Linux Commands

Question 1

Is Linux same as UNIX?

Answer 1

No, they are not the same.
They are different OS and UNIX was created long before Linux, which is said to be UNIX-like OS.
Linux is used pretty much everywhere, PCs, servers, game dev, mobiles etc while unix is mostly used in servers, workstations

Question 2

What is:
cd ~

Answer 2

Go to home dir of current user

Question 3

Absolute path vs relative path?

Answer 3

Absolute always starts from root /root/home/kmylonas/Document
If a path starts with / then its an absolute path.
Relative is from where you are now for example if you’re kmylonas:
cd Documets or cd ./Documentrs

Question 4

rmdir can only remove empty directories?

Answer 4

Yes

Question 5

How to remove non empty dir?

Answer 5

rm -r

Question 6

Whats the difference between && and ; ??

Answer 6

&& executes sequence of commands only if all of them succeed
; executes sequence of commands independently, some may fail

Question 7

What does:
chmod 753 file.txt

Answer 7

file.txt permissions:
user: rwx
group: rx
others: wx

It is equivalent to:
chmod u=rwx,g=rx,o=wx file.xt

Question 8

Use case of chmod -R ?

Answer 8

-R is used to change permission to directory and its contents

Question 9

What is a softlink and how to create it?

Answer 9

A softlink is like a shortcut in windows.
Create it with ln -s original_file shortcut_file

Question 10

Changes to softlinks apply to original file?

Answer 10

Yes

Question 11

What happens to soft links when you delete their original file?

Answer 11

They have no use..

Question 12

Can you have a softlink for directories?

Answer 12

Yes

Question 13

How to create hardlinks?

Answer 13

ln sourcefile hardlinkname

Question 14

ls -l shows the number of softlinks or hardlinks?

Answer 14

Number of hardlinks

Question 15

Can you create hardlink for directories?

Answer 15

No.

Question 16

Should you use absolute path or relative when creating links?

Answer 16

According to giatrakos, absolute path

Question 17

If you have an original file and a hardlink, what happens when you delete the original file?

Answer 17

You can still access the original data through the hardlink.

Question 18

What fits to the ?
find -type ? -name “file*”

Answer 18

? can be f if you want files whose name start with “file”
? can be d if you want directories …
if no type set then find command returns everything

Question 19

find -type f -perm 701 -name “file*”

Answer 19

find files with perm rwx—–x and name prefix file

Question 20

Whats the use of []?

Answer 20

Regex like [a-Z] or [abcd] etc

Question 21

Whats the use of {}?

Answer 21

echo file{1..10..2} will create file1 file3 file5 … file10

Question 22

Whats the usage of tee command?

Answer 22

tee file1 file2…
Takes input from stdin and outputs to each file (file1, file2..) and to the stdout
For example if you want to echo a message and log it to a logfile tee would be a way to go

Question 23

How to use “more”

Answer 23

spacebar -> next page
b -> previous page
enter -> next line
q -> exit

Question 24

Whats the main advantage of less against more?

Answer 24

Less doesn’t have to load the whole input file at once so it’s the way to go for big files.

Question 25

grep -n “Stat” sample_data/

Answer 25

Search whole directory for line matches containing Stat
-n will return the line number of each match

Question 26

!grep -r “Stat” sample_data/

Answer 26

Search subdirectories recursively

Question 27

!grep -l “Stat” sample_data/

Answer 27

Instead of returning line numbers return the filenames that you found the match

Question 28

!grep -w “Census” sample_data/README.md

Answer 28

search for a specific word
-cw would return the number lines that match
-cvw return the number of lines that didnt match

Question 29

Defaults about sort command?

Answer 29

delimiter is considered whitespace
sorts alphabetically, not numerically

Question 30

What’s the use of cp -R (equivalent to cp -r)

Answer 30

Use -R to copy entire directory with its contents

Question 31

mv -i
cp -i

What’s the use of -I flag?

Answer 31

With -I those commands will prompt you before proceeding, so you make sure that you don’t override any preexisting files

Question 32

chmod a=rw file.txt

Answer 32

all: owner, group,others = rw permissions

Question 33

usermod -a -G my_group newperson

Answer 33

Add new user to the new group

Question 34

usermod -g my_group newperson

Answer 34

change the primeary group of the user

Question 35

mkdir testchown
touch ./testchown/file.txt
chown -R newperson testchown

Answer 35

newsperson is the user of the testchown directory and its contents like file.txt

Question 36

!find . -type f -perm 701 -print -exec chmod 644 {} \;

Answer 36

In current directory find all files with permissions 701 and change their permissions to 644. Its common to use exec for this use case

Question 37

!find . -user root -iname “*.JSON”

Answer 37

in current directory find files where user owner is root and name ends in .json case insensitive

Question 38

!find -L /content -samefile testchmod/myfile.fd

Answer 38

find softlinks and hardilnks of my file.fd under testchmod

Question 39

!grep -w “Census” sample_data/README.md

Answer 39

Search for a distinct word Census.

Question 40

wc -c

Answer 40

write total number of bytes

Question 41

wc -m

Answer 41

write the total number of characters

Question 42

sort -r +2 file.txt

Answer 42

sort in reversed alphabetical order and the key is the third column

Question 43

!sort -t” “ -k 3.3,3.5 sortmylines.txt

Answer 43

delimiter = “ “
Start sorting from 3d char of 3d field till 5th char of 3d field

Question 44

!sort -c sortmylines.txt

Answer 44

check if a file is already sorted

Question 45

cut -f2-3 -d” “ sortmylines.txt

Answer 45

show from 2 until 3d column
if it was -f2-5 would show column2,3,4,5

equivalent would be -f2,3