1034 lines
36 KiB
HTML
1034 lines
36 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=iso-8859-1">
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<title>Debian Tutorial (Obsolete Documentation) - Advanced topics</title>
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<link href="index.html" rel="start">
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<link href="ch-troubleshooting.html" rel="prev">
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<link href="ch-next.html" rel="next">
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<link href="index.html#contents" rel="contents">
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<link href="index.html#copyright" rel="copyright">
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<link href="ch-about.html" rel="chapter" title="1 About this manual">
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<link href="ch-introduction.html" rel="chapter" title="2 Introduction">
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<link href="ch-start.html" rel="chapter" title="3 Getting started">
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<link href="ch-basics.html" rel="chapter" title="4 The Basics">
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<link href="ch-docs.html" rel="chapter" title="5 Reading documentation and getting help">
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<link href="ch-shell.html" rel="chapter" title="6 Using the shell">
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<link href="ch-files.html" rel="chapter" title="7 More on files">
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<link href="ch-editor.html" rel="chapter" title="8 Creating and editing text files">
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<link href="ch-custom_shell.html" rel="chapter" title="9 Customizing the shell">
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<link href="ch-X.html" rel="chapter" title="10 The X Window System">
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<link href="ch-text_tools.html" rel="chapter" title="11 Text tools">
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<link href="ch-file_tools.html" rel="chapter" title="12 File tools">
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<link href="ch-disks.html" rel="chapter" title="13 Using disks">
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<link href="ch-dpkg.html" rel="chapter" title="14 Removing and installing software">
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<link href="ch-troubleshooting.html" rel="chapter" title="15 Troubleshooting">
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<link href="ch-advanced.html" rel="chapter" title="16 Advanced topics">
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<link href="ch-next.html" rel="chapter" title="17 Where to from here?">
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<link href="ch-helping.html" rel="chapter" title="18 Contributing to Debian: How can I help?">
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<link href="ap-apps_appendix.html" rel="appendix" title="A A brief survey of available applications">
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<link href="ap-components_appendix.html" rel="appendix" title="B Summary of GNU/Linux system components">
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<link href="ap-booting_appendix.html" rel="appendix" title="C Appendix C: Booting the system">
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<link href="ap-misc_appendix.html" rel="appendix" title="D Miscellaneous">
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<link href="ch-about.html#s-acknowldgements" rel="section" title="1.1 Acknowledgements">
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<link href="ch-introduction.html#s-introduction-debian" rel="section" title="2.1 What is Debian?">
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<link href="ch-introduction.html#s-introduction-DFSG" rel="section" title="2.2 What's free software?">
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<link href="ch-introduction.html#s-introduction-how" rel="section" title="2.3 How to Read This Book">
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<link href="ch-introduction.html#s-introduction-ldp" rel="section" title="2.4 The Linux Documentation Project">
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<link href="ch-start.html#s-start-multi" rel="section" title="3.1 A multiuser, multitasking operating system">
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<link href="ch-start.html#s-start-login" rel="section" title="3.2 Logging in">
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<link href="ch-start.html#s-start-keys" rel="section" title="3.3 Keys">
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<link href="ch-start.html#s-start-commandline" rel="section" title="3.4 Command history and editing the command line">
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<link href="ch-start.html#s-start-root" rel="section" title="3.5 Logging in as root">
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<link href="ch-start.html#s-start-VC" rel="section" title="3.6 Virtual consoles">
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<link href="ch-start.html#s-start-shutdown" rel="section" title="3.7 Shutting down">
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<link href="ch-basics.html#s-basics-commandline" rel="section" title="4.1 The command line and <samp>man</samp> pages">
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<link href="ch-basics.html#s-basics-files" rel="section" title="4.2 Files and Directories">
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<link href="ch-basics.html#s-basics-processes" rel="section" title="4.3 Processes">
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<link href="ch-basics.html#s-basics-shell" rel="section" title="4.4 The shell">
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<link href="ch-basics.html#s-basics-jobs" rel="section" title="4.5 Managing processes with Bash">
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<link href="ch-basics.html#s-basics-bash" rel="section" title="4.6 A few Bash features">
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<link href="ch-basics.html#s-basics-identity" rel="section" title="4.7 Managing your identity">
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<link href="ch-docs.html#s-docs-sources" rel="section" title="5.1 Kinds of documentation">
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<link href="ch-docs.html#s-docs-info" rel="section" title="5.2 Using info">
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<link href="ch-docs.html#s-docs-textfiles" rel="section" title="5.3 Viewing text files with more and less">
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<link href="ch-docs.html#s-docs-howtos" rel="section" title="5.4 HOWTOs">
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<link href="ch-docs.html#s-docs-support" rel="section" title="5.5 Getting help from a person">
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<link href="ch-docs.html#s-docs-sysinfo" rel="section" title="5.6 Getting information from the system">
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<link href="ch-shell.html#s-shell-variables" rel="section" title="6.1 Environment variables">
