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System Commands > Week 3 > Flashcards

Week 3 Flashcards

1
Q

executing multiple commands?

A

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

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2
Q

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

A

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

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3
Q

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

A

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

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4
Q

What are file descriptors?

A

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.

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5
Q

What is the redirection operator?

A

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

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6
Q

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

A

hwinfo

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7
Q

what is the output for cat > file1

A

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)

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8
Q

what happens when we execute just the cat command?

A

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

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9
Q

How to append to a file?

A

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;

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10
Q

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

A

ls $HOME / blah 2>error.txt

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

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

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11
Q

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

A

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

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12
Q

How to redirect output and error to the same file?

A

command > file1 2>&1

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13
Q

What happens when we use pipes with commands?

A

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.

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14
Q

How to use redirection, pipes together?

A

command1 | command2 > file1

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

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15
Q

How is /dev/null special?

A

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.

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16
Q

what does tee command do?

A

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

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17
Q

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

A
  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.
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18
Q

What are major package types?

A

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.

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19
Q

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

A

lsb_release -a

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20
Q

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

A
  1. amd 61 | x86_64
  2. i386 |x86
  3. ARM
  4. ppc64el | Open POWER
  5. all |noarch|src
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21
Q

What are some of the package tools?

A
  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
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22
Q

How to inquire package database?

A

search packages for a keyword
apt - cache search keyword

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23
Q

How to list all packages currently installed in our system?

A

apt -cache pkgnames
apt-cache pkgnames nm

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24
Q

How to display package recrods of a package?

A

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

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25
Q

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

A

RPM - Package-version-release.architecture.rpm

DEB - package _version-revision_architecture.deb

26
Q

What are package priorities?

A
  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.
27
Q

What are package sections?

A

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

28
Q

What are some of the checksums ?

A
  1. md5sum (128 bit)
  2. SHA1 (160 bit)
  3. SHA256(256 bit)
29
Q

In Ubuntu, what is the administrators group referred as?

A

sudo

Only sudoers can install /upgrade/remove packages

30
Q

Where is all sudoers listed?

A

/etc/sudoers

31
Q

how a superuser can find out unsuccessful sudo attempts

A

through log file /var/log/auth.log

32
Q

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

A

/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.

33
Q

How to downloaded the updates and keep it in cache?

A

sudo apt-get update

The updates applicable only to your installed packages are used .

34
Q

How to install the updates downloaded to cache?

A

sudo apt-get upgrade

35
Q

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

A

sudo apt-get autoremove

-removes outdated versions and unwanted files.

apt-get clean
clean local repository of retrieved package files

apt-get purge <package>
purge package files from system</package>

36
Q

How to uninstall a package?

A

sudo apt-get remove <pkgname></pkgname>

37
Q

How to install a package?

A

sudo apt-get install <package_name></package_name>

sudo apt-get reinstall <package-name></package-name>

38
Q

where could we find details about the dpkg package manager?

A

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

39
Q

How to list all dpkg packages?

A

dpkg -l pattern

40
Q

How to list installed files that came from packages?

A

dpkg -L package

41
Q

How do find dpkg status of packages?

A

kpkg -s package

42
Q

How to search installed packages for a file?

A

dpkg -S pattern

43
Q

How to query dpkg database?

A

dpkg-query -W -f=’${section} {binary:Package}\n’

-w : show
-f ; format

lists the package section and naame of the executable/package.

44
Q

How to filter an output?

A

grep <filter></filter>

45
Q

What is “Sleep” command do?

A

Delays/sleep for specific seconds

46
Q

how to run processes in background?

A

coproc sleep 10

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

47
Q

which command lists all the processes currently running ?

A

ps

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

jobs

ps –forest (tree view)

48
Q

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

A

because for shell commands we need to use help command.

help coproc

49
Q

how to get the process id of the current shell?

A

echo $$

50
Q

how to kill a process?

A

kill -9 <process_id></process_id>

kill -9 <process_id> & (backgound kill)</process_id>

If process is running in foreground then we could use ctrl +c to kill the process.

51
Q

How to see all processes of OS?

A

top (Screen gets refreshed based on cpu activity)

q or Ctrl+c to exit

Ctrl +z for suspend to cmd line

52
Q

How to suspend a process?

A

Ctrl + z

53
Q

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

A

command ‘fg’

54
Q

What does the shell variable $- contain?

A

says under what selected options the current bash is running.

bash -c “echo $-“ - displays different text as the options are modified.

55
Q

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

A

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 !<shortcut/line no>

56
Q

What is expansion command?

A

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

57
Q

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

A

ls ; date; wc -l /etc/profile

”;” - aka command separator

58
Q

how to find the status of the previously run command?

A

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.

59
Q

how to open a calculator in ubuntu?

A

bc

Ctrl + d to come out of calc

60
Q

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

A

declare -A caparray=( [Switzerland]=Bern [Germany]=Berlin [Canada]=Ottawa [Tokyo]=Japan [Mongolia]=Ulaanbaater)

echo ${!caparray[@]}
echo ${caparray[Tokyo]:1:2}

61
Q

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.

A

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

62
Q
A

System Commands (8 decks)

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