Linux Tutorials

7/27/2019 Linux Tutorials

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Linux TutorialsNetwork Design/Computer

visionby Helal Saghir


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The Command Prompt Commands are the way to do things in Unix A command consists of a command name and options

called flags Commands are typed at the command prompt In Unix, everything(including commands) is case-

sensitive

[prompt]$ helal:~$ ls l -a unix-tutorial

Command Prompt Command

(Optional) flags

(Optional) arguments

Note: Many Unix commands will print a message only if something

went wrong. Be careful with rm and mv.


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Getting help with man man (short for manual) documentscommands

man retrieves detailed informationabout

man k searches the man pagesummaries (faster, and will probably givebetter results)

man K searches the full text ofthe man pages

helal:~$ man k password

passwd (5) - password file

xlock (1) - Locks the local X display

until a password is entered

helal:~$ passwd


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ssh (secure shell)

Open secure shell from windows.

Enter server name , username andpassword . You can login remotely.

In linux use this command ssh [email protected]

Will ask your password and let you enter.


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yppasswd

To change the password on linux use

passwd

In lab you have to use yppasswd as we

are running NIS.


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Quota Quota limit is 100 MB.

Check quota by using du du disk usage. du cms

-c : total -m: megabytes -s: summary There is quota command also but it wont work

because of NIS running on clients. It shouldwork on your home computer, if you have Linuxinstalled on it and quota rpm on.


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Linux Commands cd change directory

cd .. changes to one up-level

cd / - goes to the root

cd $HOME takes to you home directory cd $home does the same thing

Handy directories to know~ Your home directory


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ls command ls shows the list of files/directory

ls al shows all files including hiddenwith detail.

ls l - shows all files not including hidden. ls - shows the list in that

directory


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cp cp copy files

cp [source_file1] [des_file1]

-p : preserve=mode,ownership,timestamp

-R r : copy directories recursively


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Move / mkdir mv : move files /directory

mv [source] [destination]

mkdir : make directory Syntax: mkdir

rmdir : remove directory which must beempty.

Syntax: rmdir


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rm Rm remove non-empty directory or file

-r: recursive

-f : force

-v: verbose


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Shell Shortcuts Tab completion

Type part of a file/directory name, hit , and the shell will

finish as much of the name as it can

Works if youre running tcsh or bash

Command history

Dont re-type previous commands use the up-arrow to access

them Wildcards

Special character(s) which can be expanded to match otherfile/directory names

* Zero or more characters? Zero or one character

Examples: ls *.txt

rm may-?-notes.txt


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Editing Text Which text editor is the best is a holy war. Pick one

and get comfortable with it.

Three text editors you should be aware of: pico Easy! Comes with pine (Dantes email

program)

emacs/xemacs A heavily-featured editorcommonly used in programming

vim/vi A lighter editor, also used in

programming Your opinion is wrong.


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Commands Contd.. Cat : concatenation

-cat [file1] [file2]

Attach both file and gives the output

more : views a file, pausing everyscreenful

more [file1]


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Redirection Standard Input is from keyboard

Standard Output is the monitor

Able to change this!

command < infile > outfile

command < infile

command > outfilecommand >> outfile attach at end


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Pipes

Output of one command piped intoanother

command1 | command2

Do the first command command1

Feed its output as input for command2


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WildcardsGroup of files accessed using wildcards

* matches everything

ls g* lists all files beginning with g

ls go.* lists files named go with any extensionls * lists all files


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Commands cond.

history gives a history of yourcommands

date displays date

>date who who is on the system

finger get more info about another user. jobs one of a number of job control

commands. Gives the job No. of your jobs.

kill kill a specified job number


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Find command find: find the file

find [place to find] name filename print

. current directory

~ home directory / whole root directory


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chmod chmod : change mode

ugo : user, group, other

rwx: read, write, execute

777 :read, write, execute for all chmod 755 test.c


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printing lpr : send print to default

printer a2ps p :

format text file in postscript and prints


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Compilation in c gcc o

Make different executable file name fordifferent files compiled.

Default executable is a.out Run using ./a.out

Or a.out in 410 lab


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Detail on Compilation in c g++ syntax:

g++ []

Some useful options:

-g produce debugging info (for gdb)

-Wall Warnings all

-ansi force ANSI compliant compilation-D define in all source files

-o output filename

Example:

g++ -g Wall ansi o hello hello.cpp


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Editor Choice is on you :

emacs

vi

gedit pico

other


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Make The make utility is a software engineering tool

for managing and maintaining computerprograms.

make provides most help when the program

consists of many component files. As thenumber of files in the program increases so todoes the compile time, complexity of compilation

command and the likelihood of human errorwhen entering command lines, i.e. typos andmissing file names.


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make cont. By creating a descriptor file containing

dependency rules, macros and

suffix rules

you can instruct make to automatically rebuildyour program whenever one of the program'scomponent files is modified.

make is smart enough to only recompile the filesthat were affected by changes thus savingcompile time.


