Introduction to Linux

Introduction to Linux • What is Linux? • Why [Not] Linux? • Linux Distributions • Shells • Editors • Commands • Creating a makefile Supplementary Slides 2

What is Linux? • Unix-like operating system • Primarily a command-line OS requiring user to enter commands on the keyboard • Has a GUI called X Windows that operates similarly to MS Windows • Traditionally designed for more technical,hands-on computer users • High performance and system flexibility are major concerns • Rapidly joining the group of OS for the general consumer • More often used as a filer server, Web server, or e-mail server than as a desktop OS. Supplementary Slides 2

Origins of Unix/Linux • 1969: Initial version of Unix written in assembly language • By Ken Thompson and D. Ritchie at AT&T Bell Labs • Later rewritten in C for easier modification • Also to increase the number of mainframe and minicomputers on which Unix can run • 1974: Unix licensed to interested universities as a development tool • Students at licensed universities began developing and sharing utility programs to perform small but important tasks • some of these programs still part of the Unix kernel • Kernel: part of the OS code closest to the machine and activates h/w components directly or through another s/w layer Supplementary Slides 2

Origins of Unix/Linux (cont’d) • Many universities began to use Unix to share research data with other universities and industry over telephone lines • marked the beginning of the Internet and Internet Protocols (IP) • Unix "clones" developed over the years • typically modified Unix to make it accessible to the desktop user • One such successful clones resulted in the Linux OS • Initiated by Linus Torvalds, then a student at the Univ. of Helsinki Supplementary Slides 2

Linux Strengths • Stability • Linux rarely crashes • Written by programmers writing for other programmers (no deadlines, show one’s ingenuity etc) • Designed to be upgraded and modified "on the fly" • Security • Interested developers help identify & fix security problems • Developers said to create software updates to fix problems within 24 hours! • Resource-use economy • Designed to use limited hardware resources efficiently • Makes better use of hardware than almost any other OS • Translates into speed when more extensive resources available • A Mutitasking, Multiuser, Multiprocessing System • Can run many programs at the same time through preemptive multitasking • Many users can log in to the same system over a network and run programs • Supports multiple CPUs on the same computer using symmetric multiprocessing Supplementary Slides 2

Linux Strengths (cont’d) • Flexibility, extensibility • Source code available to users • Allows technically able folks to modify and extend the system the way they want • Open source • Faster and better software testing and refinement process - large number of software authors and beta testers • Linux is “free” • Acquired with little amount money • Generally handles network connections better than Windows • Developed by a team of programmers over the Internet; networking features given high priority • Supports most of the major protocols, and quite a few of the minor ones Supplementary Slides 2

Linux Weaknesses • Is Linux trustworthy? • How can something that is free also be reliable? • Installation & use • Can be difficult to install particularly for users who are not familiar with Unix commands • Can be difficult for casual users to operate • Documentation can be spotty • Inconsistent User Interface • Because of different design goals, no consistency enforced among the various programs and utilities included in a standard Linux distribution. • Replication of packages • For example, a distribution may contain Mail Transfer Agents: sendmail, smail, qmail, and exim • Lack of commercial applications • Not many applications are available for Linux on the desktop Supplementary Slides 2

Linux Distributions • The Linux kernel does not provide the full functionality of a commercial OS • A Linux distribution typically has the kernel as its core with many other tools like • Networking utilities • System administration tools • Installation and documentation packages • Graphical user interfaces • Etc • Most of the added system utilities are drawn from the GNU project of the FSF • Linux distributions often called GNU/Linux OS • There are many Linux distributions available Supplementary Slides 2

Popular Linux Distributions • Red Hat Linux - www.readhat.com • The most widely used distribution in the world • Main goal was to provide an easy-to-install, pre-configured system that comes complete with many applications • OpenLinux - www.calderasystems.com • The first to provide a fully graphical installation procedure • Aimed at Business users. Produced by Caldera Systems • Mandrake - www.linux-mandrake.com • Built on Red Hat Linux • Additions include installation procedure in many languages • Stampede - www.stampede.com • A distribution optimized for speed Supplementary Slides 2

Popular Linux Distributions • Debian - www.debian.org • Debian does not have company behind it:created and maintained by developers of free software • Primary goals are provision of an easy upgrade technique and high-secure components • Slackware - www.cdrom.com • One of the first Linux distributions; still maintained by its original creator, Patrick Volkerding • Probably the second-best known Linux distribution • SuSE - www.SuSE.com • The leading German distribution - with a huge collection of packages • Full installation requires > 4GB Supplementary Slides 2