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<link href="ch-shell.html#s-shell-path" rel="section" title="6.2 Where commands live: the <samp>PATH</samp> variable">
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<link href="ch-shell.html#s-custom_shell-aliases" rel="section" title="6.3 Aliases and shell functions">
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<link href="ch-shell.html#s-shell-io" rel="section" title="6.4 Controlling input and output">
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<link href="ch-shell.html#s-shell-modifiers" rel="section" title="6.5 Specifying how and when to run commands">
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<link href="ch-shell.html#s-shell-wildcards" rel="section" title="6.6 Filename expansion ("Wildcards")">
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<link href="ch-shell.html#s6.7" rel="section" title="6.7 Interactive/non-interactive">
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<link href="ch-files.html#s-files-permissions" rel="section" title="7.1 Permissions">
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<link href="ch-files.html#s-files-tour" rel="section" title="7.2 What files are on my system? Where can I put my own files?">
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<link href="ch-files.html#s-file-manager" rel="section" title="7.3 Using a filemanager">
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<link href="ch-editor.html#s-editor-textfile" rel="section" title="8.1 What's a text file?">
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<link href="ch-editor.html#s-editor-editor" rel="section" title="8.2 Text editors">
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<link href="ch-editor.html#s-editor-vi" rel="section" title="8.3 Creating and editing a text file with <code>vi</code>">
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<link href="ch-custom_shell.html#s-custom_shell-dotfiles" rel="section" title="9.1 .rc files and <samp>ls -a</samp>">
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<link href="ch-custom_shell.html#s-custom_shell-etc" rel="section" title="9.2 System-wide vs. user-specific configuration">
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<link href="ch-X.html#s-X-starting" rel="section" title="10.1 Starting the X environment">
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<link href="ch-X.html#s-X-intro" rel="section" title="10.2 Intro: What is X">
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<link href="ch-X.html#s-X-basics" rel="section" title="10.3 Basic X operations">
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<link href="ch-X.html#s-X-startup" rel="section" title="10.4 Customizing your X startup">
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<link href="ch-text_tools.html#s-text_tools-regexps" rel="section" title="11.1 Regular expressions">
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<link href="ch-file_tools.html#s-file_tools-backup" rel="section" title="12.1 Backup tools">
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<link href="ch-file_tools.html#s-file_tools-compression" rel="section" title="12.2 File compression with <samp>gzip</samp>">
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<link href="ch-file_tools.html#s-file_tools-split" rel="section" title="12.3 Splitting files into smaller pieces">
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<link href="ch-file_tools.html#s-file_tools-find" rel="section" title="12.4 Finding files">
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<link href="ch-file_tools.html#s-file_tools-file" rel="section" title="12.5 Determining a file's contents">
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<link href="ch-disks.html#s-disks-concepts" rel="section" title="13.1 Concepts">
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<link href="ch-disks.html#s-disks-mount" rel="section" title="13.2 <samp>mount</samp> and <samp>/etc/fstab</samp>">
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<link href="ch-disks.html#s-network-ppp" rel="section" title="13.3 PPP">
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<link href="ch-dpkg.html#s-dpkg-dpkg" rel="section" title="14.1 The <code>dpkg</code> package utility">
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<link href="ch-dpkg.html#s-dpkg-what" rel="section" title="14.2 What a package maintenance utility does">
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<link href="ch-dpkg.html#s-dpkg-apt" rel="section" title="14.3 Apt">
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<link href="ch-dpkg.html#s-dpkg-dselect" rel="section" title="14.4 Using dselect.">
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<link href="ch-dpkg.html#s-dpkg-commandline" rel="section" title="14.5 Using dpkg manually">
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<link href="ch-dpkg.html#s-dpkg-compiling" rel="section" title="14.6 Compiling software yourself">
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<link href="ch-dpkg.html#s-dpkg-proprietary" rel="section" title="14.7 Proprietary software not in .deb format">
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<link href="ch-troubleshooting.html#s-troubleshooting-frozen" rel="section" title="15.1 Debian is frozen or crashed!">
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<link href="ch-troubleshooting.html#s-troubleshooting-termgarbage" rel="section" title="15.2 My terminal isn't behaving properly">
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<link href="ch-troubleshooting.html#s-troubleshooting-vi" rel="section" title="15.3 The computer beeps when I press a key, and my screen shows a text file and/or a lot of <samp>~</samp> symbols.">
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<link href="ch-advanced.html#s-advanced-scripting" rel="section" title="16.1 Introduction to shell scripting">
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<link href="ch-advanced.html#s-advanced-files" rel="section" title="16.2 Advanced files">
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<link href="ch-advanced.html#s-advanced-kernel" rel="section" title="16.3 Compiling the kernel">
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<link href="ch-advanced.html#s-advanced-security" rel="section" title="16.4 A few words on security">
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<link href="ch-advanced.html#s-advanced-programming" rel="section" title="16.5 Programming on Linux">
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<link href="ch-next.html#s-next-manuals" rel="section" title="17.1 Other Debian manuals to read">
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<link href="ch-next.html#s-next-resources" rel="section" title="17.2 Other resources">
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<link href="ch-helping.html#s-helping-bugs" rel="section" title="18.1 Submit bug reports">
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<link href="ch-helping.html#s-helping-other" rel="section" title="18.2 Other things">
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<link href="ap-apps_appendix.html#s-A_apps-whatever" rel="section" title="A.1 (Subheadings could copy the structure of the menu system)">
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<link href="ap-misc_appendix.html#s-unix-history" rel="section" title="D.1 Unix History">
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<link href="ap-misc_appendix.html#sD.2" rel="section" title="D.2 GNU/Linux History">
