Systems Programming/ C and UNIX

Transcription

1 Systems Programming/ C and UNIX Alice E. Fischer September 6, 2017 Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

2 Outline 1 Booting into Linux 2 The Command Shell 3 Defining a Command Language Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

3 Booting into Linux Booting into Linux The BIOS The bootdisk The boot sector The boot loader The kernel Run systemd Install system services Open a desktop or shell Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

4 Booting into Linux Start in the BIOS: Find and read the boot sector. The Basic Input/Output System is stored in the compute s firmware. Run the POST (Power On Self Test) to ensure that the hardware components have been initialized correctly. The order in which to search for devices is controlled by a configuration file. Locate the first attached device that has a boot sector. The boot program must be very small because it AND the partition table must fit into the first 512-byte sector on the boot device. Load that boot sector into RAM and transfer control to it. It loads the next stage of the boot loader, called GRUB. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

5 Booting into Linux Read, execute the boot loader The current Linux bootloader is GRUB2: GRand Unified Bootloader 2 GRUB2 is located between the end of the boot sector and the first partition on the disk drive. This space is 31,744 bytes long ( byte sectors). It is just smart enough to find the kernel and load it into memory. This code contains a few common dilesystem drivers (EXT, FAT, NTFS). There are always at least two versions of the Linux kernel on the disk: the newest one and the prior version. The prior version is there to revert to when the new one.did not install correctly or has a bug. GRUB allows the user to choose from among several kernel images or foreign OS kernels. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

6 Booting into Linux Read and decompress the kernel, give it control. The files for Stage 2 of the boot loader are stored in /boot/grub2 and its subdirectories. The code consists mostly of runtime kernel modules, loaded as needed. It lets the user select which kernel to load. GRUB stage 2 loads the selected kernel into memory and turns control over to it. All of the kernels are in a self-extracting, compressed format to save space. When the kernel begins running, its first job is to extract the full code from the compressed format. When extraction is done, the kernel runs systemd, which starts up and initializes the session. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

7 Booting into Linux Run systemd. This initialization code was formerly, this was called init. It is the mother of all processes and is responsible for making the host machine ready to use. A configuration file lists the required services. This typically includes: Mounting the filesystems. Starting system daemons (sshd, cupsd, xinetd, crond, named, systemd-logind, apcupsd, etc. ) Starting system services (the Xorg server, postgress server, mail server, power manager) Presenting a login window. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

8 The Command Shell More on the Command Shell Quotes Compiling Commands Search Paths Changing your Environment Shell Resource Files Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

9 The Command Shell More on the Command Shell When you log into the system, or double-click your terminal-window icon, Unix will create a shell process for you. It executes in your user space, not kernel space This process displays a prompt in the window and waits for you to enter a command. It then reads and parses your command line. The shell will look at the directories in your search path to find a shell script or executable program that matches your command name. The first match, from the left end of the search path, will be used. The shell then forks off another process to execute your command, and sends the argc and argv that you typed to your new process. When your process finishes, control returns to your shell process and it displays another command prompt. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

10 The Command Shell Quotes: bash shell Quoting removes the special meaning, to the shell, of special characters and reserved words. It can be used to prevent parameter expansion. There are three mechanisms: the escape character (\), single quotes, and double quotes. The escape character is a backslash. It preserves the literal value of the following char. A \<newline> is treated as a line continuation. Enclosing characters in single quotes preserves the literal value of each character. A single quote may not occur between single quotes. Enclosing characters in double quotes is trickier. It preserves the literal value of characters within the quotes, with many exceptions and special cases. Use it for scripting and to quote single quotes. "You can t do this with single quotes." You can \ t do this with single quotes. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

11 The Command Shell Compiling Commands Compile-time search path (open log.txt, search for... ). Preprocess only: c++ -E mycode.cpp Compile only (don t link): c++ -c mycode.cpp Compile and link all: c++ -o mycommand -std=c++14 mycode.cpp mypackage.cpp Compile and link in C: cc -o mycommand mycode.c mypackage.c Load and run a program:./mycommand [any necessary switches and arguments] Load and run if you have extended your path: mycommand [any necessary switches and arguments] Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