Which Distribution to Use? • All Linux distributions do more-or-less the same things • Choice is sometimes a matter of preference • Linux newbies • Best served by Xandros, SuSE, Fedora, Mandrake, or possibly Red Hat • Experienced administrators • Able to cope with any of the Linux distributions. • Non-x86 computers • For a non-Intel computer, you must select an appropriate distribution. • For PowerPC (Macintosh) systems, your main choices are Mandrake 9.1, SuSE 7.3, Debian 3.0 Supplementary Slides 2

The Shell • While the kernel interacts with the hardware and other software, the shell interacts with the user and the kernel • A shell is a program that provides the traditional, text-only user interface for Unix-like operating systems. • Primary function: read and execute commands • Makes it easy for users to launch programs and work with files • The shell derives its name from the fact that it is an outer layer of an operating system. • Every Unix-like operating system has at least one shell program, and most have several. Supplementary Slides 2

Common Shells • sh (the Bourne Shell) is the original Unix shell, and it is still in widespread use today. • Written by Stephen Bourne at Bell Labs in 1974 • Missing functions expected:file name completion, command editing, command history etc • cshor C shell: the syntax of this shell resembles that of the • Syntax similar to that of the C programming language. • Sometimes asked for by programmers. • tcshor Turbo C shell: • A superset of the common C shell • Enhancing user-friendliness and speed • kshor the Korn shell: sometimes appreciated by people with a UNIX background. • A superset of the Bourne shell • Proprietary, written by David Korn • bash (Bourne-again shell) - the default shell on Linux. • Has many more commands than sh, making it a powerful tool for advanced users. Supplementary Slides 2

Editors • Everyone needs a text editor • Pick one and learn how to use it well • People are fanatical about their editor • No single best editor? • Several choices available: • vi Standard UNIX editor • EmacsExtensible, Customizable Self-Documenting Display Editor • pico Simple display-oriented text editor • TheXEDIT-like editor • XeditX windows text editor • NeditX windows Motif text editor Supplementary Slides 2

Commands • See P. Baumann slides Supplementary Slides 2

Introduction to Makefiles • Managing a project consisting of many source files can be tedious • For example, consider a project with tens of files • source files, header files and libraries all of which may change during development • How do we know which files to recompile or relink with when some files change? • Fortunately, Linux provides a tool, the make program, that greatly simplifies this • make keeps a record of dependencies between files • Recompiles/relink only those files that changed since the last update • Thus, instead of managing these tasks manually, make can be your automatic dependency manager • gives you more time to do other important things Supplementary Slides 2

Introduction to Makefiles (cont’d) • make is best suited for C programming • can be used with any programming language whose compiler can be run with a shell command • It has knowledge of how compilers work and can sometimes figure things out for itself (see later) • make generates commands using a description file known as a makefile. • These commands are then executed by the shell • make expects the file name to be Makefile or makefile • The makefile is basically a set of rules for make to follow to do its job • The major elements of a makefile are: • Dependency lines or rules • Targets • Dependents • Macros (user-defined and internal) • Comments Supplementary Slides 2

A Sample Makefile CC = mpicc FC = mpif77 # SOURCE CODE SRC=inner.c OBJ=inner.o inner: $(OBJ) $(CC) -c $(SRC) $(CC) -o $@ $(OBJ) run: make inner mpirun -np 4 inner clean: rm -f *.o *% *~ a.out inner Supplementary Slides 2

Components of the Makefile • The first two and the last two of the following lines are macros: CC = mpicc FC = mpif77 # SOURCE CODE SRC=inner.c OBJ=inner.o • The middle line above, starting with #, indicate make comments • Macro names are traditionally given in uppercase letters • The macro value (at the right of the equality) can be a file name, a directory, a program to execute or just about anything • Macros can have empty right-hand sides (assigned the null string) • make ignores undefined macros and those with null string values • Macros are referenced by prefixing them with dollar sign, as in $(SRC) • Macros can include other macros as in: LFLAGS = -L$(PVM_ROOT)/lib/$(PVM_ARCH) Supplementary Slides 2

Components of the Makefile (cont’d) • A makefile consists of entries like: inner: $(OBJ) $(CC) -c $(SRC) $(CC) -o $@ $(OBJ) • The first line of an entry is the dependency line consisting of a target and dependents • The target is at the left of the colon (inner in this example) and the dependents to the right (inner.c in this example) • The second line is one or more command lines, which tell make what to do if a dependent is newer than the target • The syntax of a makefile requires that each command line must be indented using a tab. • A dependency line can have a series of commands associated with it, like: run: make inner mpirun -np 4 inner Supplementary Slides 2

