Essential tools

Question 1

Piping and redirection

Answer 1

|
>
>>
2>/dev/null
<
du -h / 2>/dev/null | sort -nr | head -n 5

./script.sh 2>errors.log 2>&1
all output and errors will be shown on a screen despite the first redirection of errors to errors.log file

file< commands > file2
ls nosuchfiles 2>/dev/null
all errors will be sent to nowhere

ls -l /etc | wc -show how many lines, words and characters are in the output of ls -l /etc commannd

Question 2

Show environment vars

Answer 2

env

Question 3

Stdin, stdout, stderr

Answer 3

stdin: 0
stdout: 1
stderr: 2

Question 4

Show users logged in to the system

Show name of current user

Answer 4

who

whoami

Question 5

grep

Answer 5

-R -recurcive
-l -show only file names
-w -match whole word
-A number -print number of lines after the match
-B number- print number of lines before the match
n- add line number

grep ‘^$’ -search for blank lines
Regexp should be surrounded by single quotes.

Question 6

Man pages

Answer 6

1. exec programs and shell commands
2. system calls- function provided by kernel
3. library calls
4. special files typically found in /dev
5. files formats and conventions
6. games
7. miscellaneous
8. for sysadmins
9. for non-standard kernel routines

man -k command= apropos command

mandb- this command is used to update man pages db

Question 7

Vim

Answer 7

dd -delete line
yy -copy line
p -paste copied 
v- visual block
y -copy block which was visualized 
u -undo
crtl-r -redo
gg/+shift+g -go to the beginning  
G/shift+g - go to the end
cw- while in command mode immediately switches to insert mode removes current word and allows to change this to something else
cc-removes the line and put into insert mode
shift+r -replaces current chars with new ones while typing over

/text -forward search
?text- backwards search
n/?- moving forward/backward to the next occurrence of the search
^ -move the cursor to the beginning of the line
$- move the cursor to the end of the line
:%s/old/new/g - search and substitute from old value to new globally

Question 8

Cockpit

Answer 8

http://localhost:9090

Question 9

cat options

Answer 9

• A -show non-printable symbols
• b -number of lines
• s suppress repeated empty lines

Question 10

cut
sort
tr

Answer 10

cut -f 1,2,3 -d : /etc/passwd | sort -n
-n- numerical sort
tr- translate
cut -f 1 -d : /etc/passwd | tr [a-z] [A-Z] - accounts will be shown in uppercase
the same: tr [:lower:] [:upper:]

Question 11

sed

Answer 11

sed -n 4p file -print fourth line of file
sed -i s/old/new file
-i - overwrite the file
sed -i -e ‘2d’ file1- delete second line of file1
-e -edit

Question 12

Run a command being substituted, similar to sudo command

Answer 12

su -c “fdisk -l”

Question 13

Redirect both errors and output to the file

Answer 13

vim myfile:
hdhjdjd
ls -la
\:wq
chmod u+x myfile
./myfile &> alloutput
when running only ./myfile > alloutput without &, an error message will be shown on display and output from ls -la will be added to alloutput file.

Question 14

Redirect standard error to stdout

Answer 14

fdshfs 2>&1
benefit from this- using output from this command and redirecting it to somewhere else:
dshfs 2>&1 | grep -i ‘failed to search’
or
ls -la huj&raquo_space; myfile 2&>1
cat myfile => invalid option j…

Question 15

Login as root into login shell

Answer 15

su -
su -l
su –login

Question 16

Find files

Answer 16

find / -type f -size +100M
find / -exec grep -l user {} \; 2>/dev/null

find / -name "*.tar" -ctime 1
use weak quotes when using file globbing
\_\_\_\_\_
-name -name
-ctime n -last change time (in days)
-mtime n-modified within n days
-mtime +3 -modified more than 3 days ago
-atime -access time (in days)
-empty
-uid
-type f or -type -d
-user user_name- files belonging to specified user

find . -name myfile{1..100} -exec rm -rf {} \;

find /text -empty | xargs rm -f

Question 17

Show differences between archive file and current files/dirs having the same name as contents of the archive

