There are two kinds of links: hard and soft links. We learned about hard links last time

They do not use any disk space, since they are only a new name to the same disk location

They are symmetrical: once you create a link, the new name and the old name have the same importance

If the original file is deleted, the second name still points to the same data

The file is only deleted when the number of links is 0

cp science.txt a.txt
ln a.txt b.txt
ls -l ?.txt

-rw-r--r--  2 anaraven  staff  7767 21 Nov 09:08 a.txt
-rw-r--r--  2 anaraven  staff  7767 21 Nov 09:08 b.txt

rm a.txt
ls -l ?.txt

-rw-r--r--  1 anaraven  staff  7767 21 Nov 09:08 b.txt

Link work only on one physical disk, since disk locations are valid only inside the same disk

In Unix there is only one “folders tree”, even crossing to different disks

In Windows each disk has a separated tree

C: is one disk, C:/Users/student is a folder inside a folder in the C: disk

The other kind of links is called soft links or symbolic links

They are asymmetrical: the new link is different from the original

They can link across different disk

They can link files and folders

If the original file is deleted, the link becomes broken

echo "Current date and time is"; date

Current date and time is
Wed 21 Nov 2018 09:08:05 +03

for i in 1 2 3; do echo $i; done

1
2
3

Single and double quotes can be used to write space inside an argument

for i in '1 2' "3 4" 5; do echo $i; done

1 2
3 4
5

echo '$USER'

$USER

echo "$USER"

anaraven

The symbol ` is called backquote

It is used to run a command and use its output as input to another command

echo "Now is `date`"

Now is Wed 21 Nov 2018 09:08:05 +03

Sometimes ` is hard to see

It is better to use something that reminds us of getting the value of something

echo "Now is $(date)"

Now is Wed 21 Nov 2018 09:08:05 +03

Up to 5

seq 5

1
2
3
4
5

From 2 to 6

seq 2 6

2
3
4
5
6

From 2 to 10 in steps of 3

seq 2 3 10

2
5
8

for i in $(seq 1 3 15); do echo $i; done

1
4
7
10
13

All UNIX systems come with a text editor called vi

Modern UNIX systems have also an improved version called vim

It is a very powerful system, but hard to learn

(with great power comes great responsibility)

Here we will learn to use nano, which is easier to learn

Open the editor with the command nano

The last two lines show the commands you can use

When it says ^G it means CTRL-G

When it says M-U it means ESC U

CTRL and G are pressed at the same time

ESC is pressed first, then released, then press U

Traditional Unix editor
=======================

All UNIX systems come with a text editor called `vi`

Modern UNIX systems have also an improved version
called `vim`

It is a **very powerful** system, but hard to learn

(with great power comes great responsibility)

Here we will learn to use `nano`, which is easier to learn