A first short look at the linux command line: the power and the glory

1. Introduc)on*to*the*Linux*Command*Line*
Interface
Prepared'for'the'Nova'Sco0a'Linux'Users'Group'by'Corrie'Wa='(h=p://www.corriewa=.com)
2014

2. The$Linux$CLI$(I$am$using$the$Bash$shell)
CLI!=!Command!Line!Interface
“Graphical)user)interfaces)make)easy)tasks)easy,)
while)command)line)interfaces)make)difficult)
tasks)possible.”
(original)source)unknown,)quoted)in)Sho6s,)W.)E.)(2012).)The)Linux)Command)Line.)

3. What%is%“The%Shell”?
The$shell$is$a$program$in$your$linux$distro$which$takes$keyboard$
commands$and$passes$them$to$the$opera8ng$system$to$perform.In$
the$good$old$days,$it$was$the$only$user$interface$available.
Now$we$have$GUI's$and$mice.$Good$(intui7ve),$but$not$be;er.
For$instance,$imagine$a$directory$with$200$images$of$large$(but$
unknown)$size$that$must$be$put$in$a$gallery$page:$large$size$of$512$x$
512,$with$thumbnails$of$256$x$256.
How$would$you$do$this$quickly?
...Use%the%CLI!

4. Open%a%terminal%window%and%enter:
!~$ mkdir 512 256
~$ for i in *.jpg ; do convert -resize 256x256
"$i" "256/$i" ; done
~$ for i in *.jpg ; do convert -resize 512x512
"$i" "512/$i" ; done
...Faster(than(using(a(GUI(by(several(orders(of(magnitude!
How$can$you$get$comfortable$doing$this?!Play!with!your!CLI:!make!
test!files!and!muck!around!with!them.!Move!them,!copy!them,!put!
them!in!another!directory,!delete!them,!list!them,!etc.
...Learn'more'at'h"p://linuxcommand.org/index.php

5. Ge#ng&Started
1. Start'the'terminal'emulator'(this'program'opens'a'window'to'let'
you'interact'with'the'shell).'A'shell'prompt'will'appear:
me@linuxbox: ~$
2. Type'in'some'stuff,'press'Enter:
me@linuxbox: ~$ WTF
bash:WTF: command not found
me@linuxbox: ~$'...A#new#command#prompt#is#returned#to#you
Your%shell%could%not%find%the%command%'WTF'%because%it%does%not%
exist.%No%surprise%here.%Let%us%do%something%more%useful.

6. Ge#ng&Around
Use$the$up)arrow$to$show$previous$commands$you$used$(command$
history).
Use$the$arrow$keys$to$move$the$cursor$(to$go$forward$or$backward$
in$a$line$of$text).$Your$mouse$will$not$work$for$this!
Copy%Paste+with+your+mouse+usually+works+(but+do+not+copy+a+
blank+space+at+the+end+of+your+text+string%%the+'Enter'+key+will+be+
assumed+to+have+been+pressed).
...Hint:'File'names'in'Linux'are'case3sensi4ve.

7. Safe%Commands%to%Explore
Format'of'a'CLI'command:!command -option filename
Display(current(/me(and(date(or(this(month's(calendar:
me@linuxbox: ~$ date
me@linuxbox: ~$ cal
See(the(current(amount(of(free(space(on(your(disk(drives:
me@linuxbox: ~$ df
See(amount(of(free(memory:
me@linuxbox: ~$ free
Ending(a(terminal(sesssion:
me@linuxbox ~$ exit!...Or$type$in$'Control/D'

8. Naviga&ng(the(File(System
Files!in!Linux!are!hierarchical!(i.e.!in!an!upside2down!tree!structure)
A!file!may!contain!data!in!some!format!(with!or!without!a!file!
extension!label)!or!it!may!contain!other!files!2!making!it!a!directory.
Directories)are)files!that!have!files!in!them:!sub2directories!and!
data!files!(eg.!Downloads,!home,!Recipes).
We!need!to!know!what!directory!we!are!currently!in,!so!that!we!
can!change!our!working!directory!to!where!we!want!be:
me@linuxbox: ~$ pwd!...Print/my/current/working/directory
/home/me!...This/is/your/current/working/directory
me@linuxbox: ~$!...As/usual,/a/new/command/prompt/follows

