cheatCommands.txt.save

------------------------------------------ FIND AND REPLACE TEXT WITHIN A FILE WITH COMMAND -------------------------------------------------------------------

sed -i 's/original/new/g' file.txt
Explanation:

sed = Stream EDitor
-i = in-place (i.e. save back to the original file)
The command string:

s = the substitute command
original = a regular expression describing the word to replace (or just the word itself)
new = the text to replace it with
g = global (i.e. replace all and not just the first occurrence)
file.txt = the file name




------------------------------------------------ CHANGE DEFAULT PYTHON VERION FOR LAUNCH ---------------------------------------------------------------------

if it is your first time follow these instruction


1. Creating alias in bashrc:
We can create an alias within user’s home directory and add it to the bashrc. But it will only work for the current user.
Open the bashrc file by using the following command:

>> nano ~/.bashrc

2. Add the following line to the below of the bashrc file .

>> alias python=‘/usr/bin/python3.7’

3. Now if we use check python version using ‘python -V’, it will return 3.7.2. And if we run the program.py file using ‘python program.py’, then it will be run by python3.5 executable.
Using update-alternatives:
To switch between python version over the all users, we can use update-alternatives command.
We will set priority of each version using update-alternatives. Python executable with the highest priority will be used as default python version.
We will set the priority by the following commands:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 3
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.7 4
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8 5
Here I set the priority of python 2.7, 3.5, 3.6, 3.7, 3.8 as 1, 2, 3, 4, 5. As the python3.8 has the highest priority, the default python executable is python3.8 now.

4. To switch between any versions, we can use the following command:
>> sudo update-alternatives --config python


you can simply follow number 1, 2, 4
and if you've done this before, you can simply follow number 4


-------------------------------------------------------------- DISK CLEAN ---------------------------------------------------------------------------------------

apt-get clean : flushes the cache of packages that are to be installed, right? So isn't the worst scenario that can happen is that I have to download the package again?

apt-get autoclean : does the same thing as apt-get clean except it does not remove the cache of the latest version, so it's even "safer" than the worst 
case scenario of apt-get clean, right?

apt-get autoremove : "removes orphaned packages that were installed as dependencies, but are no longer.

Thanks.





--------------How to pipe ls to grep, and delete the files filtered by grep?
--------------------

Since you're dealing with a list coming to STDIN and rm expects parameters, you need to use xargs.

Thus:

ls | grep chrome | xargs rm
Should give you what you want.

Note that if you want to delete everything other than the chrome file, you can simply add -v to the grep statement.

Note that, per the other answers to this question, this is probably a bad way of accomplishing what you want to accomplish.








-------------------ADD A STRING OR WORD IN BEGININING OF ALL LINES IN A TEXT FILE USING PERL---------------------------------------

You have a file that contains a great number of lines, let's say 183 lines, and I want to add: echo " to the beginning of each line. What is the easiest way to do it?


Code:
cat filename.txt  | perl -ne 'print "echo " . $_' > output.txt

NOTE: "echo " is our string used here.


USING SED COMMAND


This example shows how to add a character to the beginning of each line using a sed command and bash shell.
Let's create example file.txt with some text:

add 
character
at the
beginning of
each line
Add character at the beginning of each line using sed command. For example to add # in front of each line we can use sed command with following syntax:

$ sed 's/^/#/' file.txt
#add
#character
#at the
#beginning of
#each line
replace # with ' ' ( space ) to add space in front of each line:

$ sed 's/^/ /' file.txt
 add
 character
 at the
 beginning of
 each line
Redirect the output produced by sed command to save it to a file:

$ sed 's/^/ /' file.txt  > new-file.txt
$ cat new-file.txt
 add
 character
 at the
 beginning of
 each line



------------------------------------------ QUICK COMMANDS --------------------------------------------

ls -d */    - list all directories in a directory

ls -f */     - list all files
ls -f /home  - list all files on the home directory

ls -l */     - list all files and their permission levels in a directory.

shift+ctrl+f   - find a string in the displayed strings on terminal.