Git. Work with Bitbucket/GitHub repositories without username/password

Initially, you should have two generated keys: private - id_rsa and public - id_rsa.pub.

To do this, you need to run the command:

$ ssh-keygen -t rsa -b 4096 -f $HOME/.ssh/id_rsa

Next, you need to add your public key (its contents) via the web interface.
After that, you can work with your repositories without having to enter a password for every action you take.
But it's important to remember that you must work through ssh:// not through https://

If initially you cloned your repository like this

$ git clone https://git@REPOSITORY.git

then you need to change the remote URL to ssh://
To do this, run the command:

$ git remote set-url origin ssh://git@REPOSITORY.git

To look at current remote URL it's possible either that

$ git remote -v

or that

$ git config remote.origin.url

Git. Adding information about date and time to commit message

Sometimes it might be useful to add to commit information about current date and time.

So following commands show how you can do it:  

$ git add .
$ git commit -m "[feature-branch] Added utility files `date +'%Y-%m-%d %H:%M:%S'`"
$ git push origin feature-branch

Git clients for browsing git repository on Ubuntu

Nothing can replace the work with git in the console, but sometimes it's convenient to look at the repository from a bird's eye view. In this we can get help from different gui-clients.

There are a sufficient number of clients for different operating systems.
You can see them on this link - https://git-scm.com/downloads/guis.

There is a standard client on Ubuntu - gitk. But I didn't like it because it doesn't allow me to look at the entire repository. Only within one branch.

The following client - gitg - seemed to me quite good (look at the bellow picture).

You can test it. Install it on your unix system:

$ sudo apt-get install gitg

That's all. Go to the git repository, run next command and enjoy :)

$ gitg

Ubuntu. How to debug unix sockets like telnet

If you need to debug unix sockets like telnet service you can use netcat utility.

The syntax is:

$ nc -U /var/run/.../unixsocket.sock

Math. Combinatorics

The rule of sum.

If two actions A and B mutually exclude each other and action A can be performed in
k ways, and B - by n ways, then perform one of these actions (either A or B)
can be in k + m ways.

Example.

Let there be given two baskets, in one basket there are 8 red balls, in the other basket - 5 black
balls. How many ways can you pull out one ball? We can pull out either a red or a black ball.
By the rule of the sum we get that one ball can be pulled out in 8 + 5 = 13 ways.

The rule of product.

Let it be required to execute k actions sequentially. If the first action can be performed by N1
ways, the second action N2 ways, the third - N3 ways and so to the k-th action, which
can be performed in Nk ways, then all k actions can be performed in:

N = N1 * N2 * ... * Nk

ways.

Example.

Consider the baskets from the previous example. How many ways can you get two balls out?
First we can pull out either a red or a black ball. From the above example we know
that this can be done in 13 ways, so we can take out the second ball in 13 - 1 ways (one
the ball we pulled out at the first action), so we can take out 13 balls in 13 * 12 = 156 ways.

Combinations.

In combinations, the order of the elements is not important. That is, the following pairs of two symbols A and B in this case are identical: AB == BA.

Combinations without repetition.

Usually in this combination we can asks yourself: how many ways can I choose m elements from n elements?

 

Let's consider an example, let we have 4 books: A, B, C and D. We need to choose 2 books as a gift(!). How many ways can you do this? We need to choose from 4 items 2 items, and the order is not important. Therefore you need to find the number of combinations of 4 elements of 2.

       

As a result, we get 6 combinations: AB AC AD BC BD CD.
Note that here there are no repetitions (AA, BB, ...) and in fact AB is the same as BA.
The main thing is that in the set are two elements A and B.

Combination with repetition.

How many ways can be made up of n different types of combinations of m elements, if you do not take into account the order of the elements in the combination but elements of the same type can repeat?

 

Let's consider an example. In the confectionery shop were sold 4 varieties of cakes: A, B, C and D.
How many ways can we buy 2 cakes?

 

As a result, we get 10 combinations: AB AC AD BC BD CD AA BB CC DD. As we see, in combinations there are repetitions: AA BB CC DD.

Permutation.

In the layouts, unlike the combinations, the order of the elements is important. That is, the following pairs of two symbols A and B in this case are different: AB <> BA, because they have different orders of location.

Permutation without repetition. 

In this case, we ask the question: how many ways can I choose and place n different items in m places?

 

Let there be 4 symbols: A, B, C and D. How many ways you can combine two symbols from this set
avoid duplicating characters?

 

We got 12 ways:

AB AC AD BC BD CD
BA CA DA CB DB DC

Permutations with repetitions. 

Accordingly, how many ways can I choose and place n different objects in m places, not excluded among them the same

 

For example, let's use the condition of the problem from the previous example, but duplication of symbols is acceptable. That is, how many ways can the symbols A, B, C and D be combined in two characters?

 

We got 16 ways:

AB AC AD BC BD CD
BA CA DA CB DB DC
AA BB CC DD

Transpositions.

In transpositions, absolutely all elements of the set are considered. As well as in permutations
the order of the elements is important.

Transposition without repetition.

In this case, we ask the question: how many ways can I place n different subjects on n different places?

 

In fact, a transposition without repetitions is the permutation of n elements in n places.

 

Let there be given a string of 4 symbols - ABCD. How many ways can I rearrange the characters in this line?

 

In total 24 ways we can rearrange the characters in this line:

ABCD BACD CBAD DBCA
ABDC BADC CBDA DBAC
ADCB BDCA CDAB DACB
ADBC BDAC CDBA DABC
ACBD BCAD CABD DCBA
ACDB BCDA CADB DCAB

Transposition with repetition.

If there are identical elements among the n elements to be sorted, then we can ask:
How many ways can I rearrange n items that are sold in n different places, if there are among them
k of different types (k < n), that is, of the same types?

 

Example. 

How many different letters can be made from the letters of the word "mississippi"?
Here 1 letter "m", 4 letters "i", 4 letters "s" and 2 letters "p", only 11 letters.
Hence the number of permutations with repetitions is

 

Below is a diagram of the algorithm for choosing the appropriate way of working with a given set of elements.