9. Working(with(Directories
me@linuxbox: ~$ ls!...List&the&files&in&the&current&directory
08pdfs work.tgz Desktop project5 recipes!...&files&
in&your&home&directory
Change'the'working'directory'using'Absolute'Pathnames:
me@linuxbox: ~$ cd /usr/bin
me@linuxbox:/usr/bin$!...You%are%now%in%directory%'bin'
Change'the'working'directory'using'Rela5ve'Pathnames:
me@linuxbox:/usr/bin$ cd ..!...go$UP$one$level$in$the$file$tree
me@linuxbox:/usr$

10. Learn&more&about&traversing&the&linux&directory&tree&at:!h"p://
linuxcommand.org/index.php
Naviga&on)Shortcuts
cd!...Change(to(your(own(home(directory
cd)2!...Change(to(the(previous(working(directory
cd)~jane!...Change(working(directory(to(user(Jane's(home(directory
cd)..!...Change(to(directory(one(level(higher(up(in(the(file(tree
me@linuxbox:,~$,ls,0l!...Get%more%informa-on%by%using%the%op-on%5l
drwxr-xr-x 2 jane users 4096 May 2 2011
my_c_scripts
-rw-r--r-- 1 jane users 2313 Sep 21 2012
vidlist.html

11. What%does%it%all%mean?
drwxr-xr-x 2 jane users 4096 May 2 2011
c_scripts
drwxr$xr$x%...d#says#this#is#a#file#is#a#directory#(-#means#a#plain#file)
drwxr$xr$x%...rwx#says#you#have#permission#to#read,#write#and#execute
drwxr%xr$x%...r:x#says#your#group#has#read#and#execute#permissions
drwxr$xr%x%...#This#r:x#means#the#'world'#read#and#execute#permissions
drwxr$xr$x%2%...#number#of#hard#links#to#the#file
drwxr$xr$x%2%jane+users%...Owner,#Group#name
4096+May+2+2011%...size#of#file#in#bytes,#date#last#modified
c_scripts%...name#of#directory#(filename)

12. Less$is$More

13. Using&the&less&program
The$less$program$is$an$improved$replacement$of$the$earlier$Unix$
program$called$more.$Many$Linux$files$have$human<readable$text$
content,$and$less$lets$you$look$at$them.
It$is$a$paging&program$useful$for$reading$the$actual$programs$(scripts)$
the$system$uses$and$for$checking$out$what$is$in$the$system$
configuraDon$files.$It$allows$you$to$navigate$page$by$page,$
backwards$and$forwards.
Try$this$at$home:!me@linuxbox:/etc$ less hosts.allow

14. Sample'less'commands
PAGE%UP%or%b%...Scroll'back'one'page
PAGE%DOWN%or%spacebar%...Scroll'forward'one'page
Up%Arrow;%Down%Arrow%...Scroll'up'one'line,'scroll'down'one'line
G%...Go'to'end'of'text'file
g%...Go'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

15. The$Linux$File$System
Directories,*data*files,*devices*and*peripherals*such*as*a*USB*
devices,*etc.,*are*treated*as*files*in*Linux.
See#this#link#for#a#simple#explana2on:#h4p://www.cyberci2.biz/2ps/
understanding?unixlinux?file?system?part?i.html
The$Linux$and$Unix$file$systems$are$very$much$alike.$This$is$a$good$
beginner's$tutorial:
h=p://www.ee.surrey.ac.uk/Teaching/Unix/

16. Doing&Things&with&Files
Copy%files%and%directories!(many!'op*ons'!available!with!cp)
$ cp -i myfile1 myfile2!...Make&a&copy&of&one&file
`$ cp myfile1 myfile2 myfile3 dest-directory
...Copy&several&files&into&the&dest3directory
$ cp -u *.mp3 dest_directory!...Simple'in'the'CLI,#not#so#in#a#
graphical#file#manager!
This#command#only#copies#up#the#.mp3#files#that#do#not#exist#in#the#
des9na9on#directory#or#are#newer#than#the#exis9ng#version#in#the#
directory.#Totally#cool.