12 The Command Shell Useful C/C++ flags: -E: Preprocess only -c : Compile only (don t link) -Wall : display all warnings (highest level of warnings) -v : verbose; print all commands used during compilation. --version : Display version number and copyright information for your compiler. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

13 The Command Shell The Environment Your environment is a set of strings that is global to your processes. It contains varied information about you and your hardware, and it influences many things. An initial set of definitions and settings is installed when you install your system. However, you can add to that set of definitions by using a.bashrc file. When the.bashrc file EXPORTs something, it goes into the environment and will be inherited by all commands executed in that shell. PATH, your list of search paths, is defined in the environment. When you use a shell to execute a command, the shell s environment is inherited by the shell process it forks off. This new process executes in the same environment as its parent shell. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

14 The Command Shell Understanding your Environment Typically, each string in the environment has the form NAME=setting. Within a program, you can access the environment using getenv() or through the global variable char** environ. You may create a new environment variable in a shell: setenv name value (tcsh) or export name=value (bash) You may remove an environment variable: unsetenv( name ) (tcsh) or unset name (bash) Example: Add the working directory to your search PATH: bash-3.2$ PATH=$PATH:. bash-3.2$ export PATH Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

15 The Command Shell Search Paths When you execute a command from the shell, the shell-process must find the definition of that command on your machine. To do so, it uses a search path that was loaded when you logged on. The search path lists several directories on your machine where shell scripts and executable code are stored. Initially, the search path lists the directories used for executables when the system was installed. To see your current search path in your environment, type this: echo "$PATH" printenv PATH Type a $ in front of the name of an environment variable or a shell variable to get its value. My current search path is: /usr/bin /bin /usr/sbin /sbin /usr/local/bin /usr/x11/bin Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

16 The Command Shell Search Paths Continued There is no special place for storing the UNIX commands. Each installation will have a default search path that is appropriate for the way its resources are organized. Built-in commands are typically found in these places: /bin: the most fundamental commands. /usr/bin: most commands and utility programs. /sbin: commands for system administrators. Some commands are built into the shell. Initially, your current working directory is NOT on the search path. To execute a command you just compiled, you need to change the search path or specify the current working directory as part of the command name:./mycommand myarg1 myarg2 Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

17 The Command Shell Changing your environment. A UNIX system has a predefined standard environment that defines a long list of basic variables, including who you are and what shell you use. Environment variables have upper-case names like One of those is PATH, your default search path. To see your environment, type printenv LOGNAME, USER In addition, your shell will load a resource file each time you open a shell window. The result will be that all the default variables are loaded first, followed by the ones you defined. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

18 The Command Shell Creating a Shell Resource File If you have never created your own shell resource file, you don t have one. Instead, the default profile will be used when you open a shell Use a text editor to create your own resource file, then store the file in your home directory with the name.bashrc (or.cshrc). If you have trouble giving the correct name to this file, create a file in your home directory with any name, then use the shell and the mv command to change the name. Thereafter, the commands in this file will be loaded for you every time you open a new shell. To see this file in a shell, execute cd to go to your home directory, then type cat.bashrc Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

19 Defining a Command Language Defining a Command Language Each application defines its own commands Use getopt() to analyze the argument vector Defining short and long options Using getopt_long() Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

20 Defining a Command Language Each Application Defines its own Legal Commands A program receives an argument vector, entered by the user, when it begins execution. Every program knows its own needs. Many programs do not take input from the command line. Some programs require arguments to work. Some have optional arguments, in addition to or instead of required ones. The first thing a program must do is determine whether it has enough command line arguments and whether they make sense. If not, it must print a usage comment to inform the user of the problem and possible solutions. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

