Week 3

Question 1

executing multiple commands?

Answer 1

1)command1;command2;command3;
each command will be executed one after other even if any one or more commands have error or failed to execute

2) command1 && command2

Command 2 will be executed only if command1 succeeds

3) command1 || command 2
command2 will not be executed if command1 succeeds

Question 2

how to execute multiple commands in a subshell and then return their result to the original shell?

Answer 2

using parenthesis, we could do this.
e.g.: (ls, date, we filename.txt)

Question 3

What is the name of the variable which gives the shell level in which the command is executed? Why do we need subshell?

Answer 3

$BASH_SUBSHELL , We need subshells so that we could confine the running environment of certain commands.

Question 4

What are file descriptors?

Answer 4

There are 3 file descriptors.
Stdin (0) - Input stream normally keyboard
Stdout(1) - Output stream- usually monitor
stderr (2) - usually points to the monitor.

In linux, we assume that all these file descriptors are streaming characters which could be redirected to a file or a command.

Question 5

What is the redirection operator?

Answer 5

command > file 1 (Stdout is redirected to a file). New file 1 will be created if it does not exist already.. if it already exists then it will be overwritten (Caution) , Note: write permission is required for the current directory else it wont work

Question 6

Which command is used for displaying the hardware infor of the system?

Answer 6

hwinfo

Question 7

what is the output for cat > file1

Answer 7

cat enables stdin and what ever we enter via keyboard will go into the file1. we could come to pwd by pressing Ctrl +D (End of File)

Question 8

what happens when we execute just the cat command?

Answer 8

cat takes input from keyboard and displays it back to screen. We could come out by pressing Ctrl+D

Question 9

How to append to a file?

Answer 9

using double redirection “»” . If file does not exist.. it will be created and if it exists, then the new content will be appended and NOT replaced, if write permissions are given.

e.g: date&raquo_space; file2; wc-l /etc/profile&raquo_space; file2; file /usr/bin/znew&raquo_space; file2;

Question 10

How to redirect error stream to file? can we send output to a file and error to a different file?

Answer 10

ls $HOME / blah 2>error.txt

ls -$HOME / blah > output.txt 2>error.txt

ls -R /etc (Recursively traverses into the directory etc)

Question 11

How to serve as a file as std input (ie., serve file as input stream when the program is expecting input from keyboard)

Answer 11

cat < error.txt (will serve error.txt as input to command cat)

Question 12

How to redirect output and error to the same file?

Answer 12

command > file1 2>&1

Question 13

What happens when we use pipes with commands?

Answer 13

command1 | command 2

Stdout of command1 will become stdin of command2

ls /usr/bin | wc-l
combines commands.

one command line with multiple commands and pipes are possible.

Question 14

How to use redirection, pipes together?

Answer 14

command1 | command2 > file1

stdout of command1 fed into command 2 as stdin and stdout of command 2 is fed as stdin of file1

Question 15

How is /dev/null special?

Answer 15

It is a sink for output to be discarded. Use! silent and clean scripts.

command > file1 2> /dev/null
this command output will be written to file 1 and any error will be written to null file and subsequently be discarded.

Question 16

what does tee command do?

Answer 16

tee command reads from stdin and writes to stdout and files.

could be combined along with pipe

Tee command could be used to write multiple files.

e.g : ls $HOME | tee file1 file2

ls $HOME //blah 2> /dev/null | tee file1 file2 | wc-l

Question 17

What are package management systems? why do we need one?

Answer 17

1. Tools for installing, updating, removing, managing software
   1a. Install new/updated software across network.
2. Package - File lookup, both ways.
3. Database of packages on the system including versions
4. Dependency checking
5. Signature verification tools.
6. Tools for building packages.

Question 18

What are major package types?

Answer 18

RPM (RedHat - CentOS, Fedora, Oracle Linux, openSUSE), SUSE Enterprise Linux
* for Maintaining Linux servers.. this will be helpful

DEB (Debian - Knoppix, Ubuntu - Mint
* for manintaining regular ubuntu installations , this will be helpful.

Question 19

How to find the current operating system running on our machine?

Answer 19

lsb_release -a

Question 20

What are some of the software architectures for which linux is available?

Answer 20

1. amd 61 | x86_64
2. i386 |x86
3. ARM
4. ppc64el | Open POWER
5. all |noarch|src

Question 21

What are some of the package tools?

Answer 21

1. RPM (YUM- Yellowdog Updater Modifier) - RedHat Package manager(RPM), Dandified YUM (dnf)
2. DEB - Synaptic(graphical), aptitute(Command line) - Advanced Package Tool (apt) - dpkg, kpka-deb

Question 22

How to inquire package database?

Answer 22

search packages for a keyword
apt - cache search keyword

Question 23

How to list all packages currently installed in our system?

Answer 23

apt -cache pkgnames
apt-cache pkgnames nm

Question 24

How to display package recrods of a package?

Answer 24

apt -cache show -a package
apt-cache show nmap

Question 25

What are the notations for package name of RPM and DEB package types?

Answer 25

RPM - Package-version-release.architecture.rpm DEB - package _version-revision_architecture.deb

Question 26

What are package priorities?

Answer 26

1. required - essential to proper functioning of the system. 2. important - provides functionality that enables the system to run well 3. standard - included in a standard system installation 4. optional - can omit if you do not have enough storage 5. extra - could conflict with packages with higher priority, has specialized requirements, install only if needed.