17. Move/rename*files*and*directories
$ mv file1 file2!...Rename'file1'to'file2;'file1'is'now'gone
$ mv file1 dest-directory!...Move'file1'to'the'dest7directory
Create&new&directories
$ mkdir my-directory1!...Make&a&new&directory
$ mkdir my-directory1 my-directory2!...Make&several&new&
directories
Remove&(delete)&files&and&directories
$ rm -i thisfile
$ rm -r directory
$ rm -i *.jpg

18. The$-i$Command$Op-on$and$a$Cau-on
-i!makes!a!command!interac.ve;!you!will!be!prompted!for!consent!
before&execu*ng!the!command!for!each!of!the!files!in!turn.!USE!IT!
Fun$Commands!(Do!NOT!try!at!home!or!on!a!friends!machine!)
$ rm -r * ~!...OOPS!&What&did&it&just&do???
$ rm -rf /!...This&command&will&recursively&(-r)&and&forcefully&(-f)&
delete&all&the&files&inside&the&root&directory.&Oy,&all&gone!
~$ mv /home/yourhomedirectory/* /dev/null!...dev/null&
is&a&special&file.&This&command&will&move&all&the&files&in&your&home&
directory&to&a&black&hole,&never&to&be&seen&again.

19. About&Shell&Commands
A"shell"command"can"be:
1.#An"executable"program#(like#the#files#in#/usr/bin).#It#can#be#a#
shellscript,#a#compiled#program#(in#C#or#C++),#or#a#script#in#Perl,#
Python,#Ruby,#etc.
2.#A"shell"func6on#(miniature#shell#scripts#that#are#part#of#the#
environment)
3.#A"command"built"into"the"shell#(ls,#cd,#cp,#mkdir#...)
4.#An"alias#–#a#command#we#can#define#in#a#special#file,#which#I#will#
not#cover#here.

20. What%type%of%command%am%I%using?
Use$this$shell$built+in$to$find$out$what$type$your$command$is:
me@linuxbox: ~$ type type
type is a shell builtin
me@linuxbox: ~$ type cp
cp is /bin/cp
me@linuxbox: ~$type ls
ls is aliased to 'ls --color=tty'

21. Echo,&a&sound&command
Echo%is%a%shell%command%that%displays%a%line%of%text.
@linuxbox: ~$ echo Hello World!Hello World
me@linuxbox: ~$ echo *
Documents Downloads medusa1.php xplanet-gnome-output.
txt mybooks
Since!the!wildcard!character!*!means!“match%any%characters%in%a%
file%name”,!the!shell!expands!the!*!into!something!else!before!the!
echo!command!is!executed.!Here!we!see!a!display!of!the!contents!
of!our!current!working!directory.
me@linuxbox: ~$ echo D*
Desktop Documents Downloads

22. The$Help$Files
Bash%has%built+in%help%for%each%shell%buil3n.%Type%in%'help'%plus%the%
command:
me@linuxbox: ~$ help cd
cd: cd [-L|[-P [-e]]] [dir]
Change'the'shell'working'directory.
Change'the'current'directory'to'DIR.'The'default'DIR'is'the'value'of'
the'HOME'shell'variable.```'...and%it%goes%on,%with%a%list%of%command%
op4ons%and%path%informa4on.
Op4onal%items%(op4ons)%are%in%square%brackets;%a%ver4cal%bar%indicates%
mutually%exclusive%op4ons.

23. More%Help
Executable*programs*that*are*usable*from*the*command*line,*such*
as*mkdir*or*chmod,*usually*have*a*manual*(“man$page”)*available.*
The*program*man*will*display*the*man*page,*which*is*broken*into*
secAons*1C8,*where*secAon*1*contains*the*user*commands.*It*lets*
you*scroll*through*the*whole*length*of*it.*(Usually,*man*uses*less*to*
display*the*man*pages).
Try$it:
Type%in%man mkdir%...type&h&for&Help&or&q&to&quit&(h&will&open&a&help&
file&with&a&nice&summary&of&all&the&*less&commands!)

24. Links
Useful'Websites'for'lists'of'commands,'examples'or'explana6ons:
h"p://ss64.com/bash//...An$A%Z$Index$of$the$Bash$command$line$for$
Linux
h"p://linuxcommand.org/lc3_learning_the_shell.php/...A$shell$intro$
for$beginners
Excellent(Fun
h"ps://www.digitalocean.com/community/tutorials/top7107linux7
easter7eggs;...ENJOY!

25. Part%2%will%cover%some%shell%buil3ns%and%command%op3ons
Contact:(h*p://www.info@corriewa*.com