Using the Makefile • Given the above makefile and assuming you are in the directory where it was saved (as Makefile or makefile): sahalu@linux>make inner mpicc -c inner.c mpicc -o inner inner.o • The above command tells Linux that you want to create a new version of the executable program inner • Notice how the macros are expanded and the commands issued to the shell for execution • inner is referred to as the target of this make operation • The object file(s) that are linked together to create the executable are known as the dependents • The only object here is inner.o referenced as $(OBJ) • The source code files that are compiled to create an object files are also its indirect dependents • The only source file here is inner.c Supplementary Slides 2

Using the Makefile (cont’d) • Observe the targets run and clean carefully in our example makefile • They both have no dependents • run is used to call make and to initiate mpirun • When a target has no dependents, make executes the supplied commands • provided there is no file with the same name as the target in the current directory • If all the dependencies of a target, target, are up to date, make does nothing except inform you of the following: sahalu@linux>make target ‘target' is up to date • You can supply more than one target to make at the same time: sahalu@linux>make run clean • If you supply a target that does not exist, you get: sahalu@linux>make ics make: *** No rule to make target `ics'. Stop. Supplementary Slides 2

Using Makefiles • Note that make executes each command line associated with a rule as if the command had its own shell. • Thus, the command cd somewhere mv *.c anotherwhere may not behave the way you may have intended. • Use the following syntax to remedy such situations: dependency line command1;command2;command3;... • or dependency line command1; \ command2; \ command3; • Note that if you use a backslash to continue a line, as above, the backslash must be the last character before the end-of-line character. Supplementary Slides 2

Internal Macros • make has internal macros that it recognizes for commonly used commands. • The C compiler is defined by the CC macro, and • The flags that the C compiler uses are stored in the CFLAGS macro. • Two other internal macros are $@ and $? • The $@ macro always denotes the current target; • The $? macro refers to all the dependents that are newer than the current target. • Both of these macros can only be used within command lines. • makeknows that C compilers compile files with .c suffixes, and generate files with .o suffixes. • This knowledge is encapsulated in a suffix rule: • make examines the suffix of a target or dependent to determine what it should do next. Supplementary Slides 2

Suffix Rules • The default suffix rules are as follows: .SUFFIXES: .o .c .s .c.o: ${CC} ${CFLAGS} -c $< .s.o: ${AS} ${ASFLAGS} -o $@ $< • First line: a dependency line stating the suffixes that make should try to find rules for if none are explicitly written in the makefile. • Second line: compile each .c file whose corresponding object file (.o) is out of date. • Third rule is a similar directive to rule 2 but for assembler files. • The new macro $< has a similar role to that of the $? directive, but can only be used in a suffix rule. • It represents the dependency that the rule is currently being applied to. Supplementary Slides 2

Makefile 2 someonehappy:main.o dothis.o itquick.o \ /usr/happy/lib/likeatree.a cc -o someonehappy main.o dothis.o itquick.o \ /usr/happy/lib/likeatree.a main.o: main.c cc -c main.c dothis.o: dothis.c cc -c dothis.c itquick.o: itquick.s as -o itquick.o itquick.s fresh: rm *.o maybe.h: yes.h no.h cp yes.h no.h /users/sue/ Supplementary Slides 2

Makefile 2 with Macros CC = /usr/bin/cc AS = /usr/bin/as OBJS = main.o dothis.o itquick.o YN = yes.h no.h LIB_FILES = /usr/happy/lib/likeatree.a someonehappy: ${OBJS} ${LIB_FILES} ${CC} -o someonehappy ${OBJS} ${LIB_FILES} main.o: main.c cc -c main.c dothis.o: dothis.c cc -c dothis.c itquick.o: itquick.s ${AS} -o itquick.o itquick.s fresh: rm *.o maybe.h: ${YN} cp yes.h no.h /users/sue/ Supplementary Slides 2

Makefile 2 with Internal Macros OBJS = main.o dothis.o itquick.o YN = yes.h no.h LIB_FILES = /usr/happy/lib/likeatree.a someonehappy: ${OBJS} ${LIB_FILES} ${CC} -o $@ ${OBJS} ${LIB_FILES} main.o: main.c cc -c $? dothis.o: dothis.c cc -c $? itquick.o: itquick.s ${AS} -o $@ $? fresh: rm *.o maybe.h: ${YN} cp $? /users/sue/ Supplementary Slides 2

Makefile 2 with Suffix Rules OBJS = main.o dothis.o itquick.o YN = yes.h no.h LIB_FILES = /usr/happy/lib/likeatree.a someonehappy: ${OBJS} ${LIB_FILES} ${CC} -o $@ ${OBJS} ${LIB_FILES} fresh: rm *.o maybe.h: ${YN} cp $? /users/sue/ Supplementary Slides 2

Introduction to Linux