Working at the Command Line

Question 1

[cmd] pwd

Answer 1

print working directory - outputs the current directory that you’re in

Question 2

[cmd] tty

Answer 2

prints the file name (because everything in linux is a file) of the terminal connected to the standard input. IE your current terminal.
Physical Terminal = tty1
Pusdo Terminal = pts/0

Question 3

[cmd] who

Answer 3

Tells you who is logged onto the system.

Question 4

[cmd] exit

Answer 4

logs you out of the current terminal.

Question 5

[cmd] ip a s

(aka ip addr show)

Answer 5

prints all the current network interfaces and their IP information.

Question 6

~

Answer 6

indicates the home directory

Question 7

[cmd] ls

Answer 7

lists (non-hidden) files and directories in your current working dir.

NOTE: ls is aliased by default to ‘ls –color=auto”.

Question 8

[cmd] ls -a

Answer 8

lists all files including hidden files.

Question 9

[cmd] ls -F

Answer 9

shows the file type in the printed output. So dir files end in a / and symbolic links end in an @ and executables end in * — useful if your terminal doesnt support colors

Question 10

/etc

Answer 10

this dir contains all the system/service configuration files.

Question 11

configuration file

Answer 11

local file used to control the operation of a program but is static and cannot be an executable binary.

Question 12

[cmd] ls -l

Answer 12

gives us a long listing of file types with metadata, which includes file type - permissions block - number of hard links - owner - group owner - size - date modified - name

Question 13

[cmd] ls -r

Answer 13

reverses the order of the printed output

Question 14

[cmd] ls -lrt

Answer 14

prints the long version, and reverses it based on time. Prints files but reversed based on time, so the oldest file will be listed first. (you can replace t with o to sort by owner)

Question 15

[cmd] ls -h

Answer 15

puts the file size in a human readable format

Question 16

[cmd] ls -d

Answer 16

lists the dir itself instead of the contents of the directory.

Question 17

[cmd] $(cmd)

ie: ls -l $(tty)

Answer 17

anytime you put $() the command line will run the cmd in ()s first and then the output of that is used as the input of the other cmd.

Question 18

[cmd] lsblk

Answer 18

lists the block devices (partitions) on the system. (which because everything is a file, these exist in the dev dir also)

Question 19

[cmd] * at end

ie: ls -l /dev/sda*

Answer 19

wildcard character that represents 0 or more characters. ie: lists anything that follows including zero characters. so the printed result will be /sda, /sda1, and /sda2

Question 20

[cmd] ? at the end

Answer 20

same as * wildcard but only looks at a single character. so /sda and /sda20 wouldn’t be returned.

Question 21

[cmd] [12] at the end

ie: ls -l /dev/sda[12]

Answer 21

this is a wildcard array, so in the case of the example it would be looking for sda1 and sda2

Question 22

/dev directory

Answer 22

this is where the device files are stored.

Question 23

linux file types

Answer 23

- : regular file
d : directory
c : character device file (ie tty)
b : block device (ie sda)
s : local socket
p : named pipe
l : symbolic link

Question 24

[cmd] cat

Answer 24

The cat command is used for:

• Display text file on screen
• Create a new text file
• Read text file
• Modifying file
• File concatenation

Question 25

[cmd] which

Answer 25

this command locates executables in the system. So you pass command names as arguments and it will return that commands file path.

Question 26

[cmd] cp

Answer 26

copies a file. *note must have read permissions on the file you want to copy and write permissions on the dir you are trying to copy to. The first argument is the file you’re copying the second is the location and name of the file you are copying to.

Question 27

ctrl+l

Answer 27

clears the screen

Question 28

[cmd] cp -i

Answer 28

the i option to the cp command takes you into interactive mode and warns you if you are about to overwrite an existing file

Question 29

[cmd] mv

Answer 29

allows you to move a file to a new location or rename a file. Works just like cp but moves instead of copies

Question 30

[cmd] rm

Answer 30

removes the file defined in the argument. Can also be used in interactive mode with -i option like cp and mv

Question 31

[cmd] mkdir

Answer 31

makes a new directory and takes the directory name and path as an argument. *NOTE all directories must already exist in the provided path argument or you will get an error that the dir cannot be created. (note: listing multiple arguments will create multiple dirs at a time)

Question 32

[cmd] mkdir -p

Answer 32

the -p option creates the parent directory even if it doesn’t already exist so that you will not get the “dir cannot be created” error that you would with just mkdir

Question 33

[cmd] rmdir

Answer 33

removes a dir. *NOTE will get a “failed to remove dir” error if the dir is not empty.

Question 34

[cmd] !rm

Answer 34

runs the last command that begins with the characters after the ! so in this case it will try to run your last rm cmd.

Question 35

[cmd] rm -rf

Answer 35

The -r option deletes recursively, and the f is forcibly, so unlike just a rmdir this will delete a dir and its contents.

Question 36

[cmd] touch

Answer 36

touch works the same way as mkdir but creates a file provided in the arg instead of a dir.

Question 37

[cmd] touch file{1..5}

Answer 37

the {1..5} will create 5 files named file1 - file 5.

Question 38

[cmd] cp -R

Answer 38

the -R option for the cp command means regression, so it will allow you to copy a dir and all its files to another dir.

Question 39

[cmd] tree

Answer 39

lists the file tree with all subfiles and sub directories of the dir provided in the arg. NOTE: tree is a separate yum package that may need to be installed.

Question 40

[cmd] mkdir -m

Answer 40

the -m option allows you to set permission on a dir when you create it.

Question 41

/etc/.

Answer 41

the . file in a dir is an actual file because everything in linux is a file, but it links back to the parent dir. So whenever you create a dir a dotfile linking to that dir is always created with it. The minimum hardlink count for a dir will always be 2, because you have the dir itself and the dotfile linking to it’s one set of metadata. (NOTE: Even though this is a file its of dir type because everything even dirs are a file in linux. )

Question 42

/etc/..

Answer 42

there will also be a file called .. in every dir that links to the metadata of the parent dir. So every time you create a sub dir in a dir the hardlink count will increase by one because of the .. file that is created that links back to it,. (NOTE: Even though this is a file its of dir type because everything even dirs are a file in linux. )

Question 43

[cmd] ls -i

Answer 43

the i option on the ls command shows you the metadata id (i node) of that file. This is what any link file like . or .. links to.

Question 44

[cmd] ln

Answer 44

makes a hard link from one file to another that are provided as arguments. Both files will show a hard link count of 2 and the i node number will be the same. Which means that these two files are actually the exact same file but they do not show as links in the file type (ie they aren’t type l) they are both individual regular files of type -

Question 45

[cmd] ln -s

Answer 45

the s option creates a symbolic link… it kinda works the same as a hard link but creates a separate file with type l and separate i node number and doesn’t affect the hard link count number.

Question 46

symlink vs hardlink

Answer 46

the advantage to symbolic links is that you can cross filesystem boundaries so for example a hard link cant be made from the boot dir to the home dir (you’d get a “invalid cross-device link” error, but a symbolic link can be because there is no dependance on the file system (ie you’re not sharing that same inode metadata that was generated on two different file systems).