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<link href="ap-misc_appendix.html#sD.3" rel="section" title="D.3 The Linux kernel's version numbering">
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<link href="ch-introduction.html#s-introduction-debian-os" rel="subsection" title="2.1.1 What's an operating system, and what sort of operating system is Debian?">
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<link href="ch-introduction.html#s-introduction-debian-how" rel="subsection" title="2.1.2 Who creates Debian?">
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<link href="ch-basics.html#s-basics-commandline-description" rel="subsection" title="4.1.1 Describing the command line">
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<link href="ch-basics.html#s-basics-files-intro" rel="subsection" title="4.2.1 Introduction to files">
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<link href="ch-basics.html#s-basics-files-using" rel="subsection" title="4.2.2 Using files: a tutorial">
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<link href="ch-basics.html#s-basics-bash-completion" rel="subsection" title="4.6.1 Tab Completion">
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<link href="ch-docs.html#s-docs-support-posting" rel="subsection" title="5.5.1 Dos and Don'ts of asking a question">
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<link href="ch-shell.html#s-shell-variables-parents" rel="subsection" title="6.1.1 Parent and child processes">
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<link href="ch-shell.html#s-interactive-shells" rel="subsection" title="6.7.1 Interactive shells">
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<link href="ch-files.html#s-files-permissions-groups" rel="subsection" title="7.1.1 File Ownership">
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<link href="ch-files.html#s-files-permissions-mode" rel="subsection" title="7.1.2 Mode">
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<link href="ch-files.html#s-files-permissions-tutorial" rel="subsection" title="7.1.3 Permissions in practice">
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<link href="ch-editor.html#s-editor-vi-creating" rel="subsection" title="8.3.1 Creating a file">
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<link href="ch-editor.html#s-editor-vi-editing" rel="subsection" title="8.3.2 Editing an existing file">
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<link href="ch-editor.html#s-editor-vi-editing-movement" rel="subsection" title="8.3.2.1 Moving around in a file">
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<link href="ch-editor.html#s-editor-vi-editing-deletion" rel="subsection" title="8.3.2.2 Deleting text">
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<link href="ch-editor.html#s-editor-vi-editing-moremoving" rel="subsection" title="8.3.2.3 Sophisticated movement">
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<link href="ch-editor.html#s-editor-vi-editing-numbers" rel="subsection" title="8.3.2.4 Repeating commands">
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<link href="ch-editor.html#s-editor-vi-editing-advanced" rel="subsection" title="8.3.2.5 Advanced reference">
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<link href="ch-X.html#s-X-basics-mouse" rel="subsection" title="10.3.1 The mouse">
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<link href="ch-X.html#s-X-basics-clients" rel="subsection" title="10.3.2 X clients">
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<link href="ch-X.html#s-X-basics-trouble" rel="subsection" title="10.3.3 Troubleshooting">
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<link href="ch-X.html#s-X-basics-leaving" rel="subsection" title="10.3.4 Leaving the X environment">
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<link href="ch-disks.html#s-disks-mount-mounting" rel="subsection" title="13.2.1 Mounting a filesystem">
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<link href="ch-disks.html#s-disks-mount-cdromexample" rel="subsection" title="13.2.2 Example: Mounting a CD-ROM">
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<link href="ch-disks.html#s-disks-mount-fstab" rel="subsection" title="13.2.3 <samp>/etc/fstab</samp>: Automating the mount process">
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<link href="ch-disks.html#s-disks-mount-removable" rel="subsection" title="13.2.4 Removable disks (floppies, Zip disks, etc.)">
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<link href="ch-disks.html#s-network-ppp-introduction" rel="subsection" title="13.3.1 Introduction">
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<link href="ch-disks.html#s-network-ppp-preparation" rel="subsection" title="13.3.2 Preparation">
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<link href="ch-disks.html#s-network-ppp-wvdial" rel="subsection" title="13.3.3 The Easy Way: <samp>wvdial</samp>">
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<link href="ch-disks.html#s-network-ppp-pppd" rel="subsection" title="13.3.4 Doing It Manually">
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<link href="ch-dpkg.html#s-apt-config" rel="subsection" title="14.3.1 Configuring Apt">
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<link href="ch-dpkg.html#s-apt-usage" rel="subsection" title="14.3.2 Using apt-get.">
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<link href="ch-advanced.html#s-advanced-scripting-why" rel="subsection" title="16.1.1 What and why">
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<link href="ch-advanced.html#s-advanced-scripting-example" rel="subsection" title="16.1.2 A simple example">
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<link href="ch-advanced.html#s-advanced-files-hardlinks" rel="subsection" title="16.2.1 The real nature of files: hard links and inodes">
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<link href="ch-advanced.html#s-advanced-files-types" rel="subsection" title="16.2.2 Types of files">
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<link href="ch-advanced.html#s-advanced-files-types-symlinks" rel="subsection" title="16.2.2.1 Symbolic links">
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<link href="ch-advanced.html#s-files-misc-types-devices" rel="subsection" title="16.2.2.2 Device files">
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<link href="ch-advanced.html#s-advanced-files-types-devices-null" rel="subsection" title="16.2.2.2.1 <samp>/dev/null</samp>">
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<link href="ch-advanced.html#s-advanced-files-types-pipes" rel="subsection" title="16.2.2.3 Named pipes (FIFOs)">
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<link href="ch-advanced.html#s-advanced-files-types-sockets" rel="subsection" title="16.2.2.4 Sockets">
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<link href="ch-advanced.html#s-advanced-files-proc" rel="subsection" title="16.2.3 The <samp>proc</samp> filesystem">
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<link href="ch-advanced.html#s-advanced-files-permissions" rel="subsection" title="16.2.4 Advanced aspects of file permissions">
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<link href="ch-advanced.html#s-advanced-files-permissions-absolute" rel="subsection" title="16.2.4.1 Using numeric arguments with <samp>chmod</samp>">