Answer 17

tar -dvf myarchive.tzr.gz

Question 18

Show information about the content of archive file

Answer 18

gzip file1 file2 file3

gzip -l file1.gz

Question 19

star

Answer 19

star -c -f=myarchive.tar dir1/ dir2/ file1 file2
-c-compress 
star -t -f=myarchive.tar
-t- list contents of archive 
then,
gzip myarchive.tar=> myarchive.tar.gz
gunzip myarchive.tar.gz

star -x -f=myarchive.tar=>
this will not override current existing files or dirs
______________
star -x -f=myarchive.tar file1

this will extract only file1 from myarchive.tar
___________
compress file with gzip using star
star -cz -f=myarchive2.tar.gz /dir1 file1

Question 20

Compress multiple individual files without deleting with the original files with bzip2

Answer 20

bzip2 -k MyFile1.txt MyFile2.txt MyFile3.txt

Question 21

Unzip archive with bzip2

Answer 21

bzip2 -d MyFile1.txt.bz2

bunzip MyFile1.txt.bz2

Question 22

Add executable permissions to dirs only.

So dedicated groups would be able to traverse through dirs, but not able to run scrips in those dirs

Answer 22

chmod ug-x -R dir1

chmod ug+X -R dir1

Question 23

Add/remove ugo privileges to a file or dir

Answer 23

chmod a+rx

chmod a-rwx

Question 24

Set suid to file

Answer 24

s is present in x position of owner of a file
=4
chmod u+s testfile
chmod 4740 testfile
setuid is used to execute files as if user was a owner
usr/sbin/passwd has sid

Question 25

Set gid to dir

Answer 25

s is present in x position of group owner of file/dir =2 That means when files are created within that directory, those new files will inherit the group ownership of that parent directory. chmod g+s testfile chmod 2750 dir setgid is used to execute files as if user was a group owner

Question 26

Set gid and sid simultaneously

Answer 26

chmod 6550 testfile

Question 27

Preventing user from deleting file or dir with sticky bit

Answer 27

``` t is present in x position of others =1 chmod 1777 myfile chmod +t testfile tmp dir has sticky bit ```

Question 28

Get user's group name | Get user's users name

Answer 28

id -gn | id -un

Question 29

umask

Answer 29

``` /etc/bashrc -interactive shell /etc/profile -login shell The value which will be subtracted from default permissions: 777=default for dirs 666=default for files ___ umask u=rwx,g=,o= => umask value=0077 ``` In scripts: echo $(id -un) or echo "`id -un`"

Question 30

info pages

Answer 30

info searches through /usr/share/info dir Used for GNU based systems use "n" keybutton to move to the next node use "p" key to move back use "u" to navigate to one level up use "H" to het help ``` info coreutils to look at exam for sections related to operations _____________ 1. info --apropos=tee=> coreutils 2. info coreutils tee ```

Question 31

/usr/share/doc

Answer 31

programs might have info inside this dir | some of them might not have man pages, but have docs inside /usr/share/doc/

Question 32

locate

Answer 32

locate passwd search in a system for files, docs, everything takes info from locate DB to do update DB run updatedb

Question 33

whatis

Answer 33

whatis passwd | Look into manual pages

Question 34

which

Answer 34

search for binaries located in PATH

Question 35

Locate binaries, source and man pages

Answer 35

whereis | whereis passwd

Question 36

Query for rpm docs

Answer 36

rpm -qd packagename

Question 37

stat of the file, when it was created, modified etc

Answer 37

stat file1

Question 38

tar

Answer 38

Add More Files To Tar File: tar -f archive.tar -r new/test2.txt Display Verbose Information: tar -cvf archive.tar Extract Tar File To Specific Directory: tar -xf archive.tar -C new/ Remove Files After Archiving: tar -cvf archive.tar test1.txt test2.txt --remove-files Extended Attribute Support, SELinux Support: tar -cf xattrs.tar test1.txt --xattrs --selinux

Question 39

Gunzip into a different directory

Answer 39

gunzip -c file.gz > /THERE/file

Question 40

Scp

Answer 40

scp myfile cloud_user@1.2.3.4:~/mydir/

Question 41

Sftp

Answer 41

sftp cloud_user@1.2.3.4 sftp>? sftp>get myfile sftp>put file1