Learning the Shell

Question 1

date

Answer 1

Outputs date and time per host settings

Question 2

cal

Answer 2

Outputs calendar for the current month per host settings

Question 3

df

Answer 3

Outputs the ‘disk free’ / the current amount of free space on your disk drives

Question 4

ln

[ … how to create a hard link ]
[ … how to create a symbolic link ]

Answer 4

Creates hard and symbolic links

ln FILE LINK => creates a hard link

ln -s ITEM LINK => creates a symbolic link where ITEM is either a file or a directory

Question 5

rm

Answer 5

Removes files and directories.

Note: use of the ‘-r’ may be required to remove directories on select Linux based OS’s.

Question 6

mkdir

Answer 6

Creates directories

Question 7

mv

Answer 7

Moves / Renames flies and directories

mv dir1/fun dir2 => moves the ‘fun’ file from directory 1 (‘dir1) to directory 2 (‘dir2’)

mv fun funner => changes the file / directory named ‘fun’ to ‘funner’

Question 8

cp

Answer 8

Copies directories or files

cp dir1/fun dir2 => copies the ‘fun’ file in directory 1 (‘dir1’) to directory 2 (‘dir2’)

Question 9

less

[ … navigation commands ]

Answer 9

View file contents within the command line interface.

Navigation commands:

b / PAGE UP : Scroll back one page

spacebar / PAGE DOWN : Scroll forward one page

up arrow : Scroll up one line

down arrow : Scroll down one line

G : Move to the end of the text file

1G or g : Move to the beginning of the text file

/characters : Search forward to the next occurrence of characters

n : Search for the next occurrence of the previous search

h : Display help screen

q : Quit less

Question 10

file

Answer 10

Outputs the file type

file FILE

Question 11

ls

Answer 11

Outputs a list of directory contents

Question 12

pwd

Answer 12

Print Working Directory : outputs / prints the name of the current working directory

Question 13

cd

Answer 13

Change Directory : Changes to the specified directory

On the desktop there is a folder named ‘dir1’

From Desktop : cd dir1 => moves the operator to the working directory to ‘dir1’

Question 14

exit

bye

Answer 14

Exit or Bye can be used to end the terminal session

Question 15

free

Answer 15

Outputs amount of free memory on the host

Question 16

type

Answer 16

Outputs how a command name is interpreted

type ls => ‘ls is /bin/ls’
[ ^ from MacOS ]

Question 17

which

Answer 17

Outputs which executable program will be executed

which ls => /bin/ls
[ ^ from MacOS ]

Question 18

man

Answer 18

Outputs a command’s manual page – i.e. the detailed documentation of the command and its options

man COMMAND

Question 19

apropos

Answer 19

Outputs a list of appropriate commands like a search function

apropos COMMAND

Question 20

info

Answer 20

Outputs a command’s info entry

info COMMAND

Question 21

whatis

Answer 21

Outputs a brief description of a command

whatis COMMAND

Question 22

alias

Answer 22

Creates an alias for a command durable for the terminal session.

if ‘cd /user; ls; cd -‘ is a common line, then

alias foo=’cd /user; ls; cd -‘, then typing

‘foo’ will execute ‘cd /user; ls; cd-‘ for the remainder of the terminal session.

Question 23

cat

Answer 23

Concatenates files

you have movie.mpeg.001 … movie.mpeg.099 =>

cat movie.mpeg.0* > movie.mpeg

^ concatenates the files into a single file

Question 24

sort

Answer 24

Sort lines of text

Question 25

uniq

Answer 25

Reports on or omits repeated lines

Question 26

wc

Answer 26

Print newline, word, and byte counts for each file

Question 27

grep

Answer 27

Outputs lines matching a pattern

grep STRING, for example ‘ls /bin | grep zip’ will return all items listed in ‘/bin’ that contain ‘zip’

Question 28

head

Answer 28

Outputs the first part of a file

Question 29

tail

Answer 29

Outputs the last part of a file

Question 30

tee

Answer 30

Read from standard input and write to standard output and files

Question 31

echo

Answer 31

Outputs a line of text

echo ‘Hello World!’ => ‘Hello World!’

Question 32

clear

Answer 32

Clears the screen

Question 33

history

Answer 33

Outputs the contents of the history list

Question 34

ps

Answer 34

Outputs a snapshot of current processes

Question 35

top

Answer 35

Outputs tasks

Question 36

jobs

Answer 36

Outputs a list of active jobs

Question 37

Long listing fields

-rw-r–r– 1 root root 32059 2012-04-03 11:05 oo-cd-cover.odf

Answer 37

• rw-r-r– : Access rights to the file.
• : The first character indicates the type of file. Among the different types, a leading dash (‘-‘) means a regular file, while a ‘d’ indicates a directory.

‘rw-‘ : The next three characters are the access rights for the file’s owner.
‘r–’ : The next three are for members of the file group.

‘r–’ : The final three are for everyone else.

1 : File’s number of hard links.

root : The user name of the file’s owner.

root : The name of the group that owns the file.

32059 : Size of the file in bytes.

2012-04-03 11:05 : Date and time when the file was last modified.

oo-cd-cover.odf : Name of the file.

