Databases, Systems & Networks

When you are using Linux command line frequently, using the history effectively can be a major productivity boost. In fact, once you have mastered the 15 examples that I’ve provided here, you’ll find using command line more enjoyable and fun.

1. Display timestamp using HISTTIMEFORMAT

Typically when you type history from command line, it displays the command# and the command. For auditing purpose, it may be beneficial to display the timepstamp along with the command as shown below.

# export HISTTIMEFORMAT='%F %T '
# history | more
1 2008-08-05 19:02:39 service network restart
2 2008-08-05 19:02:39 exit
3 2008-08-05 19:02:39 id
4 2008-08-05 19:02:39 cat /etc/redhat-release

2. Search the history using Control+R

Lire la suite…

Reverse SSH is a technique that can be used to access systems (that are behind a firewall) from the outside world.

As you already know SSH is a network protocol that supports cryptographic communication between network nodes. Using this protocol, you can do a secure remote login, secure copy from/to a remote machine etc.

You’ll typically do the following to connect to a remote server securely using ssh command.

$ ssh [your-account-login]@[server-ip]

What is Reverse SSH?

SSH is very good tool to access remote machine or server securely. But, the problem arises when you try to connect to a remote server which is behind a firewall and this firewall denies any incoming connection or data transfer request that has no prior outgoing request. This means that only those connections would be allowed which are initiated by the remote server machine. This is a real problem for those who want to access this server machine remotely. Lire la suite…

How to check iptables traffic on the fly?

Here are a few commands that can help:

watch --interval 0 'iptables -nvL | grep -v "0 0"'

watch 'iptables -vL'

while true; do iptables -nvL > /tmp/now; diff -U0 /tmp/prev /tmp/now > /tmp/diff; clear; cat /tmp/diff; mv /tmp/now /tmp/prev; sleep 1; done

watch -d -n 2 iptables -nvL

source: http://ubuntuforums.org/showthread.php?t=625132
On the command line, you can use the « find » command to select certain files, and then use the « -exec » switch to cause some other command to be run on those files. So if you use « -exec rm » you will delete the files that are found. So, for example:

 find -mmin +4 -exec rm {} +

will delete any files in the current directory (and sub-directories) that are older than 4 minutes. This is because the « -mmin +4 » switch causes find to return files older than 4 minutes. There are other options, like « -mtime » that return files based on modified date in days. You can use + or – depending on what kind of behavior you are trying to achieve. For a more complete explanation of all the options, see the manual page:
http://unixhelp.ed.ac.uk/CGI/man-cgi?find

Be careful when using the rm command! It’s usually a good idea to test your command first, before using it. So, for instance, use something like:

 find -mmin +4 -exec ls {} +

which will just list the files that you are selecting for. If the list looks right, then you can switch the « ls » to « rm » in the command and it will delete the files.

When you want to restore your packages that you’ve backup-ed with :

sudo dpkg --get_selections > selections.txt

And when you restore them with:

dpkg --set-selections < selections.txt

And the result is a whole bunch of warnings like “warning: package blabla not in database….”

Then theres a easy fix:

$ sudo apt-get install dselect
$ sudo dselect 
 -> Update
 -> Install