21 Defining a Command Language Use getopt() to Analyze the Argument Vector. Command shells provide tools for extracting the switches, or options, and their parameters from argv. Without automation, this is an annoying and picky job because of the need for validation and for handling errors. The getopt function automates the process. int getopt( int argc, char* argv[], const char* opts ); argc and argv are the parameters received from the shell by your program. opts is a string containing all the short switch options that your program supports: "i:avrou" An option letter followed by a colon requires a parameter. An option letter followed by a double colon has an optional parameter. Your program uses getopt() to read and validate the switches that are present. Then it must store that information in its own variables. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

22 Defining a Command Language Using getopt(): options.c. for (;;) { ch = getopt(argc, argv, "i:-abrou"); if( ch == -1 ) break;... getopt() returns the next known option character in the optstring. It will search argv for all the switches in the command line. Each time you call it, the next switch will be returned. (This is a char, even though the type is technically int.) The order of the items on the command line will be changed so that all the non-switch arguments follow all the switches. The global variable optind contains the subscript of the first non-option argument. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

23 Defining a Command Language getopt() Return Values On return from getopt(), optarg points to an option argument, if it is expected, optind contains the index to the next argv argument for a subsequent call to getopt(). optopt saves the last known option character returned by getopt(). getopt() function returns -1 when the argument list is exhausted. If getopt()encounters a character not found in optstring or if it detects a missing option argument, it returns? (question mark). optopt is set to the character that caused the error. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

24 Defining a Command Language Defining short and long Options Each program defines its own command language. A string defines the short switches: "i:avrou" A structure is used for the long switches. struct option longopts[] = { { "verbose", no_argument, NULL, b }, { "output", required_argument, NULL, o }, { "recursive", no_argument, NULL, 0 }, { "debug", optional_argument, NULL, 0 }, { NULL, 0, NULL, 0 } }; The last line of the structure must be a row of 0 s. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

25 Defining a Command Language Using getopt_long() and getopt_long_only(). This function extends the old getopt() by allowing long switches with single dashes. Many long switches are equivalent to one-letter switches; many are not. int getopt_long_only( int argc, char* argv[], const char* optstring struct option longopts[], int* codep ); longopts is a table containing all the information about long switches. If there is a short equivalent for the long switch, that char will be returned (otherwise 0). If not, the subscript of the correct line of longopts will be stored in codep. If a switch requires an argument, it may be preceded by either an = or a space. An optional argument must be preceded an =. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

26 Defining a Command Language Errors with getopt() and getopt_long_only(). Technically, all of the switches are supposed to precede all of the non-switch arguments. If you write them in the wrong order, getopt() and getopt_long_only() will change the order to put non-switches at the end. Invalid switch name: If a switch is on the command line but NOT in the set of defined switches, getopt() returns? as an error code. Your program must check for and handle the error. It is customary to output a usage message when the form of the command line is wrong. If a required switch argument is missing, the next thing on the command line will be taken as its argument. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

27 Defining a Command Language The getopt_long() Demo Program. options.c Lines to look at: Line 6 includes the header file for getopt_long(). Lines 14 through 20 define the set of long switches. The columns in the table are: the switch name argument-requirements an alternate way to return the option, and the equivalent option letter, or zero. The loop on lines processes the options returned by getopt_long(). Use a switch statement to process your options. My switch statement handles four different kinds of cases. In this program, all I do is print the switch information. However, normally, each case would set a boolean flag-variable to indicate that the switch is on, and process switch-arguments appropriately. Alice E. Fischer Systems Programming Lecture /28 September 6, / 28

28 Defining a Command Language Homework P1 In the man pages, look up ten commands from the list below. Choose unfamiliar commands, and choose two from each column. Turn in a 1-sentence description of what each command is used for. alias awk cal last uname cat cd chmod chown which cp diff df emacs file finger gcc grep gzip strings kill less ln ls xxd mkdir more mv passwd man ps pwd rm rsync ping whoami sort ssh stat head sudo slogin svn touch tail touch uniq wc who uptime Alice E. Fischer Systems Programming Lecture /28 September 6, / 28