bash

In bash (bourne again shell), the logical operand AND and OR are being symbolized by && and ||. The usage example are as follow:

&& ( If first command succeed, continue with second command, else stop )

$ ls 
cat  lion  tiger

$ ls cat && echo "there is cat"
cat
there is cat
 

$ ls elephant && echo "there is elephant"
ls: cannot access elephant: No such file or directory

You can see from above that command `echo "there is elephant"` did not get executed because the command `ls elephant` did not successfully finish (non zero exit code)


|| ( If  the first command failed, execute second command, else stop )

$ ls 
cat  lion  tiger

$ ls cat || echo "there is no cat"
cat

$ ls elephant || echo "there is no elephant"
ls: cannot access elephant: No such file or directory
there is no elephant

Now you can see that, if the first command returned non zero exit code (failed), the second command will be executed.

To run the second command, while ignoring the first command's result, use ";" instead:

$ ls tiger; echo "there is tiger"
tiger
there is tiger

$ ls elephant; echo "there is no elephant"
ls: cannot access elephant: No such file or directory
there is no elephant

That's all folks.

ssh through socks proxy

This technique is very useful if you have a firewall between you and your destination, and somehow the only way you could get in to the destination is by ssh'ing into a jumpbox and ssh again to the destination. In this scenario example, I'll call the machine we initiate this technique as A.local, the jumpbox as B.local and the destination server as C.local, and we will use a user called aladdin.

A.local -> B.local (jumpbox) -> C.local

To do this, please follow below steps:

Add below settings to your ssh config in A.local, the file is usually ~/.ssh/config

Host B.local 

DynamicForward localhost:1080 

Host C.local 

ProxyCommand /usr/bin/nc -x localhost:1080 %h %p

Initiate a socks proxy connection, and leave it open (-D is for dunamic application-level port forwarding and 1080 can be any port of your choice, 1080 is socks proxy default port for nc):

[A.local]$ ssh -D 1080 aladdin@B.local

Open another terminal, and run ssh as if you have direct connection to C.local

[A.local]$ ssh aladdin@C.local

Voila, your ssh session will go through as if you have direct connection to C.local.


If you just doesn't want to put it into your config, you can use it on the fly by using below command after you have initiate the socks proxy:

[A.local]$ ssh -o "ProxyCommand /usr/bin/nc -x localhost:1080 %h %p" aladdin@C.local

Or you can also put it as alias for easy usage:

[A.local]$ alias 

alias proxyssh='ssh -o "ProxyCommand /usr/bin/nc -x localhost:1080 %h %p"'

Using curl to check on site availability

The easy way to check if a site is available, is by using curl. For example, if you want to check whether www.google.com is available or not, jut run:

$ curl -I www.google.com

HTTP/1.1 200 OK

Date: Thu, 25 Apr 2013 06:03:06 GMT

Expires: -1

Cache-Control: private, max-age=0

Content-Type: text/html; charset=ISO-8859-1

Set-Cookie: PREF=ID=f4eacf44ddfe9913:FF=0:TM=1366869786:LM=1366869786:S=pD4jQT9xbgTOjuKG; expires=Sat, 25-Apr-2015 06:03:06 GMT; path=/; domain=.google.com

Set-Cookie: NID=67=KULBN37y3Mw7TIYNurxqV3L9OAm0gaj4VhRxz0_OsayoTS8C7nPN9QLCMovAzkVxhKfoop1EcHjWiBWjv7Vxl2C5iQ-Z8J0zcVtv4YfrJXs2ypRegbp2Y8MPcJjTyX1p; expires=Fri, 25-Oct-2013 06:03:06 GMT; path=/; domain=.google.com; HttpOnly

P3P: CP="This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info."

Server: gws

X-XSS-Protection: 1; mode=block

X-Frame-Options: SAMEORIGIN

Transfer-Encoding: chunked 

with -I for fetching the HTTP-header only. In this case the return code is 200, and it means the site is available.