UNIX Concepts COMPSCI 386

Transcription

1 UNIX Concepts COMPSCI 386

2 Topics History of C and UNIX The GNU Project Linux Command-Line Basics UNIX-Style File System

3 Multics Multiplexed Information and Computing Service Time-sharing system for mainframes (1965) virtual memory and other modern OS features MIT, Bell Labs, and General Electric First (?) OS written in a high-level language (PL/1). Influential but not a commercial success. Ken Thompson and Dennis Ritchie worked on the project until Bell Labs dropped out. (Thompson at Google since 2006, where he co-invented Go.) They continued on their own. Kernighan and others at Bell Labs joined the effort. Designed UNIX for DEC PDP-7. Later reimplemented on the PDP-11.

4 UNIX The goal was to create a time-sharing OS with a simple UI and simple components that can be arranged and composed in a variety of ways. Philosophy in a nutshell: modularity. Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface. (McIlroy) The 17 Rules Originally written in assembly, but a high-level language would be needed for its proper development.

5 C First, Ken Thompson invented B. a typeless, stripped-down version of BCPL compiler squeezed into 8K bytes of memory Then C was born as B-with-types. Easy to retarget C compiler: made UNIX highly portable sparked its prominence in industry and academia C was standardized by ANSI in 1989.

6 C Why are almost all operating systems written in C? Speed Code maps efficiently to machine instructions. no threads, templates, nested functions, function or operator overloading, exception handling; limited support for OOP. Not much runtime support is needed. no dynamic type checking no bounds checking or garbage collection Low-level access to memory (and hardware) thanks to weakly typed pointers

7 The GNU Project UNIX was proprietary, licenced to universities and corporations by A&T. In 1984, Richard Stallman (* * *) at the MIT AI Lab launched the GNU project to develop a free UNIXlike OS. He wrote gcc, emacs, and other GNU tools that are still in widespread use. Created the Free Software Foundation to sponsor GNU development and pursue free software ideals.

8 Open Source Free software: like free speech or free beer? The freedom to view, copy, modify, and redistribute source code without having to pay royalties or other fees. Open source movement Best way to produce safe and reliable software? Enlist the cooperation of developers from around the world in a free exchange of ideas, opinions, and code. Stallman wrote the first copyleft licence (GNU GPL)

9 Linux GNU's kernel was never finished. Linux released in The goal was to create a UNIX-like OS for PC. Just a kernel, but can be packaged with GNU tools and libraries for complete OS, called properly GNU/Linux. There are now hundreds of distributions. Linux kernel with GNU tools and libraries, third-party software, package manager, desktop environment.

10 User Interfaces batch, shell (CLI), GUI Two strategies for implementing a shell: Interpret and execute each command. (DOS) If user types rm myfile.txt, shell makes necessary system calls. Does not interpret the command. Just invokes a system program to perform the work. (UNIX) If user types rm myfile.txt, the shell invokes a system program named rm with myfile.txt as a parameter. Two major advantages to this approach.

11 Bourne Shell (1977) /bin/sh expressive scripting language can use scripts as filters I/O redirection and pipes globbing and command substitution scripts invoked as commands by their filename synchronous/asynchonous execution of commands

12 C Shell (1979) /bin/csh (or improved version: /bin/tcsh) scripting language with C-like syntax filename and command completion history, aliases, and other features for interactive use

13 Korn Shell (1983) /bin/ksh job control command aliasing command history better for interactive work than C shell, but had to be licensed from AT&T

14 Bash Shell (1989) /bin/bash Bourne-Again SHell Written for GNU Project Default shell for Linux and macos

15 Command-Line Basics What shell am I using? Navigating the file system Viewing and finding files File permissions Copying and moving files Grepping Pipes and redirection Hard links and soft links

16 BASH Variables Environment variables inherited by child processes /etc/bash.bashrc (for interactive non-login shells) SHELL, USER, HOME, PATH, PWD Shell variables not inherited by child processes ~/.bashrc (for interactive non-login shells) PS1, BASH_VERSION, HISTSIZE

17 What Shell Am I Using? > printenv SHELL /bin/bash > echo $SHELL /bin/bash > ps $$ PID TTY STAT TIME COMMAND 3877 pts/1 Ss 0:00 /bin/bash > ps -p $$ PID TTY TIME CMD 3877 pts/1 00:00:00 bash We can use ps to obtain info about all processes in the system.

