How to run a command on a filtered file list returned by grep

I often find myself passing a file list returned by some linux command to grep for filtering like below :

ls *.jar | grep test 

Sometimes i like to run a certain command on these filtered results. I achieve this by :

ls *.jar | grep test | while read line; do sha1sum "$line"; done

The generic form is :

<command generating a file list>   |   grep <filter keyword> | while read line; do <command> "$line"; done  

How to check if a port is open on Linux

Sometimes when writing a script/program you need to know if a port your are interested in has an established connection to it or it's free.

My favorite way to do this is to run the following command ( replace PORT with the port number you want to check for ):

netstat -ln | grep :PORT

-l = Show only listening ports

-n = Show port number

 If this command doesn't return anything, it shows that your PORT is available to use.
In your script you can check $? to see the return code of the above command. If it's non-zero, the PORT is available.

One thing to note though is that, this doesn't tell you the status of your network e.g. if PORT is accessible from outside of your notwork.

How to install VNC on Redhat and avoid getting "font catalog is not properly configured" error

 yum install -y  tigervnc-server tigervnc-server-module libXfont pixman xterm xorg-x11-twm

Setting YUM proxy settings in Redhat

If you are behind a proxy YUM won't be able to connect to its repositories. To fix this you will need to add your proxy setting to /etc/yum.conf

If you are using an anonymous proxy, only the following line needs to get added to yum.conf

proxy=http://<poxy-server>:<proxy-port>/

In case your proxy server requires authentication also add the following lines in addtion to the above

proxy_username=<proxy-user>
proxy_password=<proxy-password>

How to find the 10 largest file or directories in Linux

This finds the largest 10 files :

find . -type f -print0 | xargs -0 du -s | sort -n | tail -10 | cut -f2 | xargs -I{} du -sh {}

This finds the largest 10 directories:

find . -type d -print0 | xargs -0 du -s | sort -n | tail -10 | cut -f2 | xargs -I{} du -sh {}

You can easily change -10 to -n where n is the number of files/directories you are trying to find.

Removing passphrase from a SSH key

If you have a SSH key and are tired of typing it over and over, there is an easy way to remove it.

Open a *nix terminal and type :

bash-3.2$ ssh-keygen -p

Enter file in which the key is (/home/myuser/.ssh/id_rsa):
Enter old passphrase:
Key has comment '/home/myuser/.ssh/id_rsa'
Enter new passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved with the new passphrase.