Indholdsfortegnelse for Linux Shell Handbook - 2021 i Linux Magazine Special Editions

When it comes to finding and identifying files on your system, you are spoiled for choice. Linux offers a variety of tools that can help you locate files and programs, including find, locate, whereis, and which. These tools are not particularly difficult in use, and mastering them can help you use your Linux system more efficiently. Finding Files with find The find tool lets you search for files by name or a part of the name. By default, find searches recursively, meaning it looks for files through the entire directory tree. At the very minimum, find requires two options: a path to the directory where the search should start and the name of the file to look for. The name of the file is specified with the -name switch. For…

A hard disk on a modern computer is usually divided into partitions. A partition can contain exactly one filesystem (the data structure that stores files and directories). In Linux, swap space is usually implemented as a filesystem of its own, requiring its own partition. Additionally, many users create separate partitions for the /boot, /var/, and /home directories. In theory, although Linux can be installed on one partition, likely you’ll find multiple partitions. Before you can install an operating system, you need to create partitions and format them with filesystems. Most Linux installers provide a GUI for creating and managing partitions during the installation process, but if your system is already installed, you can turn to several management utilities for configuring partitions. The Bash command line provides several utilities for creating…

An incorrectly set PC clock can be disastrous – if your computer loses track of the time, you could end up juggling files from the future or email from 30 years ago. The time warp could lead to misunderstandings, errors, or even crashes. Almost all Linux distributions set the time and time zone during the installation phase, and desktop environments such as KDE and Gnome display a clock in the panel to give users quick access to tools for configuring the computer clock (Figure 1). In the shell, cal displays a simple but neatly formatted calendar. The date command gives you the date and time, although the output itself is fairly sparse. Additionally, this program can help the administrator set the date and time. The date tool also demonstrates its…

FTP (File Transfer Protocol) is an Internet protocol that helps users transfer files between two computers: one computer is the client and the other is the server. The server listens for connection requests from across the network. An FTP client application running on the client computer initiates connections and transmits user requests to the server. Most Linux systems include a built-in, command-line FTP client that supports the commands described in this article. Many FTP servers on the Internet provide anonymous services, for which you enter anonymous or ftp as your login name and do not provide a password, although it is considered a friendly gesture to enter your own email address. Other servers prompt you to enter your username and a password. FTP transmits usernames, passwords, and other data across…

Working with images and PDFs often requires similar processes, including the need to apply an operation to multiple files or to apply multiple processing steps to one or more files. Before graphical desktops were the norm, many Linux users ran command-line programs to process their images. Although it might seem paradoxical to use a non-graphical environment to process graphical files, in many cases, the command line is the easiest place from which to operate. On another front, the ability to create and use PDF files came late to Linux users because the proprietary format was locked down by Adobe. In mid-2008, however, the specifications were released into the wild, and Linux users now have a number of tools at their disposal that allow them to create and modify PDFs. ImageMagick…

Many desktop users approach the command line as though armed with a magic spell. They have a command – complete with options – to type or paste to get the desired results, but they are unclear what else might be going on. This approach is understandable; however, if you take the time to understand something of the structure of the command line, you can increase control over your computing. By default, most Linux distributions run Bash (the Bourne Again Shell). Bash is a command-line interpreter – a program that runs macros and other utilities. These macros and utilities are the commands that you enter at the prompt. They include those built into Bash, such as cd, and many others that are external, including most of the commands that you run.…

Most computer systems have an assortment of tools for filtering and processing data. A virus scanner, a spam fighter, a web search engine, a spell checker – each is a filter that sifts though data to isolate the information you really need. Your shell provides a filter, too. For example, ls *.jpg lists only JPEG images. Because so much of Linux depends on interpreting and processing plain text files, an entire shorthand exists for creating filters. The shorthand is called regular expressions, or regex. A regex applied to text can find, dissect, and extract virtually any pattern you seek. Table 1 shows some common regex operators, which you can string together and use in combination to build arbitrarily complex filters. The origin of regex dates back some 60 years to…

Unix-like operating systems have historically been very much about text processing. Really, the Unix design religion is: Make simple tools whose output can be manipulated by others with the use of pipes and other forms of output redirection. In this article, I’ll look at the wealth of Linux command-line tools for combining, selecting, extracting, and otherwise manipulating text. wc The wc (word count) command is a simple filter that you can use to count the number of lines, characters (bytes), and, yes, even the number of words in a file. Whereas counting lines and bytes tends to be useful, I rarely find myself using wc to count words. You can count lines in a file with wc-l: If you don’t specify a file name, wc will also read the standard…

Users and groups are concepts central to multiuser operating systems. Assigning a name to a user account allows users to log in separately, set up their own environments, and control access to their private files by assigning permissions. Strategies for applying users and groups have changed through the years, but essentially, a group is a collection of users typically assigned access to a collection of resources associated with a specific function or profile. For instance, the financial group might be assigned access to a common directory with financial documents or access to the printer located in the financial office. On home systems with only a few users, groups sometimes matter so little that users mostly ignore them. In fact, you can sometimes hear suggestions that groups are obsolete and should…