Question 27

What are package sections?

Answer 27

these are pre-bundled category of packages e.g: web, admin ulities, Games, GNOME, GNU, Phython, Ruby, Video etc.

Question 28

What are some of the checksums ?

Answer 28

1. md5sum (128 bit) 2. SHA1 (160 bit) 3. SHA256(256 bit)

Question 29

In Ubuntu, what is the administrators group referred as?

Answer 29

sudo Only sudoers can install /upgrade/remove packages

Question 30

Where is all sudoers listed?

Answer 30

/etc/sudoers

Question 31

how a superuser can find out unsuccessful sudo attempts

Answer 31

through log file /var/log/auth.log

Question 32

where is authentic sources list file from where packages will be downloaded will be stored?

Answer 32

/etc/apt Files: sources.list Folder: sources.list.d (Contains sources from other programs like google, Teamviewer etc..) - as and when updates are released, package manager will update.

Question 33

How to downloaded the updates and keep it in cache?

Answer 33

sudo apt-get update The updates applicable only to your installed packages are used .

Question 34

How to install the updates downloaded to cache?

Answer 34

sudo apt-get upgrade

Question 35

After updating the existing packages. How to remove the unused but downloaded package updates in cache?

Answer 35

sudo apt-get autoremove -removes outdated versions and unwanted files. apt-get clean clean local repository of retrieved package files apt-get purge purge package files from system

Question 36

How to uninstall a package?

Answer 36

sudo apt-get remove

Question 37

How to install a package?

Answer 37

sudo apt-get install sudo apt-get reinstall

Question 38

where could we find details about the dpkg package manager?

Answer 38

/var/lib/dpkg Files: arch, available, status Folder: info

Question 39

How to list all dpkg packages?

Answer 39

dpkg -l pattern

Question 40

How to list installed files that came from packages?

Answer 40

dpkg -L package

Question 41

How do find dpkg status of packages?

Answer 41

kpkg -s package

Question 42

How to search installed packages for a file?

Answer 42

dpkg -S pattern

Question 43

How to query dpkg database?

Answer 43

dpkg-query -W -f='${section} {binary:Package}\n' -w : show -f ; format lists the package section and naame of the executable/package.

Question 44

How to filter an output?

Answer 44

grep

Question 45

What is "Sleep" command do?

Answer 45

Delays/sleep for specific seconds

Question 46

how to run processes in background?

Answer 46

coproc sleep 10 we will get prompt immediately on enter. we could run other commands and sleep command will run in background.

Question 47

which command lists all the processes currently running ?

Answer 47

ps ps -e (all process running in computer.. and not just this shell) jobs ps --forest (tree view)

Question 48

coproc is a shell keyword (found by using command 'type') but there is no man for coproc. Why?

Answer 48

because for shell commands we need to use help command. help coproc

Question 49

how to get the process id of the current shell?

Answer 49

echo $$

Question 50

how to kill a process?

Answer 50

kill -9 kill -9 & (backgound kill) If process is running in foreground then we could use ctrl +c to kill the process.

Question 51

How to see all processes of OS?

Answer 51

top (Screen gets refreshed based on cpu activity) q or Ctrl+c to exit Ctrl +z for suspend to cmd line

Question 52

How to suspend a process?

Answer 52

Ctrl + z

Question 53

How to move a process from the background to fore-ground

Answer 53

command 'fg'

Question 54

What does the shell variable $- contain?

Answer 54

says under what selected options the current bash is running. bash -c "echo $-" - displays different text as the options are modified.

Question 55

How to see the list of all historical commands run on the computer?

Answer 55

use command "history" . It will list shortcut numbers so that it could be used to repeat the operation/command. we could run the historical command by using !

Question 56

What is expansion command?

Answer 56

echo {a..z} lists all characters from a to z echo {A..D}{A..D} will give all combinations from AA till DD echo * (expansion of all directories in pwd) echo d* (expansion of all directories in pwd staring with d) - case sensitive

Question 57

How is ";" used to run multiple commands in left to right order?

Answer 57

ls ; date; wc -l /etc/profile ";" - aka command separator

Question 58

how to find the status of the previously run command?

Answer 58

echo $? Range : 1 -255 0 sucessful 1-255 - error 1 - permission denied 127 - command not found 137 - process killed using kill command we could manually send a exit code from a shell inside a shell. if number is bigger than 256 then number %256 will be displayed.

Question 59

how to open a calculator in ubuntu?

Answer 59

bc Ctrl + d to come out of calc

Question 60

How to declare a list of items and retrieve from list?

Answer 60

declare -A caparray=( [Switzerland]=Bern [Germany]=Berlin [Canada]=Ottawa [Tokyo]=Japan [Mongolia]=Ulaanbaater) echo ${!caparray[@]} echo ${caparray[Tokyo]:1:2}

Question 61

There are three files master.txt, half1.txt and half2.txt in the current working directory. Add first 2 lines of half1.txt to the file master.txt at the end(starting at a new line) then append the last 3 lines of the file half2.txt to the file master.txt at the end(starting at a new line). Append the lines in the sequence mentioned.

Answer 61

head -2 half1.txt>>master.txt tail -3 half2.txt>>master.txt