Linux

Question 1

sudo !!

Answer 1

Runs the previous command, with sudo.

In general, the two exclamation marks refer to the previous command you just ran.

Question 2

cd -

Answer 2

Returns to the previous working directory

Question 3

pushd

popd

Answer 3

pushd changes into the new directory and pushes the current working directory to a stack.

popd brings one of the items out of the stack and changes into it

Question 4

ctrl + z

fg

Answer 4

ctrl + z sends an app to the background, effectively minimizing it.
fg brings that app out.

Question 5

ctrl + r

Answer 5

Searches through the commands previously ran. Pressing ctrl + r again will go to the next best match.

Question 6

history

Answer 6

Shows the command history.

Question 7

!

Answer 7

Runs a previously ran command from history. In the Linux world, the exclamation mark is often referred to as bing.

Question 8

HISTCONTROL=ignoreboth

Answer 8

If you assign ignoreboth to this variable, your can prevent commands from showing up in history by adding a space in the beginning,

Question 9

HISTTIMEFORMAT=”%Y-%m-%d %T “

Answer 9

If you assign “%Y-%m-%d %T “ to this variable, commands stored in the history will have a specific time assigned to them. This could be placed in .bashrc. The space at the end is for formatting.

Question 10

cmatrix

Answer 10

A package and a command to show a Matrix-inspired screen saver sort of thing.

Question 11

ctrl + u

Answer 11

Clears the line.

Question 11

ctrl + a

Answer 11

Puts the cursor all the way to the front of the line.

Question 12

ctrl + e

Answer 12

Takes the cursor to the end of the line.

Question 13

What is the difference between changing commands with ; and &&?

Answer 13

When you chain commands with a semicolon, all the commands will run even if one or more of them fail.
On the other hand, when you chain commands with the double ampersand, if it encounters an error it will not run subsequent commands.

Question 14

tail -f

Answer 14

Shows the content of a file (a log file for example) and updates if anything new is added to the file.

Question 15

truncate -s 0

Answer 15

Sets the size of a file. If 0 is provided as the first argument, it will empty the file.

Question 16

column -t

Answer 16

Creates columns out on output. For example mount | column -t.

Question 17

Basic data streams in Linux

Answer 17

Standard output (stdout) and 1 is the number designation 
Standard input (stdin) 0
Standard error (stderr) 2

Question 18

echo $?

Answer 18

Gives us the return code for the previous command.
0: success
Anything else is considered an error.

Question 19

find /etc -type f 2> /dev/null

Answer 19

The find command is obvious.

What 2> /dev/null does is, it redirects all the errors to another file and prevents them from showing up in the output.

The > symbol is the redirect operator and it will override the file.
But if you use >> the output will be added to the end of the file.

2 is the designation number for stderr. If you don’t have a number designation, the default is 1 (which is stdout).

/dev/null is essentially a black hole, anything sent there will be lost forever.

Question 20

set -x

Answer 20

Prints out all the commands running for calculating the prompt.
After running that, run exec 2>zsh.err to get stderr into a file.

Question 21

grep –files-without-match “git” ~/.oh-my-zsh/themes/*

Answer 21

Shows file names that do not match a string.

Question 22

yes

Answer 22

prints a lot of output

Effectively taking up all the resources

Question 23

ps

Answer 23

Shows the running processes as part of this particular terminal session

The response is a table with the following format:
PID: the process id
TTY: the terminal that the process is running in. If the process is running as a part of a terminal, it’s going to show here. Otherwise it’s going to show a question mark.
TIME: how much time has the process been utilizing the CPU
CMD: the actual command that is running as part of that process

Question 24

ps -aux

Answer 24

This form of options is called UNIX style options, which includes a dash at the beginning followed by any number of letters. These letters each represent an option.

Question 25

Different styles of options a command can get

Answer 25

UNIX Style: ps -aux
BSD style: ps -aux
GNU style: ps –quick-pid

Question 26

ps x

Answer 26

See all processes running from the current user
There’s a STAT column. Each letter means:

s: the process is a process leader and not a child
S: the process is in an uninterruptable sleep and is waiting for user input
R: the process is actively running
T: the process has stopped. Just like we use ctrl + z to background a process

Question 27

ps -He

Answer 27

It shows a process hierarchy and process relationships

The H option formats the CMD column to format the parent-child relationships between the processes.

The e option tells the command to show all the processes that are running on this system.

Question 28

ps -axjf

Answer 28

Columns:

PPID: parent process id
SID: session-id
PGID: parent group process id
TPGID: the terminal session-id with which the terminal is associated. If there’s no terminal, a -1 is displayed
UID: user id. 0 will be root. 1000 is the first non-system user created

Question 29

ps aux

Answer 29

The most popular form of the ps command.

Shows CPU and memory usage. The start time as well.

Question 30

du -sh

Answer 30

shows a human-readable size for the current woking dir or a specified folder

Question 31

encode a value to base 64

Answer 31

echo -n | base64

Question 32

awk ‘{}’

Answer 32

runs a command on a file.

In general, the awk command runs a script on a file

Question 33

awk ‘{print $1}’

Answer 33

prints the first filed on a file (the columns should be divided by space, as it’s the default separator in awk)

Question 34

awk ‘{print $0}’

Answer 34

Prints the entire file, since the provided value for the column number is 0 and 0 represents the entire file

Question 35

awk ‘{print $1,$3}’

Answer 35

Prints the first and third fields of a file

Question 36

awk ‘{print $NF}’

Answer 36

prints the last filed of a file.

$NF means number of fields and it kinda works like arr.length

Question 37

awk -F’:’ ‘{print}’

Answer 37

changes the delimiter or separator string that awk uses (default is space)