Comparing files in Linux with diff

Comparing files

diff man

Suppose you have a file 1 with the following content

a

And a file 2 whose first line is empty

a

you can compare them with the diff command

diff 1 2

0a1 >

The output of the diff command contains instructions on how to turn the first file into the second.

0a1 means:

After the zeroth line of the first file you need to add the first line of the second file.

In other words: at the beginning of the first file you need to insert the first (empty) line of the second file and then the files will become identical

> means the second file. After it there should be the content of the line that needs to be inserted, but it in our case is empty, so there is nothing.

a - add - to add

c - change - to change

d - delete - to delete

Let's make sure that we understood everything correctly - open the file 2 and write some text into the first line that was empty. Nothing else needs to be changed.

cat 2

www.topbicycle.ru
a

Let's compare the files again 1 and 2 . We expect that nothing will change, only after > our text will appear

diff 1 2

0a1 > www.topbicycle.ru

As you can see, that is exactly what happened, because to get the second file from the first you need at the very beginning of the first file to add a line with the text www.topbicycle.ru

If we now try to compare not the file 1 with 2 but the other way round 2 with 1 . What should change: now the file 2 is taken as the basis and the diff command will tell us what needs to be done to get from the file 2 the file 1

diff 2 1

1d0 < www.topbicycle.ru

1d0 means that you need to delete the first line of the first file and then we arrive at the beginning of the second file

< www.topbicycle.ru is the content of the line of the first file in question

Consider file 3 in which the first and second lines are empty and the third contains the character a

Think for yourself what will happen if you compare the files 1 and 3. And also the other way round 3 and 1

vi 1

a

vi 3

a

diff 1 3

0a1,2 > >

0a1,2 means that at the beginning of the file 1 you need to add the first and second lines of the file 3

If we compare in the opposite direction

diff 3 1

1,2d0 < <

1,2d0 means that in the file 3 you need to delete lines 1 and 2 and then we arrive at the beginning of the file 1

0 at the end can be understood as the line by which the files will be aligned.

As an example, let's create file 4 in which the first line is the character a, the second line the character b

vi 4

a b

Let's add three empty lines in the middle and save as file 5

vi 5

a b

Let's compare 4 and 5

diff 4 5

1a2,4 > > >

To get from file 4 the file 5 you need after the first line of file 4 to add the lines from the second to the fourth of file 5

Everything is simple here, let's try the reverse

diff 5 4

2,4d1 < < <

To get from file 5 the file 4 you need to delete the lines from the second to the fourth in file 4 and then we will land on line 1 of file 4

That is, in this example, after deleting, we aligned by line 1 and not by the beginning of the file as in the previous example.

Suppose you have a file 1 with the following content

a
b
c

And an empty file 2 you can compare them with the diff command

diff 1 2

1,3d0 < a < b < c

Let's decipher the result: you need to delete the first three lines in the first file and then the files will align at the beginning of the files.

If we now add a to the first line of file 2 and compare again

diff 1 2

2,3d1 < b < c

Let's decipher the result: you need to delete the second and third lines in the first file and then the files will align after the first line of the files.

Suppose you have a file websites.txt with a list of sites

cat websites.txt

https://www.urn.su https://www.heihei.ru https://www.topbicycle.ru

Copy it with the command

cp websites.txt newsites.txt

It is easy for now to make sure that the files are identical, you can even run

cat newsites.txt

https://www.urn.su https://www.heihei.ru https://www.topbicycle.ru

And understand that nothing has changed.

If the file is too large you can use the command diff

diff websites.txt newsites.txt

While the files are identical the output will be empty

Add a new entry to the file newsites.txt

echo https://www.aredel.com >> newsites.txt

cat newsites.txt

https://www.urn.su https://www.heihei.ru https://www.topbicycle.ru https://www.aredel.com

The diff command will now show the place in which the files differ

diff websites.txt newsites.txt

3a4 > https://www.aredel.com

3a4 means that the difference was found between the third line of the first file and the fourth line of the second.

In this case the fourth line of the first file simply does not exist

> https://www.aredel.com these are the characters that are not in the first file

Make finer changes. For example, let's delete www. before heihei.ru in the second file

vi newsites.txt

https://www.urn.su https://heihei.ru https://www.topbicycle.ru https://www.aredel.com

diff websites.txt newsites.txt

2c2 < https://www.heihei.ru --- > https://heihei.ru 3a4 > https://www.aredel.com

< means - in the first file

> means - in the second file

diff finds all lines with differences and outputs them. So if you make some more changes in the same line, for example add at the end /Spain/ fundamentally nothing will change

vi newsites.txt

https://www.urn.su https://heihei.ru/Spain/ https://www.topbicycle.ru https://www.aredel.com

diff websites.txt newsites.txt

2c2 < https://www.heihei.ru --- > https://www.heihei.ru/Spain/ 3a4 > https://www.aredel.com

If we add several empty lines

https://www.urn.su https://www.heihei.ru https://www.topbicycle.ru https://www.aredel.com

Ranges with empty lines will be shown

1a2,5 > > > > 2a7,13 > > > > > > > 3a15 > https://www.aredel.com

a2,5 means that from the second to the fifth lines

a7,13 means that from the seventh to the thirteenth

If we delete several lines

vi newsites.txt

https://www.urn.su https://www.aredel.com