linux crash course chapter 04 2

Chapter 4: The Linux Filesystem Where stuff is In this chapter … • • • • • What is a hierarchical filesystem Directories and files Pathnames: absolute vs relative Permissions Links What is a filesystem? • • • • A data structure Stores data Organizes data Allows for data retrieval and manipulation What is a hierarchical filesystem? • Essentially, a filesystem that allows nesting of folders under a central point • Like a pyramid or upside-down tree • Tree analogy most common – ie the directory tree • Programmers – definition of a tree applies here Hierarchical Filesystem • Directories can contain other directories and/or ordinary files • Concept different from reality – in implementation everything is a file • Directories, devices, named pipes, ordinary files – all really just files Lingo • Root directory • Subdirectories • Parents, children Filenames • Each file within a directory must have a wholly unique filename • Can be up to 255 characters – make them longer to avoid confusion • Special characters must either be escaped out (using backslash) or in quotes • Only illegal characters are / and carriage return Filenames con’t • Spaces and other special characters a bad idea • Instead of spaces use underscores or periods – Ex: my_file or my.file Extensions • Not always essential but helps simplify and avoid confusion • Some programs like gcc depend on proper extensions • This includes case! Hidden Files • To make a file hidden, start it with a period – Ex plan • A normal ls will not show hidden files • Use ls –a to reveal ALL files • Startup files, containing configuration settings for your account, hidden rmdir – remove directory • Syntax rmdir directory • Only deletes empty directories • Not empty? Delete the files with rm and try rmdir again • Lazy? rm –r directory will recursively delete a directory and its contents (files and directories) • Use with caution! touch – create a file • Syntax: touch filename • Creates an empty file (size 0) • Useful to create placeholders or while learning the interface mv revisited • Already used mv to rename files • If last argument is a directory, mv moves files into a different directory • If given a directory as the first argument, mv moves the directory to the new name supplied (which can either be a rename or move!) Permissions • Use a ls –l (for long view) and you might see something like this: drwxr-xr-x -rw-rw-r-drwxrwxr-x jhowell jhowell 4096 Aug 18 15:46 Desktop jhowell jhowell Sep 18:08 myfile jhowell jhowell 4096 Aug 22 15:32 public_html # of links Type of file File Permissions filename group user size Date and time created / accessed File types • • • • • • - ordinary file b block device c character device d directory p named pipe l symbolic link File permissions • Three types of permissions – read, write and execute • Three sets of permissions – owner (user), group, and other (everyone else) rwx rwx rwx user (owner) group other chmod – CHange MODe • Changes permissions • Syntax: chmod [ugoa][+-][rwx] • Ex: grant everyone (all) read and write chmod a+rw myfile • Ex: remove execute permission for other chmod o-x myfile chmod – alternate syntax • Instead of [ugoa][+-][rwx], use binary equivalent rwx rwx rwx 421 421 421 • For each section, sum up the permissions, and assemble a three digit number • So full access to everyone would be 777 Alternate syntax example • Grant user full access, group read and execute, and deny access to other rwx r-x 4+2+1=7 0+0+0=0 4+0+1=5 • So we get chmod 750 myfile Permissions Caveats • To execute a shell script you must allow both read and execute permissions • To get into a directory, you must have execute permission • root can still read and write to files without read and write permissions One more exception • setuid and setguid • Allows a file to be executed with the permissions of the file’s owner or group • A way to let users perform privileged tasks without granting them general permissions • Should be used sparingly with files owned by root Links • Pointers to files • Points to an exact location on disk • When a file is created, it is the first link to a particular spot on the disk • To make a file appear in multiple directories, make additional links Working with Links • • • • • Syntax: ln [-s] existing-file new-link Without –s a hard link is created With –s a soft or symbolic link is created To delete a link use rm Delete all the hard links and the file is ‘deleted’ Hard Links • • • • Points to a precise inode on the disk Now file appears in two locations Only one copy of the data is stored When you create a file, you allocate disk space and create a hard link • Hard links can only be used on a single FS • Can’t link to directories Soft Links • Also called symbolic links or symlinks • Instead of pointing to inode, points to the pathname of a hard link • Move the original file, symlink breaks • Symlinks don’t touch the data directly – safer • When using ln existing-file should be an absolute pathname ... jhowell 4096 Aug 18 15:46 Desktop jhowell jhowell Sep 18:08 myfile jhowell jhowell 4096 Aug 22 15: 32 public_html # of links Type of file File Permissions filename group user size Date and time... myfile chmod – alternate syntax • Instead of [ugoa][+-][rwx], use binary equivalent rwx rwx rwx 421 421 421 • For each section, sum up the permissions, and assemble a three digit number • So full access... Standards, but it’s not uncommon to find things in odd places • Even less standardized going from Linux to BSD to UNIX • In other words – no guarantees Common Directories • • • • • • • / (root)