Question 38

Redirecting standard output

Answer 38

There are two options:

1: ‘>’ overwrites
e.g. ls -l /usr/bin > ls-output.txt => creates .txt
ls -l /usr/bin > ls-output.txt => overwrites the above

2: ‘»’ appends
e.g. ls -l /usr/bin&raquo_space; ls-output.txt => creates .txt
ls -l /usr/bin&raquo_space; ls-output.txt => appends a second output of /usr/bin to the .txt created, above

Note:
0 = standard input
1 = standard output
2 = standard error

Question 39

Redirecting standard error

Answer 39

‘2>’

ls -l /usr/bin 2> ls-error.txt => ‘2>’ redirects standard errors to ‘ls-error.txt’

Note:
0 = standard input
1 = standard output
2 = standard error

Question 40

Redirecting standard output and standard error

Answer 40

There are two options:

1. Original version:
   ls -l /usr/bin > ls-output.txt 2>&1^ redirects standard output, then makes standard error ‘2’ and input to standard output ‘>&1’
2. New jack version:
   ls -l /usr/bin &> ls-output.txt^ redirects standard input and error ‘&>’ to ‘ls-output.txt

Question 41

Pipelines

Answer 41

COMMAND1 | COMMAND2

COMMAND1 becomes input for COMMAND2

Pipelines are used to ‘filter’ as in:
ls /bin /usr/bin | sort | less

^ list contents of ‘/bin’ and ‘/usr/bin’, then input to ‘sort’ to order the list, the ‘less’ to output to terminal

• or -

ls /bin /usr/bin | sort | uniq | less

^ as previous, but, sort, then deduplicate (‘uniq’), then ‘less’ to output to terminal

Question 42

Expansion

1) pathname expansion
2) tilde expansion
3) arithmetic expansion
4) brace expansion
5) parameter expansion

Answer 42

Expansion is the process by which elements of command are expanded by the interpreter.

1) pathname expansion: expansion of the wildcard (‘ * ‘)
e. g. echo D* => all items starting with ‘D’

2) tilde expansion: expansion of the ‘ ~ ‘
e. g. echo ~ => /home/me (the current user)

3) ' $((2+2))' => 4
      ' + ' addition
      ' - ' subtraction
      ' * ' multiplication
      ' ** ' exponentiation
      ' / ' division (no remainder)
      ' % ' modulo / reminder (only the remainder)

4) brace expansion: use of ‘ { } ‘ to create multiple text strings from a pattern contained within the braces.
e. g. ‘ file-0{1..5} ‘ => creates five directories named file-01, file-02, file-03, file-04, and file-05

5) parameter expansion : taps the system’s ability to store small chunks of data and to give each chuck a name.
e. g. echo $USER => me (the current user)

Question 43

id

Answer 43

Display the user identity.

Question 44

chmod

Answer 44

Change a file’s mode.

Question 45

umask

Answer 45

Set the default file permissions.

Question 46

su

Answer 46

Run a shell as another user.

generic form: su [-[l]] [user]

If the ‘ -l ‘ option is included, the resulting shell session is a login shell for the specified user. This means that the user’s environment is loaded and the working directory is changed to the user’s home directory.

Question 47

sudo

Answer 47

Execute a command as another user.

Question 48

chown

Answer 48

Change a file’s owner.

Question 49

chgrp

Answer 49

Change a file’s group ownership.

Question 50

pssswd

Answer 50

Change a user’s password.

generica form: passwd [user]

Question 51

File Types

  -
  d
  l
  c
  b

Answer 51

’ - ‘ A regular file.

’ d ‘ A directory.

’ l ‘ A symbolic link. Notice that with symbolic links, the remaining file attributes are always rwxrwxrwx and are dummy values. The real file attributes are those of the file the symbolic link points to.

’ c ‘ A character special file. The file type refers to a device that handles data as a stream of bytes, such as a terminal or modem.

’ b ‘ A block special file. This file type refers to a device that handles data in blocks, such as a hard drive of CD-ROM drive.

Question 52

File Modes in Octal

  0
  1
  2
  3
  4
  5
  6
  7

Answer 52

Octal     Binary     File Mode
0            000         - - -
1             001          - - x          
2            010          - w -
3            011           - w x
4            100          r - -
5            101           r - x
6            110           r w -
7            111            r w x

^ in File Mode column, spaces added for readability: e.g. rwx becomes r w x

Question 53

CHMOD Symbolic Notation

u
g
o
a

Answer 53

’ u ‘ Short for ‘user’ but means the file or directory owner.

’ g ‘ Group owner.

’ o ‘ Short for ‘others’ but means world.

’ a ‘ Short for ‘all;’ the combination of u, g, and o.

^ if no character is specified, ‘all’ will be assumed.

Question 54

CHMOD Symbolic Notation Examples

u+x
u-x
\+x
o-rw
go=rw
u+x,go+rx

Answer 54

’ u+x ‘ Add execute permission for the owner.

’ u-x ‘ Remove execute permission from the owner.

’ +x ‘ Add execute permission for the owner, group, and world. (Equivalent to a+x.)

’ o-rw ‘ Remove the read and write permissions from anyone besides the owner and group owner.

’ go=rw ‘ Set the group owner and anyone besides the owner to have read and write permission. If either the group owner or world previously had execute permissions, remove them.

’ u+x,go+rx ‘ Add execute permission for the owner and set the permissions for the group and others to read and execute. Multiple specifications may be separated by commas.

Question 55

ps

Answer 55

Report a snapshot of current processes.

Question 56

top

Answer 56

Display tasks.

Question 57

jobs

Answer 57

List active jobs.

Question 58

bg

Answer 58

Place a job in the background.

Question 59

fg

Answer 59

Place a job in the foreground.

Question 60

kill

Answer 60

Send a signal to process.

Question 61

killall

Answer 61

Kill processes by name.

Question 62

shutdown

Answer 62

Shut down or reboot the system.