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Guide to Linux File Command Mastery by Sheryl Calish

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Guide to Linux File Command Mastery by Sheryl Calish Linux Sysadmin Guide to Linux File Command Mastery By Sheryl Calish A crash course in Linux file commands for the newly initiated Published July 2004 Although GUI desktops such as KDE and GNOME help users take advantage of Linux features without functional knowledge of the command-line interface, more power and flexibility are often required. Moreover, a basic familiarity with these commands is still essential to properly automate certain functions in a shell script. This article is a "crash course" in Linux file commands for those who are either new to the operating system or simply in need of a refresher. It includes a brief overview of the more useful commands as well as guidance regarding their most powerful applications. Combined with a little experimentation, the information included here should lead to an easy mastery of these essential commands. (Note: When a kernel tweaked with Oracle Cluster File System (OCFS2) is involved, some of these commands may behave somewhat differently. In that case, Oracle provides an OCFS2 toolset that can be a better alternative for file command purposes.) Note that all the included examples were tested on SUSE Linux 8.0 Professional. While there is no reason to believe they will not work on other systems, you should check your documentation for possible variations if problems arise. Background Concepts Before delving into specifics, let's review some basics. Files and Commands Everything is treated as a file in the Linux/UNIX operating system: hardware devices, including the keyboard and the terminal, directories, the commands themselves and, of course, files. This curious convention is, in fact, the basis for the power and flexibility of Linux/UNIX. Most commands, with few variations, take the form: command [option] [source file(s)] [target file] Getting Help Among the most useful commands, especially for those learning Linux, are those that provide help. Two important sources of information in Linux are the on-line reference manuals, or man pages, and the whatis facility. You can access an unfamiliar command's man page description with the whatis command. $ whatis echo To learn more about that command use: $ man echo If you do not know the command needed for a specific task, you can generate possibilities using man -k , also known as apropos , and a topic. For example: $ man -k files One useful but often-overlooked command provides information about using man itself: $ man man You can scroll through any man page using the SPACEBAR; the UP ARROW will scroll backward through the file. To quit, enter q,!, or CTRL-Z. Classes of Users Remember the adage "All animals are equal but some animals are more equal than others?" In the Linux world, the root user rules. The root user can log in from another username as su , derived from "superuser." To perform tasks such as adding a new user, printer, or file system, log in as root or change to superuser with the su command and root password. System files, including those that control the initialization process, belong to root. While they may be available for regular users to read, for the sake of your system security, the right to edit should be reserved for root. The BASH shell Other shells are available, but BASH, the Bourne Again Shell, is the Linux default. It incorporates features of its namesake, the Bourne shell, and those of the Korn, C and TCSH shells. The BASH built-in command history remembers the last 500 commands entered by default. They can be viewed by entering history at the command prompt. A specific command is retrieved by pressing the UP ARROW or DOWN ARROW at the command prompt, or by entering its number in the history list preceded by "!", such as: $ !49 You can also execute a command by its offset from the highest entry in the history list: $ !-3 would execute event number 51, if there were 53 events in the history list. Like other shells in the UNIX/Linux world, BASH uses special environment variables to facilitate system administration. Some examples are: HOME, the user's home directory PATH, the search path Linux uses to search for executable images of commands you enter HISTSIZE, the number of history events saved by your system In addition to these reserved keywords, you can define your own environment variables. Oracle, for example, uses ORACLE_HOME, among other variables, that must be set in your environment for an Oracle installation to complete successfully. Variables can be set temporarily at the prompt: $HISTSIZE=100 Or, set permanently, either on a system wide basis in /etc/profile (requires root privileges), or locally in .profile . The value of an environment variable can be viewed with the echo command using a $ to access the value. $ echo $HOME /home/bluher All current environment variables can be viewed with env . Regular Expressions and Wildcards Many Linux commands use the wildcards * and ? to match any number of characters or any single character respectively; regular pattern-matching expressions utilize a period (.) to match any single character except "new line." Both use square brackets ([ ]) to match sets of characters in addition to *. The *, however, has a similar but different meaning in each case: Although it will match one or more characters in the shell, it matches zero or more instances of the preceding character in a regular expression. Some commands, like egrep and awk , use a wider set of special characters for pattern matching. File Manipulation Commands Anatomy of a File Listing The ls command, used to view lists of files in any directory to which a user has execute permission, has many interesting options. For example: $ ls -liah * 22684 -rw-r--r-- 1 bluher users 952 Dec 28 18:43 .profile 19942 -rw-r--r-- 1 scalish users 30 Jan 3 20:00 test2.out 925 -rwxr-xr-x 1 scalish users 378 Sep 2 2002 test.sh The listing above shows 8 columns: The first column indicates the inode of the file, because we used the -i option. The remaining columns are normally displayed with the -l option. The second column shows the file type and file access permissions. The third shows the number of links, including directories. The fourth and fifth columns show the owner and the group owner of the files. Here, the owner "bluher" belongs to the group "users". The sixth column displays the file size with the units displayed, rather than the default number of bytes, because we used the -h option. The seventh column shows the date, which looks like three columns consisting of the month, day and year or time of day. The eighth column has the filenames. Use of -a in the option list causes the list of hidden files, like .profile, to be included in the listing. Working with Files Files and directories can be moved ( mv ), copied ( cp ) or removed ( rm ). Judicious use of the -i option to get confirmation is usually a good idea. $ cp -i ls.out ls2.out cp: overwrite `ls2.out'? The mv command allows the -b option, which makes a backup copy before moving files. Both rm and cp accept the powerful, but dangerous, -r option, which operates recursively on a directory and its files. $ rm -ir Test rm: descend into directory `Test'? y Directories can be created with mkdir and removed with rmdir . However, because a directory containing files cannot be removed with rmdir , it is frequently more convenient to use rm with the -r option. All files have ownership and protections for security reasons. The file access permissions, or filemode, comprise the same 10 characters described previously: The first character indicates the type of file. The most common are - for a file, d for a directory, and l for a link. The next nine characters are access permissions for three classes of users: the file owner (characters 2-4), the user's group (5-7) and others (8-10), where r signifies read permission, w means write permission, and x designates execute permission on a file. A dash, -, found in any of these nine positions indicates that action is prohibited by that class of user. Access permissions can be set with character symbols or, binary masks using the chmod command. To use the binary masks, convert the character representation of the three groups of permissions into binary and then into octal format: User class: Owner Group Others character representation: rwx r-x r-- binary representation: 111 101 100 octal representation: 7 5 4 To give write permission to the group, you could use: chmod g+w test.sh or chmod 774 test.sh Default file permissions are set with the umask command, either systemwide in the /etc/init.dev file or locally in the .profile file. This command indicates the number to be subtracted from 777 to obtain the default permissions: $ umask 022 This would result in a default file permission of 755 for all new files created by the user. A file's ownership can be changed with chown : $ chown bluher ls.out Here, bluher is the new file owner. Similarly, group membership would be changed as follows: $ chgrp devgrp ls.out Here, devgrp is the new group. One piece of information that ls does not provide is which files are text, and which are binary. To find this information, you can use the file * command. Renaming Files Two popular ways to give a file more than one name are with links and the alias command. Alias can be used to rename a longer command to something more convenient such as: $ alias ll='ls -l' $ ll Notice the use of single quotes so that BASH passes the term on to alias instead of evaluating it itself.
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