Linux and other Unix-based systems often offer several alternatives for solving a single problem. Although this approach can be confusing to some novice administrators, it is one of the defining features of the open source environment. The networking tool collection is no exception to this practice. You’ll find an array of useful tools – some overlapping and some unique – for configuring, managing, and troubleshooting network connections. In this article, we highlight some favorite tools in the networking collection. Of course, a full description of the complete TCP/IP networking environment could fill up a very long book. Here, we assume you have some basic knowledge of TCP/IP networking concepts such as routing, addressing, and name resolution. Interfaces The traditional method for naming Ethernet adapters and other network devices is with…

Rsync is the perfect synchronization tool for keeping your data in sync. The program manages file properties and uses SSH to encrypt your data, and it is perfect for transferring large volumes of data if the target computer has a copy of a previous version. Rsync checks for differences between the source and target versions. The tool that has been developed by the Samba team [1] uses an efficient checksum-search algorithm for comparing data; rsync only transfers the differences between the two sides and therefore saves time and bandwidth. In Sync The generic syntax for rsync is rsync [options] source target, where target can be a local target on the same machine or a remote target on another machine. The choice of source and target is critical; decide carefully in…

GNU/Linux treats everything as a file. For this reason, learning file management commands should be among your first priorities. These commands are easy to remember because their names are usually abbreviations of their actions – for example, mv for move and ls for list – but their options can take time to learn. Basically, file management commands fall into three categories: directory and file movement, navigation and editing, and compression. Commands in all three categories are typically more powerful (and potentially more dangerous) than their desktop equivalents, thanks mainly to file globbing, or the use of standard patterns to refer to multiple files. Moving and Editing Directories and Files The most basic command for moving directories and files is cp. Its structure is simple: cp <options> <filesourcefile> <target>. By default,…

The Linux command line provides hundreds of small utilities to read, write, parse, and analyze data. With just a few extra keystrokes, you can combine those utilities into innumerable impromptu applications. For example, imagine you must extract an actor’s lines. That is to say, given the text shown in Listing 1, you must produce That's what they call a sanity clause for Groucho. The grep command can find substrings, strings, and patterns in a text file. You can use grep to find all lines that begin with GROUCHO. Then you can use cut to divide the matching lines into pieces and combine the two commands with a pipe (|): The grep clause searches the file marx.txt for all occurrences of “Groucho” that appear at the beginning of a line (-E…

When you install software in Linux, dependencies (the necessary libraries and utilities) are added automatically. Years ago, the Debian package system was the first to include this feature, and a whole ecosystem of utilities has grown up around it. Soon after, most other distributions added their own dependency-resolving features. Today, software installations rarely fail because a dependency wasn’t installed. The dreaded “dependency hell” is mostly a thing of the past, unless you try to install from poorly maintained third-party sources or mix packages from different repositories. Linux includes far too many different package managers to cover them all in a single article. Presented here are only the most widely used ones, from Debian and Fedora, along with the socalled universal package managers. Debian and Debian Derivatives Debian and derivatives like…

Telnet is probably the best-known solution for providing users with console access to remote machines. However convenient this dinosaur of network communication might be, it has one major disadvantage: All the data is sent in plaintext over the wire. If an attacker sniffs the connection, he or she will quickly learn the administrative password for the server. Admittedly, it probably isn’t quite that easy, but the danger is there all the same. For this reason, all popular Linux distributions install the Secure Shell (SSH) as a safer alternative. Getting Started with SSH First, a bit of theory. The SSH client/server architecture is based on TCP/IP. The SSH server (sshd) runs on one machine, where it listens for incoming connections on TCP port 22. The client simply uses this port to…

In recent years, wireless connectivity has become nearly automatic, with a few connection settings handled mostly through the GUI interface. However, it is certainly still possible to search for wireless networks and initiate connections at the command line. The iw utility used to be the leading tool for text-based wireless configuration in Linux, but iw only works with the unsafe and obsolete Wired Equivalent Protocol (WEP). Most Linux systems today use WPA Supplicant [1] with the wpa_cli front end for terminal sessions. A new tool called the iNet Wireless Daemon (iwd) is a smaller and simpler tool that could one day replace WPA Supplicant as the standard option for command-line wireless configuration. WPA Supplicant Most distributions come with wpa_supplicant preinstalled. The standard configuration file is found in /etc/wpa_supplicant/wpa_supplicant.conf. If you…

Shell scripts are a lazy person’s best friend. That may sound strange, because writing a shell script presumably takes work, but it’s true. Writing a shell script to perform a repetitive task requires some time up front, but once the script is finished, using it frees up the time the task used to take. In this article, I will introduce you to writing shell scripts with Bash. I’ll describe Bash scripting in the context of several common tasks. Because this is an introductory discussion, some nuances are glossed over or ignored, but I will provide plenty of information for you to get started on your own scripts. Hello, Bash In its simplest form, a shell script is a file with a list of commands. For example, a user created this…