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<link href="ch-advanced.html#s-advanced-files-chattr" rel="subsection" title="16.2.5 chattr">
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<link href="ch-advanced.html#s-advanced-files-copying" rel="subsection" title="16.2.6 Large-scale copying">
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<link href="ch-advanced.html#s-advanced-files-undocumented" rel="subsection" title="16.2.7 Other concepts not yet covered, but should be">
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</head>
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<body>
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<p><a name="ch-advanced"></a></p>
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<hr>
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<p>
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[ <a href="ch-troubleshooting.html">previous</a> ]
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[ <a href="index.html#contents">Contents</a> ]
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[ <a href="ch-about.html">1</a> ]
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[ <a href="ch-introduction.html">2</a> ]
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[ <a href="ch-start.html">3</a> ]
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[ <a href="ch-basics.html">4</a> ]
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[ <a href="ch-docs.html">5</a> ]
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[ <a href="ch-shell.html">6</a> ]
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[ <a href="ch-files.html">7</a> ]
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[ <a href="ch-editor.html">8</a> ]
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[ <a href="ch-custom_shell.html">9</a> ]
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[ <a href="ch-X.html">10</a> ]
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[ <a href="ch-text_tools.html">11</a> ]
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[ <a href="ch-file_tools.html">12</a> ]
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[ <a href="ch-disks.html">13</a> ]
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[ <a href="ch-dpkg.html">14</a> ]
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[ <a href="ch-troubleshooting.html">15</a> ]
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[ 16 ]
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[ <a href="ch-next.html">17</a> ]
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[ <a href="ch-helping.html">18</a> ]
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[ <a href="ap-apps_appendix.html">A</a> ]
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[ <a href="ap-components_appendix.html">B</a> ]
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[ <a href="ap-booting_appendix.html">C</a> ]
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[ <a href="ap-misc_appendix.html">D</a> ]
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[ <a href="ch-next.html">next</a> ]
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</p>
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<hr>
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<h1>
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Debian Tutorial (Obsolete Documentation)
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<br>Chapter 16 - Advanced topics
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</h1>
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<hr>
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<p>
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(Should advanced topics be here? I think it would be nice to have some of
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these, just to show people the possibilities and give some conceptual
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explanation that won't really be in a reference manual. Also it always feels
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nice to make it to a chapter called "advanced topics". Self-esteem
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booster for the newbie. :)
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</p>
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<hr>
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<h2><a name="s-advanced-scripting"></a>16.1 Introduction to shell scripting</h2>
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<hr>
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<h3><a name="s-advanced-scripting-why"></a>16.1.1 What and why</h3>
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<p>
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Automate simple tasks.
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</p>
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<hr>
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<h3><a name="s-advanced-scripting-example"></a>16.1.2 A simple example</h3>
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<p>
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Ideas?
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</p>
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<hr>
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<h2><a name="s-advanced-files"></a>16.2 Advanced files</h2>
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<hr>
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<h3><a name="s-advanced-files-hardlinks"></a>16.2.1 The real nature of files: hard links and inodes</h3>
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<p>
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Each file on your system is represented by an <em>inode</em> (for Information
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Node; pronounced "eye-node"): an inode contains all the information
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about the file. However, the inode is not directly visible. Instead, each
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inode is linked into the filesystem by one or more <em>hard links</em>. Hard
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links contain the name of the file, and the inode number. The inode contains
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the file itself, i.e., the location of the information being stored on disk,
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its access permissions, the type of the file, and so on. The system can find
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any inode once it has the inode number.
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</p>
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<p>
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A single file can have more than one hard link. What this means is that
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multiple filenames refer to the same file (that is, they are associated with
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the same inode number). However, you can't make hard links across filesystems:
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all hard references to a particular file (inode) must be on the same
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filesystem. This is because each filesystem has its own set of inodes, and
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there can be duplicate inode numbers between filesystems.
