Here are a few important commands you would need to check the disk usage on a linux machine

A simple df (disk filesystem) command can help us get important info on linux file system. -h will make the data more human readable.

$ df
$ df -h

More variations of df -h command

http://www.tecmint.com/how-to-check-disk-space-in-linux/

Another important command would be du (disk usage) for a particular folder. du -h would give same data in human readable form.

Other useful variation of du command

du -sh
du- sh *
du -Pshx /* 2>/dev/null

If you want to send an email with attachment through linux server.

echo “This is the message body” | mutt -a “/path/to/file.to.attach” -s “subject of message” — recipient@domain.com

source –http://stackoverflow.com/questions/17359/how-do-i-send-a-file-as-an-email-attachment-using-linux-command-line

:%s/oldstring/newstring/gc 

This command will replace all the occurences for oldstring with newstring after confirming for each existence (c in the end tells vim to ask for confirmation).

More on search replace – http://vim.wikia.com/wiki/Search_and_replace

Some other useful commands are

/ for search 
n find next while search (after /)
N find previous while searcg (after /)

:$ last line of file
:0 first line of file

http://www.cs.colostate.edu/helpdocs/vi.html

There can be times that a process becomes unresposive and you want to kill it forcefully.

In linux, we will need to first find out the process id

ps command- it gives a list of all processes running.

ps aux- gives all process details with, a = show processes for all users, u = display the process’s user/owner, x = also show processes not attached to a terminal

to fetch only required process- use grep, ps aux| grep ‘name’

Then kill it forcefully- kill -9 PID

Putting it all together.

Lets say I want to kill eclipse

kamalmeet@System:~$ ps aux | grep eclipse

1101 6070 1.3 1.8 46584 5500 ? Sl 12:34 1:38 /usr/bin/java -Dosgi.requiredJavaVersion=1.6 -XX:MaxPermSize=256m -Xms40m -Xmx512m -jar /home/kamalmeet/eclipse//plugins/org.eclipse.equinox.launcher_1.3.0.v20140415-2008.jar

kamalmeet@System:~$ kill -9 6070

There are three type of groups who can access a file in linux.

User: who created the file
Group: Group to which user belongs
Others: Other users (not belonging to above 2 categories).

To add/ modify permissions use following format

chmod u=rwx,g=rw,o=rx abc.txt

or a simpler option

chmod 765 abc.txt

Both of the above means same.

How to interpret these numbers

Read=4
Write=2
Execute=1

hence Read+Write+Exceite=4+2+1=7 or Read+ execute=4+1=5 and so on.

Creating shortcuts in linux- It can be a pain to write down long linux commands again and again. So here is how you can create quick shortcuts for all reusable commands.

1. Edit bashrc: You will have a “.bashrc” file in your root folder. open it up for editing (emacs .bashrc) and add commands to be added for shortcut at the bottom of this file like
alias connect=’ssh kamal@kamal.com’

This will simply create a shortcut for the command ‘ssh kamal@kamal.com’ which will get executed by typing connect.

2. Compile bashrc: Before you can actually use your shortcuts, you will need to compile the .bashrc file by
. .bashrc

3. Use shortcuts: Just type the shortcut command connect and you will see that it actually executes the command for which ‘connect’ is an alias.