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make cont.. Make goes through a descriptor file starting with

the target it is going to create. Make looks at each of the target's

dependencies to see if they are also listed astargets.

It follows the chain of dependencies until itreaches the end of the chain and then beginsbacking out executing the commands found ineach target's rule.

Make builds object files from the source files andthen links the object files to create theexecutable.


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The Make Command make syntax:

make [-f ] []

argument descriptions:

-f use as the makefile

default: Makefile or makefile

compile configuration targetdefault: first target in the makefile


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Lets Make to make breakfast So, let's say we need (boiled) eggs, toast, and coffee forour breakfast. Then our breakfast rule will be simply:

breakfast : boiled_eggs toast coffee butter

peel boiled_eggs put toast on a plate

pour coffee into cup

So far so good, but we don't have boiled_eggs, we onlyhave raw eggs. So we need another rule:

boiled_eggs : raw_eggs water pot stove

pour water into pot

put eggs in the water put pot on the stove

boil until eggs ready


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Make cont. Now we need a rule to make a toast, and

another to make coffee.I won't go through the excercise of writingthese rules out: I think you get the idea.


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Makefile Syntax

Rule Syntax:

:

The may be files and/or other targets It mustbe a tab before , or it wont work

The first rule is the default for make

Variable Syntax:

=

All variable values default to the shell variable values


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Simple example for Makefile This is an example descriptor file to build an

executable file called prog1.

It requires the source files file1.cc, file2.cc, andfile3.cc.

An include file, mydefs.h, is required by files file1.ccand file2.cc.

You can run it in a normal way by :% CC -o prog1 file1.cc file2.cc file3.cc

Descriptor file will run it like this :

% make prog1or if prog1 is the first target defined in the descriptor file

% make


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Simple example of Descriptor file#top level rule to compile the whole process

all:prog1

#program is made of several source files

prog1 : file1.o file2.o file3.o

gcc -o prog1 file1.o file2.o file3.o

#rule for the file file1.o

file1.o : file1.cc mydefs.h

gcc -c file1.cc



gcc -c file2.cc


file3.o : file3.cc

gcc -c file3.cc

#rule for cleaning files generator during compilation

clean :

rm file1.o file2.o file3.o


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Making Descriptor file smallerOBJS= file1.o file2.o file3.o

Prog1:${OBJS}

gcc -o prog1 ${OBJS}file1.o : file1.cc mydefs.h

gcc -c file1.cc


gcc -c file2.cc

file3.o : file3.cc

gcc -c file3.cc

clean :

rm ${OBJS}


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Making Descriptor file even smaller

#this is macro

OBJS= file1.o file2.o file3.o

CC=gccCFLAGS = -g wall

prog1:${OBJS}

${CC} -o $@ ${OBJS}

file1.o file2.0: mydefs.h

clean :

rm ${OBJS}


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Make Power Features Fake targets

targets that are not actually files can do just about anything, not just compile

like the clean target

Forcing re-compiles

touch the required files

touch the Makefile to rebuild everything


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Debugging The ideal:

do it right the first time

The reality:

bugs happen

The goal:

exterminate, quickly and efficiently


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Debugging Methods The best way: read the code

If you dont understand it, itll happen again

Consistent use of error checking & handling

most uncaught errors are not where they appear to be

Examine the state at key points in the code

wrap debug output with #ifdef DEBUG

use a debugger, like gdb, to view the state


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Efficiency in Debugging

The goal is to isolate the problem and fix it

Dont try random things, looking for a solution

if you dont understand ititll be back

half the time, this takes an insanely long time you dont learn anything from it

Look for the problem, not the solution

figure out two points in code that the problem isbetween, and close the gap from there

once you know the line the bug is on, its easy


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Breakpoints What are they?

A break point: a point in the code whereexecution pauses, or breaks, then continues

What are they for?

To examine the state of the running program

How do you make them?

modifying code to print information and pause use a debugger to interactively step through code


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Debuggers

Advantages over the old fashionedway:

you can step through code as it runs you dont have to recompile to set a

breakpoint

you can examine the entire state of theprogram

call stack, variable values, scope, etc.

you can modify values in the runningprogram

you can view the state of a crash using

core files


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GDB, the GNU DeBugger Text-based, invoked with:

gdb [ [|]]

Argument descriptions: executable program file

core dump of program

process id of already running program

Example:

gdb ./hello

Compile with gfor debug info


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Basic GDB Commands General Commands:

file [] selects as the program to debugrun [] runs selected program with arguments

attach attach gdb to a running process

kill kills the process being debuggedquit quits the gdb program

help [] accesses the internal help documentation

Stepping and Continuing:c[ontinue] continue execution (after a stop)

s[tep] step one line, entering called functions

n[ext] step one line, without entering functions

finish finish the function and print the returnvalue

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Linux Tutorials