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</p>
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<p>
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Since all hard links to a given inode are referring to <em>the same file</em>,
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you can make changes to the file, referring to it by one name, and then see
|
|
those changes when referring to it by a different name. Try this:
|
|
</p>
|
|
<ol type="1" start="1" >
|
|
<li>
|
|
<p>
|
|
<samp>cd; echo "hello" > firstlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
<samp>cd</samp> to your home directory and create a file called
|
|
<samp>firstlink</samp> containing the word "hello". What you've
|
|
actually done is redirect the output of <samp>echo</samp> (<samp>echo</samp>
|
|
just echoes back what you give to it), placing the output in
|
|
<samp>firstlink</samp>. See the chapter on shells for a full explanation.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="2" >
|
|
<li>
|
|
<p>
|
|
<samp>cat firstlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Confirm the contents of <samp>firstlink</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="3" >
|
|
<li>
|
|
<p>
|
|
<samp>ln firstlink secondlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Create a hard link: <samp>secondlink</samp> now points to the same inode as
|
|
<samp>firstlink</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="4" >
|
|
<li>
|
|
<p>
|
|
<samp>cat secondlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Confirm that <samp>secondlink</samp> is the same as <samp>firstlink</samp>
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="5" >
|
|
<li>
|
|
<p>
|
|
<samp>ls -l</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Notice that the number of hard links listed for <samp>firstlink</samp> and
|
|
<samp>secondlink</samp> is 2.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="6" >
|
|
<li>
|
|
<p>
|
|
<samp>echo "change" >> secondlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
This is another shell redirection trick - don't worry about the details. We've
|
|
appended the word "change" to <samp>secondlink</samp>. Confirm this
|
|
with <samp>cat secondlink</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="7" >
|
|
<li>
|
|
<p>
|
|
<samp>cat firstlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
<samp>firstlink</samp> also has the word "change" appended! It's
|
|
because <samp>firstlink</samp> and <samp>secondlink</samp> refer to <em>the
|
|
same file</em>. It doesn't matter what you call it when you change it.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="8" >
|
|
<li>
|
|
<p>
|
|
<samp>chmod a+rwx firstlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Change permissions on <samp>firstlink</samp>. Do <samp>ls -l</samp> to confirm
|
|
that permissions on <samp>secondlink</samp> were also changed. This means that
|
|
permissions information is stored in the inode, not in links.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="9" >
|
|
<li>
|
|
<p>
|
|
<samp>rm firstlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Delete this link. This is a subtlety of <samp>rm</samp> --- it really removes
|
|
links, not files. Now type <samp>ls -l</samp> and notice that
|
|
<samp>secondlink</samp> is still there. Also notice that the number of hard
|
|
links for <samp>secondlink</samp> has been reduced to one.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="10" >
|
|
<li>
|
|
<p>
|
|
<samp>rm secondlink</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Delete the other link. When there are no more links to a file, Linux deletes
|
|
the file itself, that is, its inode.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
|
|
<p>
|
|
All files work like this --- even special types of files such as devices (e.g.
|
|
<samp>/dev/hda</samp>).
|
|
</p>
|
|
|
|
<p>
|
|
A directory is simply a list of filenames and inode numbers, that is, a list of
|
|
hard links. When you create a hard link, you're just adding a name-number pair
|
|
to a directory. When you delete a file, you're just removing a hard link from
|
|
a directory.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-types"></a>16.2.2 Types of files</h3>
|
|
|
|
<p>
|
|
One detail we've been concealing up to now is that the Linux kernel considers
|
|
nearly everything to be a file. That includes directories and devices: they're
|
|
just special kinds of files.
|
|
</p>
|
|
|
|
<p>
|
|
As you may remember, the first character of an <samp>ls -l</samp> display
|
|
represents the type of the file. For an ordinary file, this will be simply
|
|
<samp>-</samp>. Other possibilities are:
|
|
</p>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>d</samp> (directory)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>l</samp> (symbolic link)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>b</samp> (block device)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>c</samp> (character device)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>p</samp> (named pipe)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
<samp>s</samp> (socket)
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
|
|
<hr>
|
|
|
|
<h4><a name="s-advanced-files-types-symlinks"></a>16.2.2.1 Symbolic links</h4>
|
|
|
|
<p>
|
|
Symbolic links (also called symlinks or soft links) are the other kind of link
|
|
besides hard links. A symlink is a special file that "points to" a
|
|
hard link on any mounted filesystem. When you try to read the contents of a
|
|
symlink, it gives the contents of the file it's pointing to rather than the
|
|
contents of the symlink itself. Since directories, devices, and other symlinks
|
|
are types of files, you can point a symlink at any of those things.
|
|
</p>
|
|
|
|
<p>
|
|
So a hard link is a filename and an inode number. A file is really an inode: a
|
|
location on disk, file type, permissions mode, etc. A symlink is an inode that
|
|
contains the name of a hard link. A symlink pairs one filename with a second
|
|
filename, while a hard link pairs a filename with an inode number.
|
|
</p>
|
|
|
|
<p>
|
|
All hard links to the same file have equal status. That is, one is as good as
|
|
the other; if you perform any operation on one it's just the same as performing
|
|
that operation on any of the others. This is because the hard links all refer
|
|
to the same inode. Operations on symlinks, on the other hand, sometimes affect
|
|
the symlink's own inode (the one containing the name of a hard link) and
|
|
sometimes affect the hard link being pointed to.
|
|
</p>
|
|
|
|
<p>
|
|
There are a number of important differences between symlinks and hard links:
|
|
</p>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
Symlinks can cross filesystems. This is because they contain complete
|
|
filenames, starting with the root directory, and all complete filenames are
|
|
unique. Since hard links point to inode numbers, and inode numbers are unique
|
|
only within a single filesystem, they would be ambiguous if the filesystem
|
|
wasn't known.
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
You can make symlinks to directories, but you can't make hard links to them.
|
|
Each directory has hard links --- its listing in its parent directory, its
|
|
<samp>.</samp> entry, and the <samp>..</samp> entry in each of its
|
|
subdirectories --- but to impose order on the filesystem, no other hard links
|
|
to directories are allowed. Consequently, the number of files in a directory
|
|
is equal to the number of hard links to that directory minus two (you subtract
|
|
the directory's name and the <samp>.</samp> link).
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
You can only make a hard link to a file that exists, because there must be an
|
|
inode number to refer to. However, you can make a symlink to any filename,
|
|
whether or not there actually is such a filename.