18 Navigating the File System Listing directory contents with ls options lapf man ls pwd cd ls.. ls../.. ls / ls ~ Change to parent, root, and home without specifying absolute paths.

19 Filesystem Hierarchy Standard Defines directory structure for UNIX-like systems. Most Linux distributions are FHS-compliant.

20 Navigating the File System /bin (command binaries) /boot (files used by the kernel during start-up) /dev (device files) /etc (system configuration files) /home (home directories for individual users) /lib (shared libraries and kernel modules) /lost+found (recovered files, as after system crash) /media (mount point for removable media)

21 Navigating the File System /mnt (mount point for temporary file system) /opt (add-on software not part of default installation) /proc (virtual file system containing system info) /root (home directory for system administrator) /sbin (binaries for system maintenance and admin tasks) /tmp (temporary files) /usr (user-land programs and data) /tmp (temporary files, like lock files) /var (system log files)

22 Viewing Files cat /etc/shells cat bash_lab/episode4.txt more or less

23 Determining File Types file /etc/shells file /etc/p* file /bin/cat

24 Finding Files

25 Finding Files

26 File Permissions - rwx r-x --x type user group other

27 File Permissions Do a long listing after each of the following: cd art chmod u-w 1* chmod g+w [2-4].jpg chmod u=rwx,go=r 5* chmod 751*

28 File Permissions

29 File Permissions Default file creation mask umask umask -S Type umask 135 and touch a file. umask is a built-in command. man umask

30 Copying and Moving Files cp /etc/shells. cp /etc/shells ~ cp -r bash_lab/art ~ Try to remove art.

31 Grepping Using grep the most common UNIX filter. grep womp episode4.txt grep the droids episode4.txt grep sand episode4.txt grep -c LUKE episode4.txt grep [A-Z]-[0-9] episode4.txt grep ^Eight episode4.txt

32 Pipes and Redirection A pipe connects one process to another. Redirection connects a process to a file. grep ^Luke episode4.txt grep Ben grep droid episode4.txt wc -l man less less ls /etc > etc history > history

33 Pipes and Redirection Exercise Write a C program that prompts the user for two integers and displays the average. Compile and test it. Create a text file with input for the program. Run the program with input redirected to the file. Redirect the output to a file.

34 Hard Links Create directories tempdir1 and tempdir2. Create a text file in tempdir1 and display contents.

35 Hard Links Make a hard link and display the link count. Move the link to tempdir2 and delete the text file. Change to tempdir2. cat indignation Surprised?

36 UNIX File System Directories do not contain files. That's an illusion. A directory is a file containing a list of (name, inum) pairs. art episode4.txt episode5.txt Each inum is an index into a table of inodes. Do a directory listing with i and ail options.

37 UNIX File System The inode table contains inodes. protection mode owner and group file size link count timestamps direct blocks indirect blocks double indirect triple indirect Note that file name not stored. DATA DATA DATA DATA DATA DATA DATA DATA

38 UNIX File System /usr/src/linux-headers /include/linux/fs.h struct inode { umode_t i_mode; unsigned short i_opflags; kuid_t i_uid; kgid_t i_gid; unsigned int i_flags; }

39 Hard Links ln episode4.txt e4 art episode4.txt episode5.txt e Move link to another directory. Now you can find the file with -inum option.

40 Soft Links Make a soft (symbolic) link. Move soft link to tempdir2. Remove original file (yoda). Now in tempdir2, try: cat slink.

41 Hard vs. Soft Links Which is like a Windows shortcut? Hard links more space efficient faster access to link target Soft links can link to directories can link across file systems