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
<ul>
|
|
<li>
|
|
<p>
|
|
Removing a symlink removes only the link. It has no effect on the linked-to
|
|
file. Removing the only hard link to a file removes the file.
|
|
</p>
|
|
</li>
|
|
</ul>
|
|
|
|
<p>
|
|
Try this:
|
|
</p>
|
|
<ol type="1" start="1" >
|
|
<li>
|
|
<p>
|
|
<samp>cd; ln -s /tmp/me MyTmp</samp>
|
|
</p>
|
|
|
|
<p>
|
|
<samp>cd</samp> to your home directory. <samp>ln</samp> with the
|
|
<samp>-s</samp> option makes a symbolic link; in this case, one called
|
|
<samp>MyTmp</samp> which points to the filename <samp>/tmp/me</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="2" >
|
|
<li>
|
|
<p>
|
|
<samp>ls -l MyTmp</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Output should look like this:
|
|
</p>
|
|
|
|
<pre>
|
|
lrwxrwxrwx 1 havoc havoc 7 Dec 6 12:50 MyTmp -> /tmp/me
|
|
</pre>
|
|
|
|
<p>
|
|
The date and user/group names will be different for you, of course. Notice
|
|
that the file type is <samp>l</samp>, indicating that this is a symbolic link.
|
|
Also notice the permissions - symbolic links always have these permissions. If
|
|
you attempt to <samp>chmod</samp> a symlink, you'll actually change the
|
|
permissions on the file being pointed to.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="3" >
|
|
<li>
|
|
<p>
|
|
<samp>chmod 700 MyTmp</samp>
|
|
</p>
|
|
|
|
<p>
|
|
You will get a "No such file or directory" error, because the file
|
|
<samp>/tmp/me</samp> doesn't exist. Notice that you could create a symlink to
|
|
it anyway.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="4" >
|
|
<li>
|
|
<p>
|
|
<samp>mkdir /tmp/me</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Create the directory <samp>/tmp/me</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="5" >
|
|
<li>
|
|
<p>
|
|
<samp>chmod 700 MyTmp</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Should work now.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="6" >
|
|
<li>
|
|
<p>
|
|
<samp>touch MyTmp/myfile</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Create a file in <samp>MyTmp</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="7" >
|
|
<li>
|
|
<p>
|
|
<samp>ls /tmp/me</samp>
|
|
</p>
|
|
|
|
<p>
|
|
The file was actually created in <samp>/tmp/me</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="8" >
|
|
<li>
|
|
<p>
|
|
<samp>rm MyTmp</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Remove the symbolic link. Notice that this removes the link, not what it
|
|
points to. Thus you use <samp>rm</samp> not <samp>rmdir</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="9" >
|
|
<li>
|
|
<p>
|
|
<samp>rm /tmp/me/myfile; rmdir /tmp/me</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Clean up after ourselves.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
|
|
<hr>
|
|
|
|
<h4><a name="s-files-misc-types-devices"></a>16.2.2.2 Device files</h4>
|
|
|
|
<p>
|
|
Device files refer to physical or virtual devices on your system, such as your
|
|
hard disk, video card, screen, or keyboard. An example of a virtual device is
|
|
the console, represented by <samp>/dev/console</samp>.
|
|
</p>
|
|
|
|
<p>
|
|
There are two kinds of devices: <em>character devices</em> can be accessed one
|
|
character at a time, that is, the smallest unit of data which can be written to
|
|
or read from the device is a character (byte).
|
|
</p>
|
|
|
|
<p>
|
|
<em>Block devices</em> must be accessed in larger units called blocks, which
|
|
contain a number of characters. Your hard disk is a block device.
|
|
</p>
|
|
|
|
<p>
|
|
You can read and write device files just as you can from other kinds of files,
|
|
though the file may well contain some strange incomprehensible-to-humans
|
|
gibberish. Writing random data to these files is probably a Bad Idea.
|
|
Sometimes it's useful, though: for example, you can dump a postscript file into
|
|
the printer device <samp>/dev/lp0</samp>, or send modem commands to the device
|
|
file for the appropriate serial port.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h5><a name="s-advanced-files-types-devices-null"></a>16.2.2.2.1 <samp>/dev/null</samp></h5>
|
|
|
|
<p>
|
|
<samp>/dev/null</samp> is a special device file that discards anything you
|
|
write to it. If you don't want something, throw it in <samp>/dev/null</samp>.
|
|
It's essentially a bottomless pit. If you read <samp>/dev/null</samp>, you'll
|
|
get an end-of-file (EOF) character immediately. <samp>/dev/zero</samp> is
|
|
similar, only if you read from it you get the <samp>\0</samp> character (not
|
|
the same as the number zero).
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h4><a name="s-advanced-files-types-pipes"></a>16.2.2.3 Named pipes (FIFOs)</h4>
|
|
|
|
<p>
|
|
A named pipe is a file that acts like a pipe. You put something into the file,
|
|
and it comes out the other end. Thus it's called a FIFO, or
|
|
First-In-First-Out: the first thing you put in the pipe is the first thing to
|
|
come out the other end.
|
|
</p>
|
|
|
|
<p>
|
|
If you write to a named pipe, the process which is writing to the pipe doesn't
|
|
terminate until the information being written is read from the pipe. If you
|
|
read from a named pipe, the reading process waits until there's something to
|
|
read before terminating. The size of the pipe is always zero --- it doesn't
|
|
store data, it just links two processes like the shell <samp>|</samp>.
|
|
However, since this pipe has a name, the two processes don't have to be on the
|
|
same command line or even be run by the same user.
|
|
</p>
|
|
|
|
<p>
|
|
You can try it by doing the following:
|
|
</p>
|
|
<ol type="1" start="1" >
|
|
<li>
|
|
<p>
|
|
<samp>cd; mkfifo mypipe</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Makes the pipe.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="2" >
|
|
<li>
|
|
<p>
|
|
<samp>echo "hello" > mypipe &</samp>
|
|
</p>
|
|
|
|
<p>
|
|
Puts a process in the background which tries to write "hello" to the
|
|
pipe. Notice that the process doesn't return from the background; it is
|
|
waiting for someone to read from the pipe.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="3" >
|
|
<li>
|
|
<p>
|
|
<samp>cat mypipe</samp>
|
|
</p>
|
|
|
|
<p>
|
|
At this point the <samp>echo</samp> process should return, since
|
|
<samp>cat</samp> read from the pipe, and the <samp>cat</samp> process will
|
|
print <samp>hello</samp>.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
<ol type="1" start="4" >
|
|
<li>
|
|
<p>
|
|
<samp>rm mypipe</samp>
|
|
</p>
|
|
|
|
<p>
|
|
You can delete pipes just like any other file.
|
|
</p>
|
|
</li>
|
|
</ol>
|
|
|
|
<hr>
|
|
|
|
<h4><a name="s-advanced-files-types-sockets"></a>16.2.2.4 Sockets</h4>
|
|
|
|
<p>
|
|
Sockets are similar to pipes, only they work over the network. This is how
|
|
your computer does networking: you may have heard of "WinSock", which
|
|
is sockets for Windows.
|
|
</p>
|
|
|
|
<p>
|
|
We won't go into these further, because you probably won't have occasion to use
|
|
them unless you're programming. However, if you see a file marked with type
|
|
<samp>s</samp> on your computer, you know what it is.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-proc"></a>16.2.3 The <samp>proc</samp> filesystem</h3>
|
|
|
|
<p>
|
|
The Linux kernel makes a special filesystem available, which is mounted under
|
|
<samp>/proc</samp> on Debian systems. This is a "pseudo-filesystem"
|
|
--- it doesn't really exist on any of your physical devices.
|
|
</p>
|
|
|
|
<p>
|
|
The <samp>proc</samp> filesystem contains information about the system and
|
|
running processes. Some of the "files" in <samp>/proc</samp> are
|
|
reasonably understandable to humans (try typing <samp>cat /proc/meminfo</samp>
|
|
or <samp>cat /proc/cpuinfo</samp>) while some others are arcane collections of
|
|
numbers. Often, system utilities use these to gather information and present
|
|
it to you in a more understandable way.
|
|
</p>
|
|
|
|
<p>
|
|
People frequently panic when they notice one file in particular ---
|
|
<samp>/proc/kcore</samp> --- which is generally huge. This is (more or less) a
|
|
copy of the contents of your computer's memory. It's used to debug the kernel.
|
|
It doesn't actually exist anywhere, so don't worry about its size.
|
|
</p>
|
|
|
|
<p>
|
|
If you want to know about all the things in <samp>/proc</samp>, type <samp>man
|
|
5 proc</samp>.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-permissions"></a>16.2.4 Advanced aspects of file permissions</h3>
|
|
|
|
<hr>
|
|
|
|
<h4><a name="s-advanced-files-permissions-absolute"></a>16.2.4.1 Using numeric arguments with <samp>chmod</samp></h4>
|
|
|
|
<p>
|
|
Earlier in this chapter, we briefly mentioned that you can set file permissions
|
|
using numbers. The numeric notation is called an absolute mode, as opposed to
|
|
the symbolic notation (e.g. <samp>u+rx</samp>) which is often called a
|
|
relative mode. This is because the number specifies an exact mode to set, and
|
|
the symbol just specifies a change to make (e.g. "add user read and
|
|
execute permissions").
|
|
</p>
|
|
|
|
<p>
|
|
The numeric mode is a series of four octal digits or twelve binary digits.
|
|
Each octal (base eight) digit represents three binary digits: one octal digit
|
|
and three binary digits are two ways to represent the decimal digits 0 through
|
|
7.
|
|
</p>
|
|
|
|
<p>
|
|
Deriving a particular mode is pretty straightforward. You simply add up the
|
|
modes you want to combine, or subtract modes you don't want. For example, user
|
|
permissions, with only read permission turned on, would be <samp>100</samp> in
|
|
binary. User permissions with write only would be <samp>010</samp> binary.
|
|
User permissions with read and write both turned on would be <samp>100 + 010 =
|
|
110</samp>. Alternatively, you could put it in octal: <samp>4 + 2 = 6</samp>.
|
|
</p>
|
|
|
|
<p>
|
|
For the full mode, simply add up digits from this table:
|
|
</p>
|
|
|
|
<pre>
|
|
0001 others, execute
|
|
0002 others, write
|
|
0004 others, read
|
|
0010 group, execute
|
|
0020 group, write
|
|
0040 group, read
|
|
0100 user, execute
|
|
0200 user, write
|
|
0400 user, read
|
|
1000 sticky bit
|
|
2000 set group id
|
|
4000 set user id
|
|
</pre>
|
|
|
|
<p>
|
|
To use the table, first decide what permissions you want to set. Then add up
|
|
the numbers for those permissions. The total is your mode. For example, to
|
|
get mode <samp>0755</samp>:
|
|
</p>
|
|
|
|
<pre>
|
|
0001 o=x
|
|
0004 o=r
|
|
0010 g=x
|
|
0040 g=r
|
|
0100 u=x
|
|
0200 u=w
|
|
+ 0400 u=r
|
|
-------
|
|
0755 u=rwx go=rw
|
|
</pre>
|
|
|
|
<p>
|
|
You'd actually call this mode simply <samp>755</samp>, without the leading
|
|
<samp>0</samp>, because <samp>chmod</samp> automatically adds zeroes at the
|
|
beginning of the mode --- <samp>7</samp> means mode <samp>0007</samp>.
|
|
</p>
|
|
|
|
<p>
|
|
To set a file to <samp>755</samp>, you'd type <samp>chmod 755 myfile</samp>.
|
|
</p>
|
|
|
|
<p>
|
|
<samp>755</samp> is a very common mode for directories, as it allows anyone to
|
|
use the directory but only the owner to create and delete files in the
|
|
directory. <samp>644</samp> is the analogous mode for files, and it is also
|
|
very common. It allows anyone to use the file but only the owner can change
|
|
it. For executable files, <samp>755</samp> is a common mode; this is just
|
|
<samp>644</samp> plus execute permissions (<samp>644 + 111 = 755</samp>).
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-chattr"></a>16.2.5 chattr</h3>
|
|
|
|
<p>
|
|
A useful tip?
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-copying"></a>16.2.6 Large-scale copying</h3>
|
|
|
|
<p>
|
|
cp -a and variants on the theme.
|
|
</p>
|
|
|
|
<p>
|
|
how to copy an old system to a new one.
|
|
</p>
|
|
|
|
<p>
|
|
FIXME whoops, I also listed this topic under Backup Tools. need to decide.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h3><a name="s-advanced-files-undocumented"></a>16.2.7 Other concepts not yet covered, but should be</h3>
|
|
|
|
<p>
|
|
fsck, dd, fdisk, etc.
|
|
</p>
|
|
|
|
<p>
|
|
what package is a file in?
|
|
</p>
|
|
|
|
<p>
|
|
MSDOS vs. Mac vs. Unix text files
|
|
</p>
|
|
|
|
<p>
|
|
sync
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h2><a name="s-advanced-kernel"></a>16.3 Compiling the kernel</h2>
|
|
|
|
<p>
|
|
How, what, and why
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h2><a name="s-advanced-security"></a>16.4 A few words on security</h2>
|
|
|
|
<p>
|
|
The basics of security from a user standpoint. Maintaining one's privacy.
|
|
What other users can see of your account.
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<h2><a name="s-advanced-programming"></a>16.5 Programming on Linux</h2>
|
|
|
|
<p>
|
|
Something about the Linux programming environment. Aimed at, say, people
|
|
taking CS101. Nothing on <em>how</em> to program, just Emacs, gcc, gdb, ddd,
|
|
etc. as programming tools.
|
|
</p>
|
|
|
|
<p>
|
|
Likely based on debug.tex
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<p>
|
|
[ <a href="ch-troubleshooting.html">previous</a> ]
|
|
[ <a href="index.html#contents">Contents</a> ]
|
|
[ <a href="ch-about.html">1</a> ]
|
|
[ <a href="ch-introduction.html">2</a> ]
|
|
[ <a href="ch-start.html">3</a> ]
|
|
[ <a href="ch-basics.html">4</a> ]
|
|
[ <a href="ch-docs.html">5</a> ]
|
|
[ <a href="ch-shell.html">6</a> ]
|
|
[ <a href="ch-files.html">7</a> ]
|
|
[ <a href="ch-editor.html">8</a> ]
|
|
[ <a href="ch-custom_shell.html">9</a> ]
|
|
[ <a href="ch-X.html">10</a> ]
|
|
[ <a href="ch-text_tools.html">11</a> ]
|
|
[ <a href="ch-file_tools.html">12</a> ]
|
|
[ <a href="ch-disks.html">13</a> ]
|
|
[ <a href="ch-dpkg.html">14</a> ]
|
|
[ <a href="ch-troubleshooting.html">15</a> ]
|
|
[ 16 ]
|
|
[ <a href="ch-next.html">17</a> ]
|
|
[ <a href="ch-helping.html">18</a> ]
|
|
[ <a href="ap-apps_appendix.html">A</a> ]
|
|
[ <a href="ap-components_appendix.html">B</a> ]
|
|
[ <a href="ap-booting_appendix.html">C</a> ]
|
|
[ <a href="ap-misc_appendix.html">D</a> ]
|
|
[ <a href="ch-next.html">next</a> ]
|
|
</p>
|
|
|
|
<hr>
|
|
|
|
<p>
|
|
Debian Tutorial (Obsolete Documentation)
|
|
</p>
|
|
|
|
<address>
|
|
29 Dezember 2009<br>
|
|
<br>
|
|
Havoc Pennington <code><a href="mailto:hp@debian.org">hp@debian.org</a></code><br>
|
|
<br>
|
|
</address>
|
|
<hr>
|
|
|
|
</body>
|
|
|
|
</html>
|
|
|